Canine Thoracolumbar Intervertebral Disk Disease: Diagnosis, Prognosis, and Treatment*

3 CE CREDITS CE Article

❯❯ John F. Griffin IV, DVM Abstract: Thoracolumbar intervertebral disk disease (IVDD) is a common, important cause of ❯❯ Jonathan M. Levine, DVM, paraspinal hyperesthesia, pelvic limb ataxia, paraparesis, paraplegia, and urinary and fecal in- DACVIM (Neurology) continence in dogs. A companion article reviewed pathophysiology, epidemiology, physical ex- ❯❯ Sharon C. Kerwin, DVM, amination, and emergency medical therapy. This article addresses the diagnosis, prognosis, and MS, DACVS treatment of dogs with thoracolumbar IVDD. ❯❯ Robert C. Cole, DVM, DACVR horacolumbar intervertebral disk better positioning and higher-quality radio- Texas A&M University College Station, Texas disease (IVDD) is a broad term, graphs but may not be necessary. Survey Tencompassing disk degeneration radiography can be conducted under anes- and clinical neurologic disease due to disk thesia immediately before myelography and herniation. Canine IVDD is the most com- can identify the primary site of disk hernia- mon cause of thoracolumbar myelopathy tion in 51% to 61% of cases; its ability to with paraspinal hyperesthesia.1,2 A thor- identify secondary sites of disk herniation ough understanding of diagnostic modali- is less reliable.2,6 Surgical outcome is poorer ties, prognosis, and treatment options is in dogs treated surgically on the basis of At a Glance crucial to medical decision making and survey radiography alone compared with comprehensive care. dogs that undergo myelography.7 Diagnostic Evaluation Radiographic signs suggestive of inter- Page E1 Diagnostic Evaluation vertebral disk herniation include narrowing Definitive Treatment The preanesthetic evaluation of dogs with or wedging of the disk space, decreased Page E4 suspected disk herniation should include size of the intervertebral foramen, reduced P r o g n o s i s a complete blood cell count, serum chem- space between articular facets, and min- Page E6 istry panel, urinalysis, and thoracic radi- eralized disk material in the vertebral Postoperative Care ography in geriatric dogs to evaluate canal or overlying intervertebral foramen6 Page E8 cardiovascular structures and rule out (Figure 1). Spondylosis deformans, while Pharmacotherapy for pulmonary metastatic nodules. A urine potentially associated with disk protrusion, Neurogenic Bladder culture should be obtained in dogs with is not seen with increased frequency at Dysfunction urine retention, regardless of whether sites of disk extrusion.8 Survey radiography Page E10 pyuria is present.3–5 does not confirm the presence of spinal cord compression or localize lesions accu- Radiography rately enough for surgery. Surgical localiza- Vertebral column radiography should be tion must be established by myelography, performed in all dogs with suspected IVDD. computed tomography (CT), or magnetic Vertebral column radiography is helpful resonance imaging (MRI). Because these in ruling out diskospondylitis, trauma, or tests are expensive, require anesthesia, vertebral neoplasia. Anesthesia allows for and may have side effects, they are gener-

*A companion article, “Canine Thoracolumbar Intervertebral Disk Disease: Pathophysiology, Neu- rologic Examination, and Emergency Medical Therapy,” is also available on CompendiumVet.com.

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FIGURE 1 ally reserved for surgical candidates and cases in which other medically treatable etiologies have been excluded.

Myelography Myelography has been the mainstay of localization of spinal cord compression caused by intervertebral disk herniation in veterinary medicine.9–13 Myelographic and surgical findings have a strong correlation, with myelography correctly identifying the site of Lateral survey radiographic image showing narrowing of the space intervertebral disk herniation in 85.7% to 98% between articular facets at L1–L2 and L2–L3. Note the appearance of of cases.14–16 Myelography also aids in deter- mineralized opacity in the vertebral canal dorsal to the L1–L2 intervertebral disk mining the lateralization of disk extrusions space (arrow). The intervertebral foramen at L2–L3 is difficult to see. causing myelopathy.17 Compressive lesions may be seen on myelography in dogs with back pain alone as a clinical sign of intervertebral FIGURE 2 disk herniation.18 Compared with CT and MRI, myelography is relatively inexpensive and does not require specialized equipment. The major disadvantages of myelography are side effects and artifacts caused by epidural contrast injection. Myelographic contrast media can cause asystole, seizures, and renal failure and aggra- vate spinal cord dysfunction.19–21 Artifacts that can confound the evaluation of myelograms include epidural, central canal, and subdural filling and direct parenchymal injection.22 In Lateral myelographic image of the dog in Figure 1. Note the dorsal a comparison of CT, survey radiography, and deviation of the ventral contrast column at the L1–2 and L2–L3 intervertebral myelography for the evaluation of vertebral disk spaces. The dorsal contrast column above the L2–L3 disk space is also and spinal cord tumors, myelography outper- attenuated. formed CT in classifying intradural lesions.23 Myelographic findings suggestive of extra- FIGURE 3 dural spinal cord compression secondary to disk herniation include dorsal deviation of the ventral subarachnoid contrast column and thinning of the dorsal contrast column dorsal to a disk space on the lateral view2,15,16 (Figure 2). On the ventrodorsal view, this appears as attenuation of one or both lateral subarachnoid contrast columns2,15,16 (Figure 3). In acute cases, an intramedullary pattern secondary to severe cord swelling may mask the radiographic signs of disk extrusion,24 causing attenuation of both contrast columns on the lateral and ventrodorsal projections. In these situations, Ventrodorsal myelo- careful evaluation for slight axial deviation graphic image of the dog of a contrast column or comparison with the in Figure 1. Note the bilat- survey radiographs may indicate the origin of eral attenuation of the con- the disk extrusion. In addition, oblique radio- trast columns and the axial deviation of the left contrast graphic projections (left ventral–right dorsal column, suggesting a left- and right ventral–left dorsal) are useful when sided disk herniation. the lateral and ventrodorsal projections only

