In Chiari-like malformation, the skull is malformed, which results in a cranial cavity that is too small to accommodate the caudal parts of the brain (the cerebellum and brainstem), resulting in overcrowding. In approximately 40% of animals with this malformation, the cerebellum will herniate (protrude) through an area called the foramen magnum. The majority of these dogs will also have syringomyelia.
Syringomyelia is a condition whereby fluid-filled cavities develop within the spinal cord, often as a result of the Chiari-like malformation. Due to the malformation, the cerebrospinal fluid is forced down the centre of the spinal cord, where it causes the tissues to distend and forms cavities.
What breeds are predisposed?
The Chiari-like malformation is typically found in small breed dogs. The Cavalier King Charles Spaniel is the most common breed affected by this condition. Other affected breeds include the Bichon Frise, Boston Terrier, Brussels Griffon, Chihuahua, Miniature Daschund, French Bulldog, Miniature Pinscher, Miniature Poodle, Pekingese, Pug, Shih Tzu, Staffordshire Bull Terrier, and Yorkshire Terrier.
What clinical signs will I see if my pet has this condition?
There is a large range of clinical signs that can develop in dogs with this condition. It is important to know that there is no relationship between the size of the syrinx (the fluid-filled cavity) and the severity of the clinical signs.
The most common sign seen is pain – which may be in the neck or lower down the back. Dogs affected with this condition may also yelp and be reluctant to jump or climb. Incessant scratching activity, particularly around the head and ears is common – but this is typically with no contact of the foot with the skin – termed ‘phantom scratching’. This is also commonly only performed on one side of the body. Some dogs will also tend to rub their faces incessantly.
Some dogs will develop weakness or wobbliness in their legs.
Not all animals will show any clinical signs, and as such the presence of syringomyelia can be an incidental finding when other conditions are being assessed.
How is this condition diagnosed?
The diagnosis is best achieved by performing an MRI scan of the brain and spine. This will allow us to see the shape of the brain and bones of the skull, and to detect any compression or herniation of the cerebellum. MRI will also show the presence of any fluid-filled cavities in the spinal cord.
What are the treatment options available?
Medical management is generally the treatment of choice for all patients suffering from Chiari-like malformation or syringomyelia. Medical management revolves around the use of three types of medications to provide pain relief and reduce the production of cerebrospinal fluid to reduce the size of the syrinx’s:
1. Analgesia (pain relief): The most common medication used that relieves scratching activity is a drug called Gabapentin, which is a relatively safe drug with minimal side effects, the most common of which is sleepiness.
2. Medications that reduce the production of cerebrospinal fluid
3. Corticosteroids have anti-inflammatory effects, and also are believed to reduce cerebrospinal fluid production
Whilst medical management can be successful in a large number of cases in alleviating clinical signs, it must be remembered that this is not curing the condition. Surgical management is the treatment of choice to alleviate the cause of concern.
The surgical procedure is called a foramen magnum decompression with a stabilisation cranioplasty. This procedure aims to improve the shape of the back of the skull and remove the compression/herniation of the cerebellum which reduces the flow of cerebrospinal fluid down the spinal cord.
What is the prognosis?
Many dogs will improve significantly following surgery, with an 80% success rate following surgery. There is a 25-50% chance of relapse however if foramen magnum decompression is performed alone, which is often a result of the development of excessive scar tissue. The recurrence rate decreases significantly when a cranioplasty is also performed. In some cases, phantom scratching will also persist.
Most dogs will respond to medical management, however the majority will relapse.