Caudal Occipital Malformation syndrome (COMS)

Caudal occipital malformation syndrome (COMS) is a neurological disorder seen most commonly in the Cavalier King Charles spaniel breed. Other terms for COMS include occipital hypoplasia and Chiari-like malformation.  Small other small breed can also affected.

COMS refers to malformation of the occipital bone causing crowding of the caudal fossa leading to cerebellar herniation, medullary “kinking”, obstruction of the dorsal craniocervical subarachnoid space, and alteration of cerebrospinal fluid flow. This alteration in CSF flow leads to syringomyelia (SM), which are fluid filled cavities within the spinal cord. The most prominent clinical sign of syringomyelia is pain. Pain is most commonly noted in the cervical region, but in some cases may be diffuse through the spine. Affected dogs may be sensitive to touch on one side of the head, neck or shoulder. Additionally, affected dogs tend to scratch frequently on one side of the head, neck or body often without making contact. Pain and scratching are thought to be due to SM affecting the dorsal horn of the spinal cord which is a relay center for sensory information transmission to the brain. Other neurological signs include thoracic or pelvic limb weakness and seizures.

The diagnosis of COMS and SM requires an MRI of the head and neck. Additional studies may be necessary based on localization of the neurological exam. Concurrent vertebral malformations may also exist.

Treatment involves medical and surgical management. Surgical management is indicated when analgesics do not control pain or when significant neurological deficits are present. Medical management may be chosen for patients with mild pain or when finances do not allow surgical management. Medical therapy is aimed at decreasing spinal fluid production and alleviating pain. In mild cases, a non-steroidal anti-inflammatory drug (NSAID) combined with gabapentin may be used. Omeprazole can be used short-term to decrease CSF production. Corticosteroids are also effective in decreasing pain and neurologic deficits by inhibiting pain mediators such as substance P and decreasing CSF production. While corticosteroids may be effective, dogs would require continuous therapy and develop adverse effects such as immunosuppression, weight gain, and skin changes. For these cases, surgical management is indicated. Surgery with a foramen magnum decompression has an 80% success rate. Some clinicians will apple a titanium mesh over the defect to reduce scar tissue formation to prevent recurrence of the clinical signs.


The diagnosis is made by visualizing the defect with MRI and also ruling-out other common diseases that can cause the same clinical signs. Because this defect is commonly identified and is not always the cause of significant problems, a spinal tap is often performed to address the possibility of another common disease in small breed dogs called meningoencephalitis.

Medical management of this condition is often with pain modulators like gabapentin (Neurontin) or pregabalin (Lyrica) for neuropathic pain, and other pain medication (opiates, NSAIDS or steroids) and medication to try to decrease fluid production (steroid, omeprazole (Prilosec), furosemide (Lasix)).

Surgery for this condition is called a foramen magnum decompression (FMD) and has the goal of alleviating the pinching of the spinal cord, smoothing out the flow of spinal fluid, and therefore eliminating the impetus for neuropathic pain and weakness. A FMD has a success rate of about 80% to improve the pain, weakness and vertigo. Many variables exist to explain why some dogs do better than others. Long term, there is a reported recurrence rate as high as 25% following surgery.