Foramen magnum decompression with cranioplasty for treatment of caudal occipital malformation syndrome in dogs - an update on 18 cases
Dominic Marino

Foramen magnum decompression (FMD) is the preferred mode of therapy for Chiari type 1 malformation of people. Surgery is often successful, but 8-30 % of patients require re-operation, due to excessive scar tissue formation at the FMD site. This scar tissue results in compression of nervous tissue and abnormal cerebrospinal fluid dynamics, perpetuating the secondary formation of a syrinx and resulting in neurologic deterioration.

A FMD procedure for canine caudal occipital malformation syndrome (COMS) has been described, the success rate of this procedure is approximately 81%. Similar to humans, about 25% of canine patients require re-operation due to excessive post-operative scar tissue formation. In our experience clinical signs of worsening associated with scar tissue impingement typically occur within 3 months of surgery.

A cranioplasty procedure developed for Chiari type 1 patients at the Chiari Institute has essentially eliminated the problem of post-operative scar tissue compression at the FMD site.

The purpose of this report is to describe a modification of that procedure for dogs with COMS and report on the results of long term follow-up.

Eighteen dogs with MRI confirmed COMS underwent FMD with cranioplasty.

Fifteen of the eighteen dogs were Cavalier King Charles Spaniels; the three remaining dogs were a Pug, Chihuahua and a Maltese. There were 7 male castrated dogs and 11 female spayed dogs. In two dogs, the surgery was a re-operation (excessive scar tissue following standard FMD).

Median age was 26 months (range 12 months - 98 months).

Following the FMD procedure previous described, guide holes for either 1.5mm diameter (1.1mm drill bit) or 2.00mm (1.5mm drill bit) titanium screws were made in the occipital bone, around the edge of the FMD defect. Self tapping titanium screws were inserted into the holes for an approximate depth of 2 - 3 mm. A skull plate was fashioned using titanium mesh and polymethylmethacrylate (PMMA), and fixed to the back of the skull, using titanium screw heads as anchor posts for the PMMA. The caudal aspect of the plate extended slightly over the dorsal defect of C1 and curved dorsally to avoid impinging on the medulla or cranial cervical spinal cord. There were no intra-operative complications.

Post operative complications included a mild head tilt in two dogs (both re-operated cases), and the need for narcotic pain medications for about 1 week in another dog (persistent neck pain). All but one of the dogs experienced clinical improvement following the procedure, although one has required a second decompression distal to the cranioplasty secondary to scar tissue formation. Mean follow-up time was 6 months (range 1 to 14 months).

Post-operatively, 1 dog continued to receive gabapentin as needed, 1 dog occasionally received carprofen, 1 dog was euthanized due to lack of improvement and 1 was euthanized due to recurrent pain around the head and neck. The latter dog also had severe FSOM and it was undetermined if the pain were due to COMS or the ear disease. One dog was re-operated because of connective tissue impingement of the cord at a site dorsal to the surgery.

Cranioplasty with titanium screws and titanium /PMMA plates appears to be well tolerated in dogs with COMMS. Although one dog required re-operation in this small case series, additional cases with more follow-up information will be needed to ascertain to what level, if any, the cranioplasty procedure mitigates compressive scar tissue compression at the FMD site.

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