E2 Compendium: Continuing Education for Veterinarians® | March 2009 | CompendiumVet.com FREE Canine Thoracolumbar IVDD: Diagnosis CE show cord swelling. If the disk material is FIGURE 4 located significantly lateral to midline, oblique views may show a classic extradural pattern, whereas the traditional views may only show attenuation of the contrast columns.24

Computed Tomography CT allows for assessment of extradural spinal cord compressive lesions.25 Herniated disk material appears on CT images as a heterogeneous, isoattenuating-to-hyperattenuating extradural mass, depending on the mineral content of the herniated disk material.25,26 With chronicity, herniated disk material may become more homogenous and hyperattenuating as the mineral content increases25,26 (Figure 4). Epidural hemorrhage typically appears slightly more attenuating than the spinal cord. CT is not very effective at differentiating structures Transverse CT images of a left-sided intervertebral disk extrusion at within the dura.23 T12–T13 (arrows). (A) Soft tissue window. (B) Bone window. CT may be combined with myelography to allow visualization of the subarachnoid space, improve accuracy in differentiating intramed- not correlate with histopathologic degenera- ullary from extradural causes of spinal cord tion in dogs.30,31 In one study, MRI was 100% swelling, and definitively determine the loca- sensitive and 79% specific for detecting disk tion of herniated disk material. The advantages degeneration in nonchondrodystrophoid dogs of CT include few side effects and the ability compared with histopathology.31 The lack of to view reformatted images. CT is useful in specificity was caused by MRI findings sug- demonstrating gross morphologic changes in gesting nuclear and annular degeneration in the disk, such as mineralization, but is not a few histologically normal disks. As disk helpful in assessing subtle changes suggestive degeneration almost always precedes disk of disk degeneration.27 herniation, normal MRI findings generally confirm the absence of disk herniation at Magnetic Resonance Imaging that site.28 MRI allows early recognition and classifica- MRI is helpful in the classification of tion of disk degeneration with minimal intra disk herniation as extrusion, protrusion, or observer and interobserver variability.28,29 The bulge.28,32 Disk extrusion (Hansen’s type I normal canine intervertebral disk has a clear IVDD) is defined as complete rupture of the demarcation between the annulus fibrosus annulus fibrosus with translocation of the and nucleus pulposus, with the nucleus pul- nucleus pulposus into the vertebral canal.28,32 posus on T2-weighted images appearing as an Disk protrusion (Hansen’s type II IVDD) is ovoid area of high signal intensity compared caused by rupture of the inner layers of the with the relatively hypointense annulus fibro- annulus fibrosus, partial displacement of the sus28 (Figures 5 and 6). The signal intensity of nucleus into the disrupted annulus, and annu- the nucleus pulposus on T2-weighted images lar hypertrophy.28,32 Disk bulge is defined as correlates directly with proteoglycan concentra- symmetrical hypertrophy of the annulus fibro- tion.30 T1- and proton-density−weighted images sus.28 Additional MRI findings suggesting inter- have a high signal-to-noise ratio and are useful vertebral disk herniation include fragmentation for showing anatomic detail.26 of disk material and displacement of the adja- The appearance of a dark cleft in the cent epidural fat.33 Intravenous (IV) contrast nucleus pulposus in humans correlates with material can be used to help differentiate scar- a mild increase in collagen content and a ring, granulation tissue, and residual disk mate- mild decrease in water content, but it may rial in dogs with previous surgery.26

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FIGURE 5 niation at 5 days or fewer after onset, but even in these cases it is rare to see counts greater than 5/μL or protein values exceeding 29.75 mg/dL in samples acquired from the cerebellomedullary cistern.36,37 Both leukocyte counts and protein levels are usually higher in samples acquired from the lumbar cistern than in those from the cerebellomedullary cistern.36,37 Median CSF leukocyte counts and protein levels after lumbar cistern sample acquisition in dogs with acute, severe myelopathy secondary to intervertebral disk herniation have been reported at 5/μL and 419 mg/dL, respectively.36,37 CSF samples are usually obtained in conjunction with myelography and are seldom associated with serious side effects.38

