Simon R. Platt, BVM&S, MRCVS, DACVIM (Neurology), DECVN
Small Animal Medicine & Surgery, College of Veterinary Medicine, University of Georgia, Athens, GA, USA
Chiari-Like Malformation and Syringomyelia (CM/SM)
Chiari-like malformation (CM) and syringomyelia (SM) often occur together, although both may occur independently of the other. Syringomyelia is a condition characterized by the presence of a fluid-filled cavity (syrinx) or cavities within the parenchyma of the spinal cord. SM is secondary to abnormal cerebrospinal fluid movement and is usually associated with Chiari-like malformation, although it may be associated with other conditions such as congenital malformations, trauma, inflammation, and neoplasia. Chiari-like malformation is defined as a decreased caudal fossa volume with herniation of the cerebellum and often brainstem into or through the foramen magnum. In people, this condition is referred to as Chiari malformation, which has several types.
The term syringomyelia is accepted to describe fluid accumulation within the spinal cord, whether it be secondary to central canal dilation (hydromyelia) or secondary to fluid accumulation within the spinal cord parenchyma (syringomyelia or syringohydromyelia). It is difficult to determine the location of the fluid using magnetic resonance imaging (MRI) and these cavities often communicate with each other. Syringomyelia frequently occurs with Chiari-like malformation in dogs and the terms Chiari-like malformation and syringomyelia (CM/SM) have been adopted to describe the canine condition.
Onset of signs may be acute or chronic in dogs ranging from 6 months to 10 years of age. The most common sign of CM/SM is pain, predominately isolated to the cervical region, occurring in 35% of affected dogs and 80% of people with the similar condition.
These structural abnormalities are best diagnosed with MRI, but they may be clinically silent; therefore, their significance must be carefully considered when such abnormalities are discovered.
Treatment may not be necessary in asymptomatic dogs or dogs with mild non-progressive signs. Dogs exhibiting pain, more severe neurological deficits, or progressive signs can be treated either medically or surgically. Typically, medical therapy is pursued initially involving the use of analgesics and drugs that reduce CSF formation. Furosemide (1–2 mg/kg orally q12h) and prednisone (0.5–1 mg/kg orally 24h, tapering dose) are frequently used. Treatment of neuropathic pain with drugs such as gabapentin (10 mg/kg PO q8 h) is also an important aspect of therapy.
Approximately 70% of patients show some improvement, but it is rarely complete. If medical therapy does not alleviate the clinical signs, surgical decompression of the foramen magnum has been suggested (suboccipital craniectomy) and is the treatment of choice in people. Foramen magnum decompression has been performed in dogs with a success rate reported at about 80%; however, recurrence is common and neuropathic pain may persist requiring continued medical therapy. Additionally, multiple surgeries may be required if scar tissue develops at the surgical site obstructing CSF flow, although cranioplasty may reduce the likelihood of this complication. Improvement may not be a result in the reduction of syrinx size, which usually persists.
A severe form of steroid-responsive meningitis-arteritis (SRMA) has been reported in beagles, Bernese mountain dogs, boxers, German short-haired pointers, and sporadically in other breeds. This condition has a worldwide distribution and represents one of the most important inflammatory diseases of the canine CNS. Beagles, especially but not exclusively those in laboratory-bred colonies, appear at risk. In the beagles, the condition has been termed beagle pain syndrome, necrotizing vasculitis, polyarteritis, panarteritis, juvenile polyarteritis syndrome, and primary periarteritis. In other breeds, this condition previously appears under the terms necrotizing vasculitis, corticosteroid-responsive meningitis, aseptic suppurative meningitis, and corticosteroid-responsive meningomyelitis.
Affected animals usually are most commonly young adults between 8 and 18 months of age, although the age range may extend from 4 months to 7 years. The clinical course is typically acute with recurrences. Signs include recurring fever, hyperesthesia, cervical rigidity, and anorexia. There may be a creeping gait, arching of the back with head held down, and crouched posture.
