Syringomyelia (SM) is characterised by fluid-filled cavities within the spinal cord. SM occurs secondary to obstruction of cerebrospinal fluid (CSF) flow especially if that obstruction is at the foramen magnum. The most common predisposing cause in the dog is Chiari-like malformation (CM) (Figure 1). The primary clinical sign of CM/SM is pain, either due to obstruction of the CSF pulse pressure and/or a neuropathic pain syndrome due to damage to the spinal cord.

Figure 1. T1W mid-sagittal magnetic resonance imaging (MRI) scan from a Cavalier King Charles Spaniel (CKCS) with CM/SM (fluid = dark grey*).

CM is a condition characterised by disparity in volume between the cerebellum (too big) and the caudal cranial fossa (too small) such that the cerebellum and brainstem are herniated into or through the foramen magnum (Figure 2).

Figure 2. Comparison between the caudal cranial fossa and foramen magnum in a Staffordshire Bull Terrier (left) and a CKCS (right). The images on the top are mid-sagittal T2W images of the caudal cranial fossa and C1 (cerebrospinal fluid = white). The Staffordshire Bull Terrier has a well defined cisterna magnum and unobstructed foramen magnum. In the CKCS the foramen magnum is obstructed by herniated cerebellar vermis. The images on the bottom are transverse T2W images at the level of craniocervical junction. In the Staffordshire bull terrier an unobstructed subarachnoid space around the spinal cord can be appreciated. In the CKCS the CSF space cannot be appreciated and the neural tissue sectioned is the medulla and vermis - structures which are normally within the skull.

Pathogenesis

The pathogenesis of canine CM/SM is not fully understood. An important contributory factor is thought to be the mismatch in size between the brain and skull volume and obstruction of CSF flow through the foramen magnum. The precise pathogenetic mechanism of development of SM is also much debated. A popular theory is that obstruction of CSF flow results in relative increase in spinal cord pressure and decrease in pressure in the CSF space around the spinal cord, the consequence of which is repeated mechanical distention of the spinal cord. This in turn results in dilatation of the central canal and accumulation of tissue fluid which eventually coalesces into cavities (Figure 3).

Figure 3. The intramedullary pulse pressure theory of syringomyelia. Chiari malformation and obstruction of the foramen magnum prevents transmission of the systolic CSF pulse pressure wave to the distal CSF spaces. The pulse pressure is instead transmitted and reflected into the spinal cord tissues resulting in a relative increase in intrathecal pressure and decrease in subarachnoid pressure. In addition, because of the partial obstruction, the CSF displaced by each systole is forced through the narrower opening resulting in high-velocity jets of CSF ventrally within the foramen magnum. These high-velocity jets decrease the hydrostatic pressure in the subarachnoid space (Venturi effect/Bernoulli theorem). The changes in pressure have a 'suction effect' on the spinal cord. Repeated spinal cord distension results in extracellular fluid accumulation and eventually syringomyelia.

Prevalence

Brachiocephalicism and miniaturisation appear to be risk factors for CM. The condition is most commonly reported in CKCS, King Charles Spaniels, Griffon Bruxellois, Affenpinschers, Yorkshire Terriers, Maltese, Chihuahuas, Pomeranians and Papillons. Partly because of its popularity as a pet, the CKCS is overwhelmingly overrepresented. Studies into the inheritance of SM associated with CM in CKCS have shown it to be a complex trait with a moderately high heritability (h² = 0.37 ± 0.15 standard error). It has a varying age of onset - there is 46% prevalence in asymptomatic breeding CKCS but prevalence increases with age and may be as high as 70% in dogs over 6 years of age.

Clinical Signs

The most important and consistent clinical sign of CM/SM is pain; however, this may be difficult to localise. Owners may describe postural pain; for example pain on jumping or being picked up. Sleeping with the head in unusual positions may be reported. Pain is positively correlated with syrinx width and symmetry, i.e., dogs with a wider asymmetrical syrinx are more likely to experience discomfort, and dogs with a narrow symmetrical syrinx may be asymptomatic (Figure 4). Syrinxes can progressively expand and a dog which is asymptomatic in early life may eventually become painful. Dogs with a wide syrinx may also scratch, typically on one side only, while the dog is walking and often without making skin contact, such behaviour is often referred to as an 'air guitar' or 'phantom' scratching. Dogs with a wide syrinx are also more likely to have scoliosis. SM may result in other neurological deficits such as thoracic limb weakness and muscle atrophy (due to ventral horn cell damage) and pelvic limb ataxia and weakness (due to white matter damage or involvement of the lumbar spinal cord by the syrinx). Seizures, vestibular dysfunction, facial nerve paralysis and deafness may also be seen; however, no direct relationship has been proven and this association may be circumstantial.