Transverse T2-weighted MRI studies at T13–L1 (A) and L1–L2 (B). Definitive Treatment There is a large, hypointense structure in the vertebral canal at T13–L1 (blue Nonsurgical Management arrow). This is strongly suggestive of disk extrusion causing compression of the The traditional cornerstone of nonsurgical treat- spinal cord and subarachnoid space, as well as displacement of epidural fat. This ment of IVDD is strict cage confinement for 4 intervertebral disk extrusion nearly obliterates the cross-sectional area of the ver- to 6 weeks.2,9,39 Owner compliance is critical if tebral canal. At L1–L2, note the lack of T2 hyperintensity in the expected region of 2,9 the nucleus pulposus (green arrow). The subarachnoid space and epidural fat are conservative management is to succeed. Strict preserved at this level (yellow arrow). cage rest should be clearly defined, and owners should be advised of the risks of failure to follow instructions.2,9 Owners must also monitor MRI has several advantages in the diagnosis for dysuria or progressive neurologic signs. of intervertebral disk herniation. In addition to Appropriate analgesia is important. Severely, demonstrating subtle degenerative changes in acutely affected dogs are not ideal candidates the intervertebral disk, it provides more detailed for nonsurgical treatment but may recover if IV localization of extruded disk material than fluid therapy and injectable analgesics are used myelography.28,33 It also allows for assessment with cage rest during hospitalization. Oral anal- of soft tissue structures such as the spinal cord gesics (e.g., tramadol) are beneficial for at-home parenchyma and ligaments, helping to deter- management. Antiinflammatory doses of pred- mine prognosis.34 Another advantage of MRI is nisone or NSAIDs may also be helpful in keep- the avoidance of complications associated with ing the dog comfortable and minimizing disk myelographic contrast agents. Disadvantages inflammation. Acupuncture has been described include limited availability and high cost. but has not been proved beneficial.40,41

Cerebrospinal Fluid Evaluation Surgical Treatment Cerebrospinal fluid (CSF) evaluation can rule Surgery decompresses the spinal cord and out meningomyelitis before myelography or removes herniated disk material. This is accom- surgery. Disk herniation can be an inciden- plished by a dorsal or dorsolateral approach to tal finding in neurologically normal animals the vertebra and hemilaminectomy, pediculec- or those with myelitis. Additionally, myelog- tomy, or dorsal laminectomy.2,9,14,42–48 Advanced raphy may exacerbate neurologic abnormali- imaging (myelography, CT, or MRI) and surgery ties in dogs with meningomyelitis.35 The mild are generally performed during the same anes- changes typically seen with disk herniation are thetic period. Meaningful comparisons among generally not helpful in differentiating among surgical procedures have not been performed. possible causes of noninfectious, noninflamma- Hemilaminectomy involves the removal tory, focal central nervous system disease.36,37 of the articular facets, lamina, and pedicle.47 Elevations in CSF leukocyte counts are most Dorsal laminectomy typically involves removal pronounced in acute, severe cases of disk her- of the spinous process, lamina, and (poten-

E4 Compendium: Continuing Education for Veterinarians® | March 2009 | CompendiumVet.com FREE Canine Thoracolumbar IVDD: Diagnosis CE tially) the articular processes or pedicle.9,6,49 FIGURE 6 Pediculectomy is the removal of the pedicle with preservation of the articular facets.50 Pediculectomy has also been described as minihemilaminectomy and modified lateral decompression.14,42,44,48,50 Hemilaminectomy offers a few potential benefits over dorsal laminectomy. Herniated disk material may be more accessible for removal.7,47 Formation of a laminectomy membrane (cicatrix) may be less likely, although this occurred in only two of 187 dogs treated by dorsal laminectomy in one study.45,51 One report comparing hemilaminectomy with dorsal laminectomy for the treatment of disk extrusion in dachshunds found a significantly improved outcome with hemilaminectomy.7 Survey radiography, with or without myelogra- Sagittal T2-weighted MRI study of the dog in Figure 5. There is a severe phy, was used for surgical localization. Forty of intervertebral disk extrusion at T13–L1 (arrow) and degeneration of each inter- 47 dogs (85%) treated with hemilaminectomy vertebral disk. The large structure of low-signal intensity in the vertebral canal at and 24 of 51 dogs (47%) treated with dorsal T13–L1 causes compression of the spinal cord and subarachnoid space, as well as displacement of epidural fat. There is equivocal evidence of disk bulge or pro- laminectomy underwent prior myelography, an trusion at T12–T13, L1–L2, L2–L3, and L3–L4. These intervertebral disks are not inconsistency that makes it difficult to interpret associated with attenuation of the dorsal aspect of the subarachnoid space and the differences between treatment groups. This epidural fat (circle). study highlights the need for advanced imaging to confirm the location of disk herniation. Lateral corpectomy has been described as a trauma.53 The potential therapeutic benefit of treatment for chronic disk herniation in dogs.52 durotomy was lost after 2 hours postinjury in This involves the creation of a slot in the ver- experimental models.54 Several surgeons have tebral body, ventral to the intervertebral fora- performed durotomy in dogs with absent deep men.52 The slot is 25% of the length of each nociception for potential therapeutic and prog- vertebra, centered over the disk. The width is nostic benefit.13,55,56 It has been shown experi- about 50% of the vertebra, and the depth is mentally in dogs that adhesions are formed about 50% of the height of the vertebral body. between the durotomy site and the spinal cord The pedicle and lamina remain intact.52 Possible 16 weeks postoperatively, but the clinical sig- advantages of this technique include enhanced nificance of this finding is unknown.57 A recent removal of herniated disk material with mini- study comparing deep nociception−negative mal spinal cord manipulation.52 Disadvantages dogs treated with hemilaminectomy alone ver- include possible destabilization, fractures, and sus those treated with hemilaminectomy plus the technical expertise required.52 A successful durotomy found no difference in outcomes.56 outcome was reported in 14 of 15 chronically The prognostic value of durotomy remains affected dogs treated by lateral corpectomy. We uncertain. To our knowledge, there are no data view this technique with caution, as benefit comparing gross surgical findings of myelomal- compared with traditional techniques has not acia with histopathology or clinical outcome. been assessed, and biomechanical evaluation A number of materials have been applied was not performed to determine potential insta- to the laminectomy site as a barrier to prevent bility, particularly when combined with facetec- adhesion formation.57–63 Autologous fat grafts tomy as the investigators suggest. have been demonstrated to be more effective than absorbable gelatin foam, cellulose mesh, Other Techniques triamcinolone suspension, polyethylene oxide/ Durotomy has been shown experimentally polybutylene terephthalate copolymer, and to improve functional spinal cord recovery untreated controls for preventing epidural and when performed immediately after spinal cord peridural adhesions.60,61 One study found that

