The answer to this question is multifactorial. The age of dog tends to be a very important consideration in many instances. Young dogs with clinical signs should be considered for surgical treatment, while older dogs with minimal to no clinical signs could be managed medically or surgically.
The severity of CLM/SM plays a role in this decision. Young dogs with mild changes should still be considered surgical candidates to minimize progression of the disease as the dog ages. Severe cases of CLM/SM, regardless of age, are still surgical candidates as many dogs do have some relief of clinical signs as they heal. Many dogs may still require management post-operatively with medication, but the variety and dosages may be reduced with surgical correction. Dogs that have been managed with medication initially and continue to have progression of the disease, may still benefit from surgical intervention.
What are the goals of surgery?
Syrynx decompression through restoration of normal CSF flow is the goal of any definitive treatment for SM. Although decompressive shunt placements have been described in an effort to deflate the syrinx, it is considered a last resort in favor of more physiologic treatments. Chiari-like malformation is the most common cause of SM and foramen magnum decompression (FMD) with a dorsal lamenectomy of the C1 vertebral body is considered the treatment of choice. Decompression or widening of the back of the skull is believed to impove CSF flow and lessen the impact on the descending cerebellar vermis. Re-shaping the back of the skull, or cranioplasty, in addition to FMD is recommended at The Canine Chiari Institute at LIVS to decrease the frequency of scar tissue formation in the operative site after surgery.
Foramen magnum decompression is the preferred treatment for Chiari type I malformation in people. Surgery is often successful, but 8-30% of patients require re-operation, due to excessive scar tissue formation at the FMD site. A FMD procedure for CLM in dogs has been described and the success rate of this procedure is approximately 81%. When performing FMD, both portions of the occipital bone and the lamina of the C1 vertebral body are removed to provide bony decompression (Fig 19). The dura mater that covers the area is frequently abnormal and is either incised (durotomy) or removed. Failure to remove the diseased dura can result in persistent compression, because it acts as a fibrous band constricting the cerebellum. Similar to humans, about 25% of canine patients require re-operation due to excessive post-operative scar tissue formation with FMD alone (Fig 20). It has been reported that worsening clinical signs associated with scar tissue impingement typically occur within 3 months of surgery. Cranioplasty offers a protective covering for the recently decompressed cerebellum and has reportedly decreased the frequency of scar tissue formation in patients after surgery.
In performing a cranioplasty, guide holes for titanium screws are made in the occipital bone, around the edge of the FMD defect. Self-tapping titanium screws are inserted into the holes for an approximate depth of 2-3 mm (Fig 21). The skull plate is fashioned using titanium mesh and polymethylmethacrylate (PMMA), and fixed to the back of the skull, using the titanium screw heads as anchor posts for the PMMA (Fig 22).
|Fig 20||Fig 21|
Currently, 73 of 103 dogs with MRI confirmed CLM, treated with FMD with cranioplasty, had a minimum 1 year follow up and were evaluated. Results of our current ongoing study revealed 62/73 dogs (85%) improved after FMD and cranioplasty, with 11/73 (15%) requiring intermittent medication to maintain quality of life. Only 5 dogs (7%) experienced scar tissue formation after FMD with cranioplasty and were re-operated. Additionally, although it has been reported the majority of syrinx formations remained unchanged on visual estimation, our objective syrinx evaluation using MRI syrinx volume calculations revealed length measurements preoperatively and 6 months post-operatively in 13 of 21 dogs (62%), had decreased in both average syrinx length and volume, 29.4% and 45.6% respectively, while 4/21 (19%) remained unchanged (Fig 23) Future studies are underway to identify reasons why most dogs experience syrinx resolution while others do not, and if there are abnormalities in the latter that are unaddressed.
|Fig 22A||Fig 22B||Fig 22C||Fig 22d|
Pain and medical therapy in dogs with CLM/SM
One of the most common and devastating aspects of CLM/SM in dogs is the pain that the patients experience. In addition, the scratching activity characteristic of the disorder is suspected to be a sensory disturbance called a paraesthesia. Most dogs with CLM/SM with clinical signs of disease exhibit varying degrees of neck pain.
|Fig 23A||Fig 23B|
It has become apparent that many patients, however, have pain throughout the length of their spine. When these dogs have MR imaging of their thoracic and lumbar spines performed, in addition to their neck region, syrinx cavities are usually found in all of the imaged locations.
There are a number of medical therapies that seem to help CLM/SM dogs with both pain management and scratching activity, but there are no controlled clinical trials published yet to objectively measure the efficacy/inefficacy of these treatments. Therefore, there are a lot of opinions regarding which drugs or drug combinations are helpful in this disorder. The three main categories of medical therapy for CLM/MS include analgesic (pain-relieving) drugs, drugs that decrease CSF production, and corticosteroid therapy (which has multiple effects).
The most widely used drug to alleviate pain and scratching activity is gabapentin. Gabapentin is a drug that works at calcium channels and decreases the release of substances from nerve terminals that help generate pain and paresthesia. The drug appears to be very effective, but dogs often appear to worsen before the next dose is due (this drug is typically given every 8 hours), suggesting that it sometimes does not last long enough in the bloodstream. The next generation of gabapentin, a drug called pregabalin, lasts longer (almost twice as long) and appears to be more potent. Pregabalin appears to be at least as, if not more effective than gabapentin, and can be given every 12 hours. A popular opiate-like drug, called tramadol, is also frequently prescribed for CLM/SM patients and is often effective in alleviating head/neck pain. Several non-steroidal anti-inflammatory drugs have been used to treat pain associated with CLM/SM. These include meloxicam, carprofen and deracoxib. Amantadine is a drug that inhibits a pain receptor called the NMDA (N-methyl-D-aspartate) receptor. Amitriptyline is a tricyclic antidepressant drug that may have some efficacy in relieving pain and paraesthesias.
A number of drugs have also been used to decrease CSF production. These include omeprazole, acetazolamide, methazolamide and furosemide. Omeprazole is a proton pump inhibitor that has been shown to decrease CSF production in dogs, but the exact mechanism is unknown. Acetazolamide and methazolamide are inhibitors of the enzyme carbonic anhydrase, which is required for the production of CSF. Furosemide is a loop diuretic that may also decrease CSF production.
Corticosteroids, like prednisone, tend to be very effective in managing pain; however, prolonged use of corticosteroids can have adverse health consequences. Non-steroidal anti-inflammatory drugs such as carprofen can also be effective, but these drugs should never be used concurrently with glucocorticoid drugs because of potential gastrointestinal damage. Corticosteroids have also been used in dogs with CLM/SM to decrease CSF production and inflammation of the CNS.
We have found that most dogs with CLM/SM respond well to medical therapy initially. Although this is definitely a positive aspect of the disorder, we estimate that between one-third and one-half of our patients will stop responding to drug therapy within 1.5 to 3 yrs, and many of these dogs are subsequently euthanized due to unrelenting pain. We have also found that dogs tend to have more positive responses to surgical intervention the earlier such intervention takes place. There is a natural tendency for dog owners to delay surgical therapy once a pet has responded favorably to medical management. Although this is an understandable approach to patient management, there is a risk that the disease may not respond as favorably to surgical management in the future if surgery is delayed. The fact that there are complications and a measurable failure rate with surgical treatment of the disorder compounds the difficulty for owners and veterinarians in making the right therapy decisions for dogs with CLM/SM.