The crooked spine: Congenital and developmental spinal disorders (Proceedings)

Gait evaluation in the pediatric patient can initially be difficult as puppies first learn to walk and the myelination process matures. As development progresses spinal disorders may become more apparent. Numerous congenital abnormalities resulting in spinal malformation exist. Spinal malformations most commonly are asymptomatic. However, spinal malformations may lead to mild or significant dysfunction.

Hemivertebrae are wedge shaped vertebrae, with the base oriented ventrally, dorsally or medially. A hemivertebra occurs when a portion of the vertebra does not form, typically the vertebral body. Failure of the central portion of the vertebra to form may result in hemivertebrae on both sides and is called a butterfly vertebra. Hemivertebrae are common in screw tail breeds. Traumatic and pathologic fractures should be distinguished from hemivertebrae as they can appear similar.1,2 Hemivertebrae can lead to severe spinal angulation resulting in kyphosis, scoliosis or lordosis.2 Plain radiographs may diagnosis vertebral anomalies. However, myelography, CT or MR imaging are required to assess the degree of spinal cord compression. Non steroidal anti-inflammatory (NSAID) therapy may be of benefit if pain or mild paresis is present. If NSAID therapy is unsuccessful or if paresis is severe then prednisone therapy at anti-inflammatory dosages (0.5 mg/kg PO BID initially and then tapered) may be successful. If paresis is severe or if progressive despite medical therapy, then surgical decompression ± stabilization is recommended. The prognosis with surgery may be guarded due to concurrent spinal malformations,2 and chronicity.1

Block vertebrae result from embryonic failure of segmentation. Block vertebrae are typically shorter than the sum of the individual segments. Differential diagnoses for block vertebrae include vertebral fusion following a previous fracture, discospondylitis or previous disk surgery.1,2

Transitional vertebrae occur at the junction of the divisions of the vertebral column. The thoracolumbar junction is commonly affected, with unilateral or bilateral defects. The thirteenth thoracic vertebra may have a short thick transverse process instead of a rib. Less commonly, the first lumbar vertebrae may have a rib instead of a transverse process.1 Transitional vertebrae at the thoracolumbar junction do not appear to cause a significant clinical problem. However, the last rib is commonly used for localization during the surgical approach for hemilaminectomy. Thus, a malformation at this location could lead to an inappropriate surgical location. Identification of thoracolumbar vertebrae is more difficult via MR imaging. With the increasing availability and use of MR imaging this should be considered.

Lumbarization of the first sacral vertebra is also common. The dorsal lamina separates with a wing like lateral process, similar to a transverse process. A new disc space at S1-S2 is also formed. Lumbosacral transitional vertebrae occur in 2.5 to 4 % of dogs. In the German Shepherd Dog the anomaly is more common with 8 % to 11% of asymptomatic dogs affected.1,3 Lumbosacral transitional vertebrae appear to be clinically significant. An increased incidence has been seen in German Shepherd Dogs with cauda equina syndrome as 38 % to 44 % of affected German Shepherd Dogs have the anomaly.3,4

Incomplete closure of the developing neural tube results in spinal dysraphism and congenital syringohydromyelia. Ataxia, a bunny hopping pelvic limb gait and pelvic limb spinal reflex deficits may be seen. Spinal dysraphism is heritable in Weimaraners. Affected Weimaraners may lack a central canal, a ventral median fissure or have incomplete separation of the ventral horns.1,5

Spina bifida is a form of spinal dysraphism in which the vertebral arches do not fuse. When a concurrent meningocele, myelocele or meningomyelocele is present the terms spina bifida cystica, manifesta and aperta are used.6 A high incidence is present in the Bulldog and Manx cat and may be associated with other vertebral malformations. Manx cats may have concurrent sacrococcygeal dysgenesis, spinal cord or sacral abnormalities. Spina bifida is frequently asymptomatic. When symptomatic, signs vary based on location of the defect. Paresis and incontinence may occur. Spina bifida cystica may be evident grossly. Abnormal direction of hair growth, dimples in the skin or a CSF draining tract may be identified. The extent of the defect may be identified with myelography or MR imaging. Treatment for spina bifida is typically not attempted.1,2

