FUNCTIONAL NEUROANATOMY in EQUINE BACK PAIN Goals: • To

FUNCTIONAL NEUROANATOMY IN EQUINE BACK PAIN

Pedro Luis Rivera, DVM, DACVSMR, FACFN, FCoAC Program Director Healing Oasis Wellness Center 2555 Wisconsin St., Sturtevant, WI 53177-1825

Goals: • To present, discuss, and describe the functional neuroanatomy of the equine back as it pertains to pain. • To discuss and describe the areas that pain (or nociception) can originate from as it pertains to the equine back. • To provide the attendees with clinically relevant cases to help with integration of the presented material. • To discuss and describe through case presentations, several modalities that can be safely used to minimize back pain in our equine patients.

Equine patients are commonly presented to be evaluated by the primary doctor with what is described as “back pain.” Back pain can originate from any of the spinal anatomical (cervical, thoracic [including ribs], lumbar, sacral, and pelvic) regions or may originate from changes within the synovial membrane, regional fascia, regional muscles, ligaments, tendons, subchondral bone changes, and joint capsule, among others. Percentage of cases that have been diagnosed with back problems ranges from 0% to up to 25% of those that are clinically evaluated and/or examined.[1-4]

Pain is described by the International Association for the Study of Pain (IASP) as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” [5] Nociception (which is a sensory stimuli) originates from peripheral receptors (free nerve endings) culmination at the somatosensory cortex of the parietal lobe in which it is recognized as pain, involving the stimulation of the limbic system. [6]

It is my personal experience that back problems in the equine patients are being overlooked and should be considered as a primary cause for decrease in performance or conditioning. Most of the presenting clinical signs of “back pain” are provided by owners descriptions and not from the primary veterinarian point of view or examination. Some clinical signs of back pain can include but are not limited to: [7, 8] • Responding adversely to a saddle or rider • Reluctance to transition from one gait to another • Reluctance and difficulty in maintaining a specific gait • Refusal to complete a task or exercise • Thoracic and or pelvic limb strength changes • Behavioral expression

Final diagnosis for equine back problems varied depending on the discipline or training that the patient was undertaking. Diagnosis that could be encountered include but are not limited to: [2, 9] • Kissing spinous processes • Dorsal spinous ligament injuries • Muscle pain • Sacroiliac problem • Concurrent conditions (polysaccharide storage myopathy) • Primary joint conditions (fetlock, hock, stifle and coxofemoral)

It should be noted; there is another aspect of the normal equine anatomy that is not examined or evaluated as often, the articular facet or zygapophysial joint. This synovial joint is composed of two facet processes of two adjacent vertebrae that are lined with hyaline cartilage, are highly innervated, and contains a large synovial fold.[10] The joint itself is surrounded by a thick joint capsule and ligamentum flavum. [11] Anatomically, each articular facet is located dorsal to the intervertebral foramen and, if affected, can influence several or all of the components of the intervertebral foramen. There are several structures that the clinician must keep in mind that traverse the intervertebral foramen, these include:[12] • Dorsal root ganglia • Spinal nerve (union of the dorsal and ventral roots) • The dural root sleeve with cerebrospinal fluid • Lymphatics • Artery, vein • Connective tissue (including fat) • Several meningeal nerves

It is important to understand that from C2-S1, there at two articular facets (zygapophyseal joints) between each vertebrae (cranial and caudal) and more synovial joints depending on the region of the spine that is being described. For example:

Vertebral segment Types of joint surfaces Total # of joints C3-6 (independently) Four facet or zygapophyseal Six joints Two discs C7 Four facet or zygapophyseal Eight joints Two discs Two costofovea or demi- facets T1-about T10 Four facet or zygapophyseal Twelve joints Two discs Four costofovea or demi- facets Two costotransverse joints Last T – about L4 Four facet or zygapophyseal Six (with some variations)[1] joints Two discs L5 Four facet or zygapophyseal Eight (with some joints variations)[1] Two discs Two caudal intertransverse joints L6 Four facet or zygapophyseal Ten joints Two discs Two cranial intertransverse joints Two caudal intertransverse joints

All of the above-listed joints are considered to be synovial with the potential, if affected, to be a source of nociception.[13-15]

Another aspect of the normal equine anatomy is the sacroiliac joints (SIJ). These joints, depending on the anatomist, can be described as having multiple innervations arising from the sciatic nerve branches (L6-S2 spinal cord level, covering the dorsal aspect of the SIJ) and the femoral nerve branches (L4-L6 spinal cord level covering the ventral aspect of the SIJ). In addition, in other mammalian species, sensory innervation to the SIJ has come as far cranial as L1 spinal cord segment.[16, 17] Changes to the SIJ can have far-reaching performance effects than anyone can imagine.