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both free fat grafts and cellulose membranes Nonetheless, arguments for fenestration were partially effective in preventing lami include a potentially decreased likelihood of nectomy membrane formation.58 This study, recurrence. Arguments against fenestration which used a dog dorsal laminectomy model, include lack of proven efficacy in decreasing also found a significant increase in neurologic overall recurrence rates, longer operative time deficits and spinal cord compression on histo- (increasing client cost, risk of wound infection, pathology in the sites treated with fat grafts; and anesthesia-related side effects), iatrogenic this was thought to be caused by migration of vertebral column instability, and numerous the fat graft into the spinal canal during the other possible complications.69,75 first 6 postoperative hours.58 However, there Percutaneous alternatives to surgical disk does not seem to be a benefit to using a pedi- fenestration have been reported, including cle versus a free fat graft.57 Early evaluation of chemonucleolysis with collagenase, partial a hyaluronic acid sheet is promising, and this percutaneous diskectomy, and percutane- product seems superior to absorbable gelatin ous laser ablation with a holmium:yttrium– foam and hyaluronic acid gel.62 aluminum–garnet laser.71,76,77 None of these Autologous olfactory glial cell transplanta- procedures has demonstrated efficacy in a tion may benefit both human and canine spi- controlled clinical trial, and they have limited nal cord injury patients in the future.64 It has availability and occasional complications.76,77 been shown that intraspinal transplantation of autologous olfactory glial cells in dogs is safe Prognosis and reliable.64 These cells were harvested from Nonsurgical Management QuickNotes the olfactory bulb using a transnasal approach Little is known about IVDD prognosis with Surgery is indi- and grown in cell culture for 3 weeks before conservative therapy, and many studies on cated in dogs with transplantation into the spinal cord via dor- this subject are older and used questionable sal laminectomy, midsagittal myelotomy, and treatment modalities (e.g., parenteral vitamin severe myelopathy injection of cell culture medium containing E, selenium) and research methods.39,40,78 One (e.g., nonambula- suspended cells. Similar procedures have been report described successful outcomes (ambu- tory paraparesis), shown to improve functional outcome in rats lation, urinary continence) in nearly 50% of rapidly progressing following experimental spinal cord injury.65,66 conservatively treated paraplegic dogs; the signs, or failure to nociception status was not noted.78 Another respond to conser- Fenestration report described success (ambulation) in vative therapy. Fenestration is the excision of the nucleus pul- about 40% of paraplegic dogs with absent posus through a surgically created window nociception.79 The remaining 60% of patients in the annulus.9,67 This is usually done at the died or were euthanized, and return of ambu- time of decompressive surgery.68 Fenestration lation without deep nociception (“spinal walk- can be performed at the site of decompres- ing”) was not described.79 One study reported sion and other sites throughout the vertebral a 51% success rate for medical management column.9 Fenestration has been used for both of intervertebral disk herniation in nonambu- therapeutic and prophylactic therapy, and is latory dogs compared with 82% for surgical accomplished via ventral, dorsal, or lateral management.80 approaches.9,18,67–71 The efficacy of fenestration may vary with the amount of nucleus removed, Surgical Treatment which depends on patient factors and surgi- Patients with Intact Deep Nociception cal technique.72,73 A recent retrospective study Successful surgical outcomes in dogs with found that herniation recurrence at sites of intact deep nociception range from 72% to fenestration was decreased.68 However, there 100%.7,10–12,14,33,42,43,81–85 Dogs with back pain was no decrease in overall recurrence rates, alone have an excellent prognosis: in one and the authors concluded that fenestration report, 24 of 25 dogs with back pain alone could have a destabilizing effect and promote improved following fenestration and (in most disk herniation at adjacent sites.68 Indeed, in a cases) decompressive surgery.18 Extensive epi- cadaver model, fenestration significantly desta- dural hemorrhage may be an indication for bilized the vertebral column in response to lat- extensive surgical decompression but does eral bending.74 not seem to negatively affect outcome.84 Two