A dermoid sinus results from failure of the neural tube to separate from the skin during embryonic development. A tubular tract of epithelial tissue may extend as deep as the dura mater. Meningitis may result. Spina bifida may be identified radiographically when the sinus extends into the dura mater. The location is cranial or caudal to the ridge in Rhodesian ridgebacks. The defect has also been identified in Boxers and Shih Tzus. A small hole may be seen with a hair extending outward.7 The tract can become swollen or abscessed. MR imaging can be used to evaluate the extent of the sinus. Surgical excision and antibiotic therapy based on culture of the sinus or CSF is recommended

Atlantoaxial instability occurs from aplasia, hypoplasia or degeneration of the dens or associated ligaments that lead to dorsal deviation of the dens. Instability may arise directly from these abnormalities or may be precipitated by trauma. Toy breed dogs are most commonly affected with the Poodle and Yorkshire Terrier being overrepresented. Most affected dogs are less than one year of age. Most commonly spinal cord compression leads to pain and tetraparesis.8 Tetraplegia and respiratory arrest can occur.9

Plain radiographs may identify the dens abnormality. Oblique views are often helpful in distinguishing the dens. Myelography identifies spinal cord compression and can identify dynamic instability. CT scan may also identify the abnormality, but is less helpful in identifying dynamic instability. MR imaging may identify the abnormality as well as spinal cord edema.1

Care should be taken to limit cervical motion particularly ventroflexion. Opiods can be used for pain control, however respiration should be monitored. NSAID therapy may be considered for mild pain control or prednisone at anti-inflammatory dosages may be considered for pain and spinal cord edema. A cervical brace could be considered if surgery is not feasible.

Surgical stabilization of the atlantoaxial joint has been reported to be 85.3% to 88.9% effective.8 A second surgery is sometimes necessary due to fixation failure, implant migration or implant failure and my reduce the reported success rates. Respiratory failure secondary to brain stem dysfunction can lead to death. Death or surgical failure typically occurs within the first 20 days post operatively. Cervical braces post operatively may improve stability. Surgical success is better when dogs are less than 2 years of age and signs are less than 10 months.1,8 Severe neurological signs are associated with a worse outcome.8

Caudal occipital malformation syndrome is similar to Chiari type I malformation in people. Hypoplasia of the occipital bone results in overcrowding of the caudal fossa and obstruction of the foramen magnum. Syringohydromyelia often results in the cervical spinal cord and sometimes the thoracic spinal cord. The condition is hereditary in the Cavalier King Charles Spaniel10 and has also been identified in the Maltese, Yorkshire Terrier and Pomeranian.11 Clinical signs include persistent scratching at the cervical and or shoulder region, thoracic limb weakness, cervical scoliosis, pelvic limb ataxia and apparent pain.10,12 Facial nerve deficits, seizures and vestibular signs have also been reported.13 Clinical signs typically arise between 6 months and 3 years of age, but have been identified as late as 10 years.10 MR imaging of the brain and cervical spinal cord is needed for diagnosis. The malformation occurs in many clinically normal Cavalier King Charles Spaniels.14

Medical treatment options include anti-inflammatory doses of prednisone, gabapentin and omeprazole. Surgical therapy is recommended if neurological deterioration and significant pain are present despite medical therapy.14 Suboccipital craniectomy with a C1 dorsal laminectomy is most commonly done. Results are variable, but are up to 81.25 % successful. Repeat surgery may be required in as many as 25 % of dogs due to constrictive scar tissue formation.11 Surgery is likely most successful when signs are less severe and of shorter duration.14

References

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Morgan JP, Bahr A, Franti CE, et al. Lumbosacral transitional vertebrae as a predisposing cause of cauda equina syndrome in German Shepherd dogs: 161 cases (1987–1990). J Am Vet Med Assoc 1993;11:1877–82.

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Dewey CW, Berg JM, Barone G, et al. Treatment of caudal occipital malformation syndrome in dogs by foramen magnum decompression [abstract]. J Vet Intern Med 2005;19(3):418.

Rusbridge C, MacSweeney JE, Davies JV, et al. Syringohydromyelia in cavalier king charles spaniels. J Am Anim Hosp Assoc 2000;36:34–41.

Lu D, Lamb CR, Pfeiffer DU, et al. Neurological signs and results of magnetic resonance imaging in 40 cavalier king charles spaniels with Chiari type 1-like malformations. Vet Rec 2003;153:260–3.

Rusbridge C. Treatment of syringomyelia. In: Proceedings of the 23rd Annual Meeting of the American College of Veterinary Internal Medicine. Lakewood (CO): American College of Veterinary Internal Medicine; 2005. p. 361–2.