Atlantoaxial instability can lead to subluxation of the first and second cervical vertebrae; the cranial aspect of the axis often rotates dorsally with respect to the atlas, into the vertebral canal. Subsequent spinal cord compression results in a variety of neurological signs, but may just cause cervical pain. The pathogenesis, diagnosis and treatment of this disease are discussed in site.
Clinicalsigns:The condition may be acute orchronic, and can occur in dogs ranging from 6 months to 10 years of age. Clinical signs include neck pain, torticollis or scoliosis, spinal hyperaesthesia and neurological deficits relating to cervical spinal cord dysfunction. Intracranial signs, such as facial paresis and vestibular dysfunction, have also been reported. Paroxysmal involuntary scratching of the neck and flank has been associated with this condition.
Pathogenesis: Chiari-like malformations are complex developmental disorders involving the caudal brainstem, cerebellum and the cranial cervical spinal cord. The human classification of Chiari type I necessitates elongation and caudal displacement of the cerebellartonsils(vermis and paravermal lobes) through the foramen magnum into the cranial cervical vertebral canal; the cord may be kinked, due to being pushed caudally by the brainstem. A similar condition has been documented in dogs and is apparently over-represented in Cavalier King Charles Spaniels (). A familial or genetic basis is suspected (). This condition seems to be associated with occipital bone dysplasia resulting in ‘overcrowding’ of the caudal fossa, obstruction of the CSF flow and secondary syringohydromyelia.
In type II malformation, there is herniation of the cerebellar vermis and sometimes portions of the lateral cerebellar hemispheres, over the dorsal aspect of the cervical spinal cord; the brainstem may also be elongated and partially located in the spinal canal, usually in association with a meningomyelocele.
Diagnosis: These structural abnormalities are best diagnosed with MRI (), but they may be clinically silent, so their significance must be carefully considered when such abnormalities are discovered ().
Treatment and prognosis: Treatment of these conditions is either medical or surgical. Medical therapy involves the use of frusemide (2 mg / kg orally q12h) in conjunction with prednisolone (0.5 mg / kg orally q48h). Approximately 70% of patients show some improvement, but it is rarely complete (Dewey, 2003). If medical therapy does not alleviate the clinical signs, surgical decompression of the caudal fossa has been suggested (suboccipital craniectomy), and is the treatment of choice in people.
Perineurial (Tarlov) cysts
Perineurial cysts are rare lesions of the nerve roots that have been recently reported in two dogs (). Tarlov cysts are distinguished from other extradural meningeal lesions on the basis that;
- • They arise at the junction of the dorsal root ganglion and the nerve root
- • They develop between the endoneurium and perineurium
- • Their lining contains nerve fibres and / or ganglion cells.
In humans, excision orfenestration of these cysts is recommended, although recurrence is possible.
In the author’s study, a 4-year-old spayed female Scottish Deerhound and an 8-month-old female St Bernard presented with chronic neck pain; MRI revealed fluid-filled structures at the level of vertebrae C6 and C7 (). The cysts, which were attached to the nerve roots, were fenestrated. Histopathology of the capsules was compatible with perineurial cysts. On re-examination one year postoperatively, neither dog had evidence of cervical pain.
Spinal cord anomalies
These diseases include spinal dysraphism, myelodysplasia, spina bifida and syringohydromyelia. The first three usually present with neurological deficits and are described in site. Although neurological deficits are common with syringohydromyelia, pain and persistent scratching of the neck and flank are often the first clinical signs.
Syringohydromyelia: Syringomyelia (cavitation of the spinal cord) and hydromyelia (dilation of the central canal) result in similar signs of spinal cord dysfunction. Syringomyelia may exist as a distinct entity or may exist in communication with the central canal. It is often difficult to differentiate between the two conditions and so the term syringohydromyelia is often used (). The causes of this condition include congenital malformations, trauma, inflammation and neoplasia but it can also be idiopathic. Clinical signs depend on the specific spinal site of the lesion but approximately 40% have spinal pain (). This condition is discussed further in site.
This may be subclinical but can cause spinal discomfort (). Neurological deficits may be present; these depend on the underlying cause.
Scoliosis may occur in animals with a hemivertebra. There are numerous reports of scoliosis occurring in animals with congenital or acquired cystic lesions involving the spinal cord, such as syringohydromyelia (). The association has been suggested to be due to progressive destruction of grey matter by cavitation, resulting in denervation and atrophy of epaxial muscles unilaterally, followed by asymmetrical contralateral muscle spasticity and subsequent vertebral deviation.
Diagnosis of the structural cause of the scoliosis necessitates imaging studies. Treatment, if any is possible, should be directed at the underlying cause; the prognosis is guarded.
Many spinal anomalies do not produce any clinical signs and are detected as incidental findings; however, if a vertebral malformation is discovered in the presence of spinal pain, with or without neurological deficits, it should be thoroughly investigated. The presence of a vertebral anomaly may be a sporadic occurrence, but the potential for heritability must be considered. Many of the documented anomalies in veterinary patients occur in the thoracolumbosacral vertebrae, but they have the potential to occur anywhere in the vertebral column. They often cause signs due to associated spinal canal stenosis, progressive spinal deformity with growth or ageing, or instability exacerbated by degenerative disc disease. Important vertebral anomalies include hemivertebrae, block vertebrae, occipitoatlantoaxial malformation, hypoplasia or aplasia of the dens, transitional vertebrae and congenital spinal stenosis.