Neurosurg Focus 8 (3):Article 1, 2000, Click here to return to Table of Contents Classification of syringomyelia THOMAS H. MILHORAT, M.D. Department of Neurosurgery, State University of New York, Health Science Center at Brooklyn, and the Kings County Hospital Center, Brooklyn, New York Syringomyelia poses special challenges for the clinician because of its complex symptomatology, uncertain patho- genesis, and multiple options of treatment. The purpose of this study was to classify intramedullary cavities according to their most salient pathological and clinical features. Pathological findings obtained in 175 individuals with tubular cavitations of the spinal cord were correlated with clinical and magnetic resonance (MR) imaging findings in a database of 927 patients. A classification system was developed in which the morbid anatomy, cause, and pathogenesis of these lesions are emphasized. The use of a disease-based classification of syringomyelia facilitates diagnosis and the interpretation of MR imag- ing findings and provides a guide to treatment. KEY WORDS • Chiari malformation • arachnoiditis • hydrocephalus • posttraumatic syringomyelia • basilar invagination • tethered spinal cord Previous attempts to classify syringomyelia have been spinal cord. There were 94 males and 81 females who had based on presumed mechanisms of pathogenesis.1–3,5,6,20 ranged in age from 1 day to 87 years (mean age 41.6 These classifications have been useful and have helped to years). In all cases a standard complete autopsy was per- shape surgical treatment. However, with the advent of MR formed. After opening the cranial cavity, the upper end of imaging it has become evident that some concepts of the spinal cord was divided at the foramen magnum and pathogenesis are no longer tenable. Chief among these is removed with the brain side of the specimen. The remain- the hypothesis that syrinx formation depends on the force- der of the spinal cord was removed via the abdominal and ful diversion of CSF from the fourth ventricle into the cen- thoracic cavities, as described elsewhere.10 The review of tral canal of the spinal cord.7,19,20 histological material was facilitated by an institutional Contributing to the difficulty of defining mechanisms policy that requires that original slides and paraffin blocks of syrinx formation has been the general lack of infor- be retained with the permanent autopsy file. Old and new mation on pathological features. Although Netsky15 has paraffin blocks were cut serially to a thickness of 6 µ reported on eight autopsy cases, with few exceptions most mounted on glass slides, and stained with hematoxylin studies prior to 1990 were case reports. In recent years, and eosin. Supplemental stains included Luxol fast blue, reports have appeared that help to clarify the pathological Weil stain, Gomori's stain, Bodian's stain, periodic-acid features of spinal cord cavitation.8,10 These data, taken to- Schiff, phosphotungstic acid hematoxylin, and glial fibril- gether with MR imaging correlates, permit a reclassifica- lary acidic protein. Histological sections were viewed tion of syringomyelia based on morbid anatomy. through a microscope at magnifications ranging from ϫ 10 to ϫ 1000. Final diagnoses were as follows: nonneo- CLINICAL MATERIAL AND METHODS plastic syringomyelia in 105 cases; syringomyelia ex vacuo in 52 cases; and neoplastic cysts in 18 cases. The Autopsy Data pathological findings in 105 cases of nonneoplastic syrin- gomyelia have been previously published.10 Between the years 1955 and 1993 autopsies were per- formed in 175 cadavers with tubular cavitations of the Clinical Data Nine hundred twenty-seven patients with cavitary le- Abbreviations used in this paper: CCS = central canal stenosis; sions of the spinal cord were evaluated between the years CSF = cerebrospinal fluid; MR = magnetic resonance. 1990 and 1998. There were 382 male and 545 female Neurosurg. Focus / Volume 8 / March, 2000 1 Unauthenticated | Downloaded 09/24/21 03:30 AM UTC T. H. Milhorat Fig. 2. Magnetic resonance imaging studies demonstrating communicating syringomyelia of the cervical spine in a 58-year- old male with posthemorrhagic hydrocephalus and syringomyelia. Left: Sagittal image demonstrating that the syrinx is continuous with the fourth ventricle. Right: Axial image revealing a symmet- rically enlarged central cavity. Fig. 1. Diagrammatic and photomicrographic depictions of the CSF pathways distal to the outlets of the fourth ventri- communicating syringomyelia. Left: Diagram illustrating the cle (Fig. 1). In typical cases, there is generalized enlarge- pathological findings obtained in a 49-year-old male with post- ment of all four cerebral ventricles, and the central canal meningitic hydrocephalus, basilar