CM alone appears to cause significant head and spinal pain in some dogs.

Figure 4. T2W transverse image through a wide syrinx (asterisk) demonstrating the asymmetrical involvement of the right spinal cord dorsal horn (cerebrospinal fluid = white).

Diagnosis

Magnetic resonance imaging (MRI) is essential for diagnosis and determining the cause and extent of SM. In the instance of CM/SM the cerebellum and medulla extend into or through the foramen magnum which is occluded with little or no CSF around the neural structures. The size of the cerebellar herniation is not correlated with severity. There is typically ventricular dilatation. SM is indicated by fluid-containing cavities within the spinal cord. The upper cervical and upper thoracic segments are typically most severely affected. Maximum syrinx width is the strongest predictor of pain, scratching behaviour and scoliosis.

Differential Diagnosis

The most important differential diagnoses are other causes of pain and spinal cord dysfunction such as intervertebral disc disease (IVD); CNS inflammatory diseases such as granulomatous meningoencephalomyelitis; vertebral abnormities such as atlantoaxial subluxation; neoplasia; and discospondylitis. IVD would be an unlikely cause of pain in a CKCS aged less than 4 years old. When scratching or facial/ear rubbing is the predominant clinical sign, ear and skin disease should be ruled out. The classic scratching behaviour for SM is to one distinct area. It is a common incidental finding for CKCS to have mucoid material in one or both tympanic bullae and in the majority of cases this is not associated with clinical signs of pain, although it may cause deafness. Some cases with scoliosis appear to have a head tilt which could be confused with vestibular dysfunction.

Treatment

A possible approach to management of CM/SM is illustrated in Figure 5. The main treatment objective is pain relief. The most common surgical management is cranial/ cervical decompression (also described as foramen magnum or suboccipital decompression) establishing a CSF pathway via the removal of part of the supraoccipital bone and dorsal arch of C1. This may be combined with a durotomy (incision of the dura with/without incision of subarachnoid meninges) with or without patching with a suitable graft material. Cranial/cervical decompression surgery is successful in reducing pain and improving neurological deficits in approximately 80% of cases and approximately 45% of cases may still have a satisfactory quality of life 2 years postoperatively. However, surgery may not adequately address the factors leading to SM and the syrinx appears persistent in many cases. The clinical improvement is probably attributable to improvement in CSF flow through the foramen magnum. In some cases scarring and fibrous tissue adhesions over the foramen magnum seem to result in reobstruction, and 25% to as many as 50% of cases can eventually deteriorate. This can be as early as 2 months postoperatively.

Due to the persistence of SM and/or spinal cord dorsal horn damage it is likely that the postoperative patient will also require continuing medical management for pain relief and in some patients medical management alone is chosen because of financial reasons or owner preference. There are three main drugs used for treatment of CM/SM: drugs that reduce CSF production, e.g., cimetidine or omeprazole or possibly diuretics such as furosemide; analgesics; and corticosteroids (Figure 5). Simple actions, for example raising the food bowl and removing neck collars, can also help. The clinical signs of CM/SM are often progressive - a recent study found that approximately three-quarters of CKCS with CM/SM-associated neuropathic pain will deteriorate on conservative treatment only.

Figure 5. Treatment algorithm for Chiari-like malformation and syringomyelia. With the exception of NSAIDs none of these drugs are licensed for treatment of syringomyelia. The drugs gabapentin, pregabalin and amantadine are not licensed for veterinary medicine.

Breeding Recommendations

It is recommended that breeders of dogs predisposed to CM/SM screen their stock. Current breeding recommendations for CKCS concentrate on removal of dogs with early onset SM from the breeding pool (for more information see www.veterinary-neurologist.co.uk/sm_screening_breeders.htm). A British Veterinary Association (BVA)/Kennel Club (KC) CMSM Scheme is due to launch in early 2012. Results of KC registered dogs will be sent to the Animal Health Trust for inclusion in the Estimated Breeding Value (EBV) calculations for a Mate Select Computer program.