E6 Compendium: Continuing Education for Veterinarians® | March 2009 | CompendiumVet.com FREE Canine Thoracolumbar IVDD: Diagnosis CE recent studies found no difference between ing surgical success in 78% of paraplegic dogs dogs with lesions causing upper and lower without deep nociception evaluated deep motor neuron pelvic limb signs.85,86 nociception by applying hard finger pressure In a report describing the prognostic value to the digits; in our view, this method only of MRI in paraplegic dogs, 65% of dogs with tests superficial nociception.86 Other studies intact deep nociception did not have an area required the application of hard pressure with of spinal cord parenchymal hyperintensity forceps or a similar instrument to digital or greater than the length of the second lumbar metatarsal bones and reported lower percent- (L2) vertebra on T2-weighted images (a find- ages of recovery to ambulation.13,55,88 ing that indicates spinal cord edema).34 All of Several recent studies reported recovery to these dogs had successful surgical outcomes. ambulation in 58% to 64% of surgically treated, In addition, 13 of 17 dogs (76%) with intact deep nociception−negative dogs.13,34,87,88 About deep nociception and an area of spinal cord 33% of such dogs that return to ambulation parenchymal hyperintensity greater than the have intermittent fecal or urinary inconti- length of L2 on T2-weighted MRI had posi- nence.88 Even with agreement on testing tive outcomes. The overall surgical success methods, assessment of nociception in dogs rate for dogs with intact deep pain perception remains subjective.86 was 92%. Another report found that the height It has been postulated that imaging find- and length of intervertebral disk extrusion as ings suggesting myelomalacia may correlate measured on MRI had no prognostic value.33 with a poor outcome.34,89 Myelographic evi- Extruded disk material may lie immediately dence of myelomalacia includes spinal cord adjacent to the disk or migrate cranially or swelling and contrast medium infiltration into QuickNotes caudally in the epidural space, and the degree the spinal cord.89 The appearance of con- Survey radiography of dispersion does not seem to affect preoper- trast medium within the spinal cord is not is helpful in ruling 33 pathognomonic for myelomalacia; it may be ative neurologic status or surgical outcome. out diagnostic dif- Mean time from surgery to ambulation in iatrogenic or represent chronic pathology (e.g., nonambulatory dogs has been reported at syringohydromyelia).90 ferentials such as 10 to 13 days in study populations of small- Information regarding the relationship diskospondylitis, breed dogs with intact deep nociception.11,82 between myelographic findings of spinal cord vertebral fracture, Another study population composed entirely swelling and prognosis is conflicting. In two luxation, infection, of large-breed dogs (93% of which had intact studies,13,87 spinal cord swelling was measured or neoplasia. deep nociception) had a mean time to ambu- by calculating a ratio of the length of the loss lation of 7 weeks.10 Most of these dogs (62%) of myelographic contrast column to the length were walking within 4 weeks of surgery.10 of the second lumbar vertebra (L2). A ratio of Another 30% were walking within 12 weeks of more than 5 was significantly related to poorer surgery.10 Time to ambulation increased with prognosis in one study, but no difference was patient weight and age.10 The time to recovery seen in the other. These studies differed in of pelvic limb function was similar in 33 non- exclusion criteria and outcome measures. The ambulatory paraparetic dogs and 19 paraple- study that showed a significant difference gic dogs with intact deep nociception.85 All of included only dogs with clinical signs of less these dogs (regardless of group) were weight- than 24 hours in duration, and a positive out- bearing with protraction (forward advance- come was defined as return of voluntary motor ment) of the pelvic limbs more than 50% of function.87 The other study included dogs with the time within 12 weeks of surgery.85 clinical signs that exceeded 24 hours in duration, and it defined a successful outcome as Patients Without Deep Nociception restoration of nociception, ambulation, and Paraplegic dogs in which deep nociception is urinary continence.13 absent have a 25% to 78% chance of recov- MRI provides important prognostic infor- ery to ambulation.13,42,55,56,86–88 This wide range mation for dogs with absent nociception.34 probably reflects different means of assessing Parenchymal hyperintensity on T2-weighted deep nociception, low case numbers, different images suggests spinal cord edema, whereas exclusion criteria, and different criteria for a parenchymal hypointensity on T2-weighted successful outcome. A recent report describ- gradient echo (T2*) images is suggestive of spi-

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nal cord hemorrhage, particularly in dogs with instead of neurologic examination. A report absent nociception.26 These findingslocated of postsurgical recurrence with a mean fol- diffusely, cranially, and caudally to a compres- low-up duration of 44.8 months suggests that sive extradural lesionmay correlate grossly about 20% of dogs exhibit pain and neurologic to ascending/descending myelomalacia.91 In deficits.93 Dachshunds may have higher post- a study in which 46% of deep nociception– surgical recurrence rates of disk herniation negative dogs did not have an area of spinal than other breeds.93 The number of opacified cord parenchymal hyperintensity greater than intervertebral disk spaces increased the risk the length of L2 on T2-weighted MRI, all of the of recurrence by 1.4 times in non-dachshund patients had successful outcomes. In contrast, breeds.93 Thus, dogs with five or six opacified only 31% (five of 16) of deep nociception–nega- disks at the time of surgery had a recurrence tive dogs with an area of spinal cord parenchy- rate of 50%.93 Reported recurrence rates in mal hyperintensity greater than the length of L2 nonsurgically managed dogs are as high as on T2-weighted MRI had successful outcomes. 40% over the ensuing 4 years.80,94 In this study, dogs with an area of T2 hyperintensity in the spinal cord had increased odds of Postoperative Care an unsuccessful outcome compared with dogs The goals of postoperative care are to decrease lacking T2 hyperintensity in the spinal cord the likelihood and severity of concurrent dis- (odds ratio: 29.87; 95% confidence interval: 3.47 ease processes. In addition to urinary bladder to 256.95). One dog in another study did not management, priorities of postoperative care have MRI findings suggesting spinal cord edema include maintaining appropriate analgesia and QuickNotes but subsequently developed myelomalacia.56 hydration status, avoiding diseases of recum- Myelography, com- Other prognostic indicators in deep bency (e.g., pressure sores), monitoring the puted tomography, nociception−negative dogs have been studied. surgical wound, and treating myelography- Longer duration between loss of deep nocicep- 35,95–97 Many clinicians use or magnetic reso- associated seizures. tion and surgery seems to correlate with poorer gastroprotective drugs, although their efficacy nance imaging is prognosis, but definitive evidence is lacking.2 in minimizing corticosteroid-induced gastroen- necessary to local- The surgeon usually does not know when deep teritis is unclear.98,99 Ice can be applied to the ize herniated disk nociception was lost, and the functional out- wound, at least for the first postoperative day.100 material and spinal come of dogs treated surgically more than 48 Any dog that deteriorates neurologically during cord compression. hours after the loss of deep nociception has not the postoperative period should be viewed as a been rigorously studied.2 Therefore, aggressive candidate for further imaging studies to deter- diagnostic measures are warranted in dogs in mine whether additional surgery is indicated.101 which nociception is absent for longer than 48 Physical rehabilitation offers potential ben- hours.87,92 In one study, dogs that deteriorated efits in the functional recovery of the dog to nonambulatory status in less than 1 hour with thoracolumbar intervertebral disk hernia- had poorer outcomes compared with dogs that tion.100 A five-step protocol has been suggested deteriorated more slowly.13 Other investigators for patients with thoracolumbar myelopathy.100 have found that dogs that regain deep nocicep- Overzealous physical rehabilitation can be detri- tion within 2 weeks after surgery have a good mental to dogs with vertebral column instability prognosis.88 The converse is also true: dogs or unresolved spinal cord compression, causing that do not recover deep nociception within 2 further spinal cord injury.100 The protocol initially weeks after surgery are less likely to recover.13 calls for cold-packing the surgical incision, pas- Subdural hemorrhage seen at surgery may be sive range-of-motion exercises, and massage of associated with a negative outcome.34 As in affected limb muscles (step 1). Once the patient dogs with intact deep nociception, increased is able to bear weight, standing exercises and age and body weight may prolong recovery to neuromuscular stimulation are recommended ambulation.88 (step 2). As limb motion returns, weight-shifting exercises, walking (assisted and/or treadmill), Recurrence and swimming can be incorporated (step 3). Early reports probably underestimated the Steps 4 and 5 include sit-to-stand exercises, bal- recurrence of disk herniation due to short dura- ance and coordination activities, and increased tion of follow-up and reliance on owner surveys intensity of walking and swimming.