The prognosis is guarded to favourable, especially in dogs with acute disease that are treated promptly using immunosuppressive doses of corticosteroids. Untreated dogs tend to have a remitting and relapsing course. Tipold recommends the following long-term therapy (e.g., for at least 6 months), especially in any dog that has had a relapse: prednisolone at 4 mg/kg/day, PO or IV initially. After 2 days, the dose is reduced to 2 mg/kg daily for 1 to 2 weeks, followed by 1 mg/kg daily. Dogs are re-examined, including CSF analysis and hematology, every 4 to 6 weeks. When signs and CSF are normal, the dose can be reduced to half of the previous dosage until a dosage of 0.5 mg/kg every 48 to 72 hours is attained. Treatment is stopped 6 months after clinical examination, CSF, and blood profiles are normal. In refractory cases, other immunosuppressive drugs such as azathioprine (at 1.5 mg/kg PO every 48 hours) may be used in combination with steroids (e.g., alternating each drug every other day). Antibiotics are ineffective.
Onset of signs in dogs with the congenital form of the disease usually occurs in young dogs (less than two years of age), although problems can develop at any age. Signs can develop acutely or gradually, and waxing and waning of signs is often reported; presumably a reflection of instability at the atlantoaxial junction causing repeated injury to the spinal cord. Signs include neck pain (variably present), ataxia, tetraparesis, and postural reaction and conscious proprioceptive deficits with normal to increased muscle tone and myotatic reflexes in all four legs. In severe cases, animals can present with tetraplegia and difficulty breathing and they may die acutely as a result of respiratory failure.
Atlantoaxial subluxation can be diagnosed from survey radiographs of the cervical spine although extreme care must be taken when restraining and moving dogs in which this disease is suspected. It is preferable to do this with fluoroscopy so that the movement can be monitored to prevent accidental iatrogenic subluxation. Recently, MRI evaluation of the spine and cord has proved beneficial in both the diagnosis and prognosis.
Dogs with mild signs can be treated conservatively by placing an external splint for at least 6 weeks. The splint must immobilize the atlantoaxial junction and so must come over the head cranial to the ears and go back to the level of the chest. The aim is to stabilize the junction while the ligamentous structures heal. The splint should be checked daily for signs of pressure sores by the owner and checked weekly by the veterinarian, with regular bandage changes if necessary. While often effective in the short term, the long-term efficacy of this approach is not known and dogs treated in this way will always be at risk of repeated injury.
Surgery is recommended in dogs with neurological deficits, although it can be associated with high perioperative morbidity and mortality. Dorsal and ventral approaches to the atlantoaxial junction have been described, but dorsal approaches are associated with a greater risk of causing spinal cord injury during surgery, and a higher incidence of implant failure. This is a serious disease but dogs with mild deficits treated surgically have an excellent prognosis if they survive the 48-hour perioperative period. Although reported surgical success rates range from 50% to 90%, the majority report a mortality rate in the region of 20% with the majority of deaths occurring either during or immediately after surgery. As with all spinal cord diseases, prognosis is worse in animals with severe and chronic neurological deficits. It has also been shown that prognosis is better in young dogs (<24 months).
Tumors affecting the spinal cord are described based on their location as extradural, intradural-extramedullary and intramedullary. Intradural-extramedullary tumors that occur include meningioma and nerve sheath tumors. Meningiomas affecting the spinal cord are most common in the neck. Meningiomas of the spinal cord in dogs and cats tend to present with a progressive paraparesis. The clinical signs represent the spinal cord region involved. Paraspinal pain may or may not be present. Spinal meningiomas are the most common primary spinal cord tumor in cats older than 8 years of age. Mean age for spinal meningiomas in dogs is 9 years of age. Spinal meningiomas in dogs are most common in the cervical spinal cord, but occur in any region of the spinal cord. Myelography typically shows an intradural extramedullary compressive lesion. With MRI, these tumours are iso- to hypointense on T1-weighted (T1W) images, hyperintense on T2-weighted (T2W) images, and demonstrate strong, uniform contrast enhancement. These lesions should be surgically explored because many meningiomas can be completely or partially removed and therefore may be associated with prolonged survival after surgery. Postoperative radiation therapy may be used adjunctively to prolong survival in dogs with incompletely excised tumors. Treatment with surgery and radiation therapy can result in an improved outcome and prevent recurrence. Surgical results are guarded when meningiomas are associated with an intumescence and tumors with ventral location and invasion of the neural parenchyma.