It would behoove the attendee to review the afferent pathways as the spinothalamic tract, which is part of the ventrolateral or anterolateral system can enter the spinal cord several segments cranial or caudal from where they originated, hence providing skewed and erroneous information of the origin of the nociceptive stimuli, making a diagnosis much more difficult.[18] As peripheral nerves contain sensory, motoric, and autonomic fibers, I personally try to keep in mind the somato-somatic, somato-autonomic, somato-visceral, viscero-somatic, or viscero- visceral implication(s) to help formulate a viable differential diagnosis.[19]

Cases presentations discussing therapeutic modalities will be utilized to help attendees integrate the presented didactic information. Some therapeutic modalities that will be discussed include manual therapies (massage, rehabilitation, spinal manipulation), and extracorporeal shock wave.

References: 1. Haussler, K.K., S.M. Stover, and N.H. Willits, Developmental variation in lumbosacropelvic anatomy of thoroughbred racehorses. Am J Vet Res, 1997. 58(10): p. 1083-91. 2. Turner, T.A., Back Lameness In Horses, N.A.V. Conference, Editor. 2007: Orlando, FL. p. 217-219. 3. van Weeren, P.R. and W. Back, Musculoskeletal Disease in Aged Horses and Its Management. Vet Clin North Am Equine Pract, 2016. 32(2): p. 229-47. 4. Ehrle, A., et al., Structure and Innervation of the Equine Supraspinous and Interspinous Ligaments. Anat Histol Embryol, 2017. 46(3): p. 223-231. 5. Anand, K.J.S., Defining pain in newborns: need for a uniform taxonomy? Acta Paediatr, 2017. 106(9): p. 1438-1444. 6. Baliki, M.N. and A.V. Apkarian, Nociception, Pain, Negative Moods, and Behavior Selection. Neuron, 2015. 87(3): p. 474-91. 7. Martin, B.B. and A. Klide, Diagnosis and Treatment of Chronic Back Pain in Horses, AAEP, Editor. 1997: Proceedings AAEP. p. 310-311. 8. Murray, R.C., et al., Identification of risk factors for lameness in dressage horses. Vet J, 2010. 184(1): p. 27-36. 9. Turner, T.A. Back Problems in Horses. in 49th Annual Convention of the AAEP. 2003. New Orleans, LA: AAEP. 10. Thomsen, L.N., et al., Synovial folds in equine articular process joints. Equine Vet J, 2013. 45(4): p. 448-53. 11. Vasseur, P.B., G. Saunders, and C. Steinback, Anatomy and function of the ligaments of the lower cervical spine in the dog. Am J Vet Res, 1981. 42(6): p. 1002-6. 12. Cramer, G., General Characteristics of the Spine, in Clinical Anatomy of the Spine, Spinal Cord and ANS, D.S. Cramer GD, Editor. 2014, Elsevier: St. Louis, MO. p. 15-64. 13. Curtis, L., N. Shah, and D. Padalia, Facet Joint Disease, in StatPearls. 2019, StatPearls Publishing StatPearls Publishing LLC.: Treasure Island (FL). 14. Kalichman, L. and D.J. Hunter, Lumbar facet joint osteoarthritis: a review. Semin Arthritis Rheum, 2007. 37(2): p. 69-80. 15. Vandeweerd, J.M., et al., Innervation and nerve injections of the lumbar spine of the horse: a cadaveric study. Equine Vet J, 2007. 39(1): p. 59-63. 16. Murata, Y., et al., Sensory innervation of the sacroiliac joint in rats. Spine (Phila Pa 1976), 2000. 25(16): p. 2015-9. 17. Murata, Y., et al., Origin and pathway of sensory nerve fibers to the ventral and dorsal sides of the sacroiliac joint in rats. J Orthop Res, 2001. 19(3): p. 379-83. 18. Swenson, R. Somatosensory Systmes. 2006 [cited 2019 2nd October]; Available from: https://www.dartmouth.edu/~rswenson/NeuroSci/chapter_7A.html. 19. Rivera, P., Spinal manipulation or Animal Chiropractic and the musculoskeletal system - its influence in quadruped locomotion. Integrative Veterinary Care, Winter 2015/16: p. 52-56.