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Swelling and discharge are the most com- muscle relaxation, α-mediated internal urethral mon surgical wound complications, occurring sphincter tone, and cholinergic-mediated exter- in 7.5% and 5.3% of dogs, respectively, in a ret- nal urethral tone.103 During voiding, adrenergic rospective study.97 Other complications (occur- input to the bladder is decreased, resulting in ring in 1.5% of cases each) included bleeding increased detrusor muscle tone and decreased from the incision, seroma formation, and par- internal urethral sphincter tone.103 Cholinergic tial or complete dehiscence.97 These complica- stimulation via the pelvic and pudendal nerves tions generally did not warrant antibiotic use results in detrusor muscle contraction and exter- or surgical wound closure, although two of 264 nal urethral sphincter relaxation.103 dogs required surgical wound management.97 In upper motor neuron bladder syndrome, the sacral spinal cord segments supplying the Urinary Bladder Management pelvic and pudendal nerves remain intact.2,103 Urinary bladder dysfunction is a common The communication of ascending and descend- preoperative and postoperative problem with ing information with the brain is decreased or IVDD.2,3,14,83,88,102,103 The nature of the dysfunc- lost.2,103 This results in urinary incontinence tion depends on lesion location and severity, with increased detrusor muscle and external degree of pelvic limb motor function, and loss urethral sphincter tone, which clinically pre of nociception, ranging from mild increases sents as a large, firm bladder that is difficult to in residual volume to distention with over- express and may overflow as intraluminal pres- flow. 2,3,14,83,88,102,103 Consequences of urinary sure overwhelms the urethral sphincters.2,103 bladder dysfunction include bacterial cystitis, Pharmacologic intervention targets relax- pyelonephritis, bladder atony, and bladder ation of the urethral sphincters (Table 1). Drugs QuickNotes fibrosis with long-term distention.3,96 may include α-adrenergic antagonists (e.g., phe- The presence or 106–108 The canine urinary bladder is innervated noxybenzamine, prazosin). Skeletal muscle absence of deep by the pelvic, pudendal, and hypogastric relaxants (e.g., diazepam, dantrolene) may be nociception has nerves.103,104 The pelvic nerve arises from the helpful in decreasing external urethral tone.105 sacral spinal cord segments, supplying the In dogs with bladder atony, bethanechol may important prognos- detrusor (smooth) muscle with parasympa- be used in addition to an α-adrenergic antago- tic ramifications; thetic motor and sensory innervation.103,104 nist after therapeutic levels of the latter have 58% to 64% of dogs Preganglionic and postganglionic parasympa- been reached.105,109 Phenoxybenzamine has a treated surgically thetic neurons synapse at the pelvic ganglia, slow therapeutic onset and may take several within 24 hours of within the wall of the bladder.105 The pudendal days to demonstrate clinical efficacy.105 the loss of deep nerves arise from the sacral spinal cord seg- In dogs with thoracolumbar interverte- nociception recover ments and provide motor and sensory function bral disk herniation and lower motor neuron the ability to walk. to the skeletal muscle of the external urethral bladder syndrome, the sacral spinal cord seg- sphincter, motor function to the perineal ments or nerve roots are dysfunctional.103 The musculature, and sensory function to the ure- first through fourth lumbar spinal cord seg- thra.105 The hypogastric nerve arises from the ments and hypogastric nerve may remain first through fourth lumbar spinal cord seg- intact.110−112 This results in the loss of local ments to provide adrenergic innervation to reflexes that maintain detrusor muscle and the internal urethral sphincter, pelvic (para- external urethral sphincter tone during urine sympathetic) ganglia, and detrusor muscle.105 storage.103 Some bladder nociceptive informa- Sympathetic input to the pelvic ganglia may tion (via the hypogastric nerve) may remain inhibit parasympathetic activity during urine intact.105 As with upper motor neuron bladder storage.105 The hypogastric nerve also contains syndrome, communication with the brain is sensory fibers from the bladder wall involved decreased or lost.2,103 This results in decreased in nociception.105 Descending motor informa- detrusor muscle and external urethral sphinc- tion includes inhibitory and excitatory upper ter tone, which clinically presents as a large, motor neuron input to the detrusor muscle soft bladder that is easy to express and over- and urethral sphincters.103,104 flows with minimal intraluminal pressure.103 Normal urine storage and voiding involves Pharmacologic intervention targets the detrusor both reflexive and conscious input.103 Urine muscle and may include cholinergic agonists storage is accomplished by β-mediated detrusor (e.g., bethanechol).106,108 Because sympathetic-