arachnoiditis, and syringomye- participates in the hydrocephalic process like a "fifth ven- lia. The syrinx is anatomically continuous with the enlarged fourth ventricle (4th) and its length has been determined by CCS. tricle." Causative factors include postmeningitic and Right: Photomicrographs of specimens obtained in the same posthemorrhagic hydrocephalus, complex hindbrain mal- patient. H & E. A: Axial section obtained through the syrinx at formations, such as Chiari II malformation and enceph- T-1, showing a central cavity lined by ependyma. Original magni- alocele, and Dandy–Walker cysts. An experimental model fication ϫ 10. B: Axial section obtained immediately below the of communicating syringomyelia can be produced by in- syrinx at T-10, demonstrating occlusion of the central canal jecting kaolin into the cisterna magna.4,21 (arrow). Original magnification ϫ 40. On histological examination, communicating syringes appear as simple dilations of the central canal, lined whol- ly or partially by ependyma (Fig. 1A). In acquired exam- patients who ranged in age from 8 months to 82 years ples, the length of the cavity is defined caudally by central (mean age 38.2 years). Each patient underwent a detailed canal stenosis (Fig. 1B), which is an age-related phenom- neurological examination and at least one MR imaging enon affecting the majority of normal individuals by the session of the spinal cord that included the area of cavita- early years of adult life.13 Holocord enlargements are most tion and the cervicomedullary junction. In many cases, often of congenital origin and may be anatomically con- additional information was provided by the whole-brain tinuous with caudal lesions such as myelomeningocele. and spinal cord MR images, cine MR imaging, computer- With distension of the central canal, the ependymal epi- ized tomography myelography, somatosensory evoked thelium becomes stretched and denuded. Nevertheless, potentials, and other neurodiagnostic tests. The protocol despite the large size of some communicating syringes, for diagnostic workup and the specific techniques of neu- these lesions are much less prone than noncommunicating roimaging have been previously published.11 syringes to rupture paracentrally, and this may explain why a significant number of communicating syringes re- main asymptomatic throughout life or are associated with RESULTS only minor neurological findings.10,12 Figure 2 illustrates Tubular enlargements of the spinal cord that are not due the MR imaging correlates of communicating syringo- to intramedullary tumors have been classified as follows myelia. on the basis of pathological findings:10 1) dilations of the central canal that are anatomically continuous with the Noncommunicating Central Canal Dilations fourth ventricle (communicating syringomyelia); 2) dila- Dilations of the central canal that do not communciate tions of the central canal that do not communicate with the with the fourth ventricle are associated with obstructions fourth ventricle (noncommunicating syringomyelia); and of the CSF pathways at or below the foramen magnum. 3) extracanalicular syringes that originate in the spinal Causative factors include the Chiari I malformation, bas- cord parenchyma and do not communciate with the cen- ilar invagination, spinal arachnoiditis, extramedullary tral canal or fourth ventricle (primary parenchymal cavi- compressions, tethered cord, and acquired tonsillar herni- tations). These lesions are distinguished from two other ation. There is accumulating evidence that the formation types of cavitation: 1) atrophic syringes occurring with of noncommuniating syringes depends on an increase of myelomalacia (syringomyelia ex vacuo); and 2) neoplas- 12 the arterial pulse wave in the spinal subarachnoid space tic cysts. that is sufficient to force CSF through anatomically con- tinuous perivascular and interstitial spaces into the central Communicating Central Canal Dilations canal of the spinal cord.14,16–18 An experimental model of Communicating syringes are caused by obstructions of noncommunicating syringomyelia can be produced by 2 Neurosurg. Focus / Volume 8 / March, 2000 Unauthenticated | Downloaded 09/24/21 03:30 AM UTC Classification of syringomyelia Fig. 4. Diagrammatic and photomicrographic depictions of noncommunicating central canal dilation. Left: Diagram illustrat- Fig. 3. Diagrammatic and photomicrographic representations of ing the pathological findings obtained in a 62-year-old female with noncommunicating central canal dilation. Left: Diagram illustrat- a Chiari I malformation and syringomyelia. The syrinx is defined ing the pathological findings obtained in a 66-year-old male with rostrally and caudally by CCS. At C-7, the syrinx has ruptured into syringomyelia occurring