Canis ISSN: 2398-2942

Spine: dysraphism

Synonym(s): Myelodysplasia, Spinal dysraphism

Contributor(s): Laurent Garosi

Introduction

  • Cause: malformation of spinal cord.
  • Hereditary in some breeds, eg Weimaraner.
  • Signs: abnormal gait, abnormal dorsal cervical hair patterns, scoliosis, depression of sternum (koilosternia), abnormal head posture may be seen.
  • Treatment: no treatment available.
  • Prognosis: usually poor.

Pathogenesis

Etiology

  • Inherited in Weimaraner with thoracolumbar myelodysplasia as a codominant lethal gene with variable penetrance. The homozygous condition is lethal. Clinically affected dogs are heterozygotes.
  • The pathological mechanisms that result in syringomyelia and hydromyelia are many and varied and no single pathological mechanism adequately explains all instances of the diseases.
  • Possible pathogenical mechanisms of cyst formation include changes in CSF pressure relationships within the spinal cord (as occurs with hydrocephalus or foramen magnum abnormalities), loss of spinal parenchyma, stenosis of the central canal, and obstruction to CSF flow via inflammation or tumor. Each of these mechanisms are possible in dogs and cats but have yet to be definitively proved.
  • Syrinx that do not communicate with the central canal (extracanalicular syrinx) at any level are often acquired due to spinal injury or damage from disease (hemorrhage and inflammation).

Predisposing factors

General
  • Animals with associated vertebral defects may be at an increased risk.
  • This is most often seen in Bulldogs.

Pathophysiology

  • Spinal dysraphism (also known as myelodysplasia) are congenital malformations of the vertebral column and spinal cord that occur secondary to abnormal closure of the caudal neuropore of the developing neural tube → defective neural arch through which meninges or neural elements may be herniated.
  • Include the overall group of dorsal midline defects derived from the secondary maldevelopment of ectoderm, mesoderm, and neuroectoderm layers of tissue, and encompassess a variety of malformation.
  • Malformations include spina bifida Spina bifida , meningocele, meningomyelocele, syringomyelia Neurology: Chiari-like malformation/syringomyelia (CM/SM) , split cord anomalies (diastematomyelia) as well as absence or duplicated central canal, abnormal distribution-migration of gray matter into its normal column and failure of formation of a ventral median fissure.
  • Abnormal formation of the spinal cord due to faulty development of neuronal and supporting cells or abnormalities in migration of differentiating cells.
  • Neural tube fails to close normally.
  • Changes within the spinal cord include absence, distension or duplication of the central canal.
  • Thoracolumbar myelodysplasia in Weimeraner primarily affects midline structures with central canal abnormalities including absence or dilation (hydromyelia), absent or forked ventral median fissure, and failure of neuronal cell bodies to migrate laterally into the ventral gray column.

Syringomyelia and hydromyelia
  • Syringomyelia and hydromyelia are cystic abnormalities of the spinal cord.
  • Syringomyelia refers to abnormal cavities filled with liquid in the substance of the spinal cord.
  • Hydromyelia refers to a pathologic condition characterized by accumulation of fluid within and enlarged central canal of the spinal cord.
  • In both of these instances, the fluid that accumulates is similar, if not identical, to cerebrospinal fluid (CSF). Some authors refer to hydromyelia as a communicating syringomyelia and syringomyelia as used to describe all intraspinal abnormal fluid accumulations. The histologic difference being that with hydromyelia the fluid cavity is lined by ependymal cells characteristic of the central canal and a syringomyelic cavity is usually lined by glial cells. Clinically and practically it is often difficult to differentiate between the two.
  • Additionally, an increasing hydromyelia may destroy or disrupt the ependyma layer with fluid rupturing into the surrounding spinal cord. These abnormal fluid-filled areas, while originating as hydromyelia, may be lined by glial tissue consistent with a syrinx.
  • These syrinx tend to occur in the central grey matter or dorsal and lateral white matter, possibly associated with changes in vascular distribution ('watershed zones').
  • Thirty-seven percent of extracanalicular syrinx ruptured through the pia-arachnoid to communicate with the subarachnoid space in one study.
  • An important cause of syringo-/hydromyelia in humans are abnormal CSF dynamics at the level of the fourth ventricle/foramen magnum area. Abnormal pressure/fluid dynamics may then result in spinal cord cavitation or dilation of the central canal.
  • Often under-recognized in animals, hydromyelia is often associated with hydrocephalus.
  • More recent studies of humans with Chiari malformation has shown that disequilibration and movement of CSF from the intracranial to the spinal subarachnoid space may be the underlying factor in perpetuating syringomyelia.
  • During systole of the cardiac cycle the brain expands slightly to accommodate the increase in cerebral blood flow.
  • Cerebrospinal fluid is then shifted across the foramen magnum to the cranial cervical spinal cord.
  • During diastole, when blood leaves the brain, brain volume decreases.
  • The direction of flow of CSF reverses and moves from the cranial cervical spinal cord to the intracranial subarachnoid space.
  • If the foramen magnum is obstructed due to caudal displacement of the cerebellum, CSF cannot not move in either direction.
  • Cerebrospinal fluid, which cannot leave the intracranial space during systole, causes increased intracranial pressure.
  • The pulsatile increase in pressure is transmitted down the spinal cord and appears to be an important factor in perpetuating the syrinx cavity.
  • Cerebrospinal fluid may enter the syrinx by multiple microscopic connections of the syrinx with the subarachnoid space.
  • Reversal of these excessive pressure pulsations occurs after decompressive surgery (craniectomy and durotomy) of the foramen magnum.
  • Spinal pain may also be associated with syringo-/hydromyelia. This is curious, as many intraspinal diseases are non-painful. The pain associated with syringo-/hydromyelia may result from expansion of the spinal cord with subsequent stretching of nerve roots or the dura. Local inflammation, either primary or associated with hemorrhage into the syrinx, may also contribute to the pain noted.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Chauvet A E, Darien D L & Steinberg H (1996) What is your neurological diagnosis? Spinal abnormalities and syringomyelia of the lumbar spinal cord.​ JAVMA 208 (9), 1387-1389 PubMed.
  • Bailey C S & Morgan J P (1992) Congenital spinal malfornmations. Vet Clin North Am Small Anim Pract 22 (4), 985-1015 PubMed.
  • Cauzinille L & Kornegay J N (1992) Acquired syringomyelia in a dog. JAVMA 201 (8), 1225-1228 PubMed.
  • Johnson L, Rolsma M & Parker A (1992) Syringomyelia, hydromyelia and hydrocephalus in two dogs. Prog Vet Neurol 3 (3), 82-86 VetMedResource.
  • Child G, Higgins R J & Cuddon P A (1986) Acquired scoliosis associated with hydromyelia and syringomyelia in two dogs. JAVMA 189 (8), 909-912 PubMed.
  • McGrath J T (1965) Spinal dysraphism in the dog. With comments on syringomyelia. Pathologia Veterinaria 2, 1-36 PubMed.

Other sources of information

  • Dewey C W (2008)Myelopathy: disorders of spinal cord.In: Dewey C D (ed)A Practical Guide to Canine and Feline Neurology. Ames, IA: Wiley-Blackwell. pp 350-361.
  • LeCouteur R A & Child G (1995)diseases of the spinal cord.In: Ettinger SJ, Feldman EC (eds)Textbook of Veterinary Internal Medicine.4th edn. Philadelphia:W B Saunders. pp 629-695.
  • deLahunta A (1983)The development of the nervous system.In:Veterinary Neuroanatomy and Clinical Neurology. 2nd edn. Philadelphia: W.B. Saunders. p 28.


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