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mediated constriction of the internal urethral For most dogs, urinary bladder evacuation sphincter may remain intact, an α-adrenergic by expression or intermittent catheterization antagonist may be helpful.2 is feasible and advantageous.3,96 The necessary frequency of evacuation depends on Urinary Bladder Evacuation the completeness of the previous evacuation Urinary bladder distention must be prevented and volume of urine produced.96 It has been in dogs with urine retention.96,105 This can be suggested that the urinary bladder should be accomplished by indwelling urinary catheter evacuated every 4 to 6 hours initially, with placement, manual expression, or intermittent the frequency adjusted as needed.96 We feel catheterization.3,96 Intermittent urinary evacua- that this may be excessive and that bladder tion (via catheter or expression) has been advo- expression every 6 to 8 hours may be more cated as a means of avoiding the increased risk appropriate. Ultrasonography can help to of urinary tract infection (UTI) resulting from determine the optimum frequency and effi- an indwelling urinary catheter.3,96,113−115 The cacy of bladder evacuation.102 Technical diffi- placement of an indwelling urinary catheter is culties involved in intermittent catheterization advantageous in dogs not amenable to inter- of female dogs preclude widespread use of mittent urine evacuation. For example, a large, this technique.3 nonambulatory, female dog with a bladder that is difficult to express may be best treated with Urinary Tract Infection an indwelling urinary catheter. Other candi- Bacterial UTI may predominate in a single site dates include aggressive dogs and dogs with (e.g., kidney, urinary bladder) or multiple sites.4 QuickNotes lower motor neuron bladder syndrome (to pre- Predisposing factors for UTI in dogs with spi- Although the prog- vent leakage and scalding). It may be best to nal cord injuries may include incomplete void- nosis is poorer avoid systemic antibiotics in dogs with indwelling, elevated intravesical pressure, catheter use, and preexisting medical conditions (e.g., cystic beyond 24 hours ing urinary catheters, as the risk of acquiring an antibiotic-resistant UTI may be increased.114–116 calculi, hyperadrenocorticism, diabetes melli- after the loss of These dogs should be monitored closely for the tus).5,118 Dogs may not have any clinical signs deep nociception, development of a UTI. An exception would be of UTI or may develop pollakiuria, stranguria, some dogs can still a dog with pyelonephritis secondary to urine dysuria, inappropriate urination, hematuria, recover, warranting retention and cystitis. Antiinfective catheter pyuria, proteinuria, fever, septicemia, abdomi- the use of aggres- materials, antibiotics, and antiseptic agents do nal pain, or renal failure.4 Many of these signs sive diagnostic not seem to be effective in the long-term pre- overlap with clinical signs of neurogenic blad- measures. vention of UTI in the setting of spinal cord der dysfunction. UTI is best diagnosed by injury and urine retention.117 urine culture and susceptibility testing.4

table 1 Pharmacotherapy for Neurogenic Bladder Dysfunction106,107,110,111

Drug Dosage Notes

Phenoxybenzamine 0.25–0.5 mg/kg PO q12–24h Decreases internal urethral sphincter tone. Therapeutic or effects take days. May cause 5–15 mg (total dose) PO q12–24h hypotension.

Prazosin HCL 1 mg/15 kg PO q8–24h Decreases internal urethral sphincter tone. May cause marked hypotension.

Bethanechol chloride 2.5–25 mg (total dose) PO q8h Enhances detrusor muscle contraction. May cause vomiting, diarrhea, excessive salivation, and anorexia.

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Antibiotics are the mainstay of UTI treat- lethargy, anorexia, vomiting, polyuria, polydip- ment.119,120 The antibiotic should reach urine sia, and abdominal pain.4 Clinical pathologic concentrations that exceed the minimum inhibi- abnormalities consistent with pyelonephritis tory concentration by at least fourfold.4 Route of include neutrophilic leukocytosis, azotemia, administration, side effects, and cost are also pyuria, and isosthenuria. Dogs meeting these important considerations.4 The duration of ther- criteria may require ultrasonographic evalua- apy in dogs with UTI secondary to neurogenic tion of the kidneys.4 Because many of these bladder dysfunction should be longer than that findings are nonspecific (e.g., stress leukogram, recommended for a simple UTI.120 Some experts prerenal azotemia, corticosteroid-induced advocate a 3- to 6-week course of antibiotics for isosthenuria), we suspect that pyelonephritis persistent UTI.120 We recommend repeating the is underdiagnosed in dogs with urine reten- urine culture 2 to 3 days after the completion tion secondary to intervertebral disk hernia- of antibiotic therapy.119,120 Using antibiotic ther- tion. It is unclear whether dogs with IVDD apy without addressing predisposing factors is benefit from the treatment of asymptomatic largely unsuccessful, with most UTIs recurring bacteriuria, but current recommendations are in less than 8 weeks.120 Therefore, in dogs that to treat with antibiotics as for symptomatic do not regain urinary continence, periodic uri- UTI.118,121,122 A long-term strategy involving a nalysis and culture should be performed. weekly oral cyclic antibiotic administration A recent study reported an overall UTI inci- program appears promising.123 dence of 27% (diagnosed by positive culture) in dogs treated surgically for intervertebral Surgical Failure disk extrusion.3 Interestingly, UTI developed In the event of an unsuccessful surgical out- in 12% of dogs that were able to urinate volun- come, many pet owners are able to success- tarily. Statistically significant factors associated fully care for a paraplegic dog.88,124 Large dogs with an increased likelihood of UTI included are more difficult to manage than small dogs female sex, urinary incontinence, nonambu- because of the increased physical demands latory status, prolonged hypothermia during of nursing care, the likelihood and severity of anesthesia, and lack of perioperative antibiotic pressure sores, and difficulty expressing the administration.3 Among dogs that were urinary urinary bladder.124,125 A variety of carts are incontinent or nonambulatory, the prevalence available to assist in ambulation. Recurrent of UTI increased with age.3 Dogs undergoing UTI may become problematic in some dogs.88 intermittent urinary catheterization tended to “Spinal walking” has been reported in some have a decreased incidence of UTI compared dogs that did not regain deep nociception after with those evacuated by manual expression. surgery and that maintained the activity for at However, no significant differences could be least 3 weeks.88 This involves the use of pelvic determined between these groups because limb reflexes, trunk muscles, and any remain- intermittent catheterization was only per- ing upper motor neurons.88 Most of these dogs formed in male dogs.3 Method of evacuation do not recover urinary and fecal continence.88 had no effect on prevalence of UTI when sex was taken into account.3 Conclusion Although pyelonephritis is rare in dogs Despite recent research, the efficacy of many with intervertebral disk herniation, it has life- common therapies remains unknown and con- threatening implications.4 Pyelonephritis was troversial. In practices that do not perform spi- reported in six of 187 dogs in one large ret- nal surgery, the decision of when to refer is of rospective study.45 Clinical signs include fever, paramount importance.

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1. Which finding suggests intervertebral a. clear distinction between the annulus a. is a negative prognostic indicator. disk herniation on survey radiography? and nucleus. b. is seen more commonly in dogs with a. widened intervertebral disk space b. symmetric annulus extending dorsally intact deep nociception than in those b. remodeling of the bone surrounding the into the vertebral canal. with absent deep nociception. intervertebral foramen c. round area of low signal intensity cor- c. does not provide meaningful prognostic c. mineralized material within the plane of responding to the nucleus pulposus. information. the vertebral canal d. hyperintense annulus fibrosus relative d. implies an area of altered biomechanical d. widening of the space between the to the nucleus pulposus. function. articular facets 6. CSF evaluation is 9. Which procedure is most appropriate for 2. Which statement is true regarding survey a. highly specific for IVDD. the treatment of thoracolumbar disk her- radiography? b. helpful in ruling out meningomyelitis. niation with severe myelopathy (absent a. It is highly sensitive for disk herniation. c. expected to show increased protein deep nociception)? b. It correlates well with surgical findings. levels in samples obtained from the cer- a. partial percutaneous diskectomy c. It can help to rule out diskospondylitis. ebellomedullary cistern compared with b. acupuncture d. It can demonstrate spinal cord the lumbar cistern. c. hemilaminectomy compression. d. an unnecessary risk to the patient with d. chemonucleolysis suspected intervertebral disk herniation. 3. A common adverse effect of myelography is 10. Which statement regarding urinary bladder a. acute hepatic failure. 7. Surgical outcome for repair of disk physiology is true? b. seizures. herniation a. Urine storage is accomplished by c. worsening of pancreatic islet cell function. a. is good to excellent in dogs with absent α-mediated detrusor muscle relaxation, d. thrombocytopenia. deep nociception. β-mediated internal urethral sphincter b . is better for dogs with absent deep noci- tone, and cholinergic-mediated external 4. Myelographic findings suggesting extra- ception that deteriorated to nonambula- urethral tone. dural spinal cord compression include tory status acutely (in less than 1 hour) b. Adrenergic input to the bladder is reduced a. widening of the contrast column in a than for those that deteriorated more during voiding, resulting in decreased “golf tee” appearance. slowly. detrusor muscle tone and increased inter- b. contrast medium infiltration into the c. is principally related to the presence or nal urethral sphincter tone. spinal cord. absence of the withdrawal reflex. c. Cholinergic stimulation via the pelvic c. a heterogeneous hyperattenuating mass d. has a success rate of 58% to 64% in and pudendal nerves results in detrusor on transverse views. dogs with absent deep nociception. muscle contraction and external urethral d. dorsal deviation of the ventral contrast sphincter relaxation during micturition. column on the lateral view. 8. The presence of an area of spinal cord d. The external urethral sphincter is made hyperintensity on T2-weighted MRI up of smooth muscle. 5. On T2-weighted MRI, the normal canine greater than or equal to the length of the disk has a L2 vertebral body

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