What is chiari-like malformation (also known as COMS)?

Chiari-like Malformation (CLM), formerly known as Caudal Occipital Malformation Syndrome (COMS) is a condition in which the rearmost  part of the brain, the cerebellum, descends out of the skull through the opening at its base, called the foramen magnum, crowding the spinal cord.

The exact cause is not completely understood, however it is believed that due to an abnormal skull shape or reduced skull size, part of the cerebellum is forced through the foramen magnum, altering the normal cerebral spinal fluid (CSF) flow in that region. (Fig. 1) Changes in CSF dynamics and pressure gradients caused by this syndrome (Syringomyelia) may result in abnormal accumulations of fluid within the substance of the cervical spinal cord called “syrinxes”. There is recent evidence to support the concept that CLM represents an overall mismatch between intracranial volume and the tissue occupying that volume (i.e. the skull is too small for the brain and vice versa) rather than a disorder confined to the back of the skull. Chiari-like malformation is considered to be a developmental abnormality resulting in a variety of clinical signs in affected patients and is commonly confused with many other conditions. It affects predominately small and toy breed dogs, in particular the Cavalier King Charles Spaniels (CKCS) and the Griffon Bruxellois, although it has recently been found in several feline patients. Based on the results of intensive studies performed at Stone Lion Veterinary Hospital in London in the CKCS breed, Chiari-like Malformation and Syringomyelia (the presence of one or more syrinxes) have a high heritability. Although it appears that CLM affects predominately CKCS dogs, this may be misleading since active breeder support has resulted in intensive scrutiny of the CKCS breed, well beyond the assessment other small breed dogs. With increased awareness in the veterinary community in recent years, almost 40% of dogs operated at The Canine Chiari Institute at Long Island Veterinary Specialists (LIVS) for CLM are not CKCS dogs.   

What is fibrous band compression and dural fibrosis?

The dura mater refers to the outer thick fibrous envelope normally covering the brain and spinal cord in people and pets.  In certain dogs with CLM, excessive thickening of the dura mater and or the associated ligament found at the junction of the skull and vertebra of the neck can result in significant compression (Fig 3).

This soft tissue compression can be found in addition to the osseous compression and can alter CSF flow resulting in pressure gradients and subsequent syringomyelia.

It is for this reason many believe both an osseous and soft tissue decompression must be performed in patients with a fibrous band.

What causes Chiari-like malformation?

Chiari-like Malformation in dogs is a condition in which the skull is malformed and does not fully accommodate the total brain volume. The most obvious abnormality seen on MR imaging in most patients is found in the area of the back of the skull, however, there is increasing evidence that the entire skull may be abnormal in these patients. Causes of CLM are not yet fully understood. One theory is that the miniaturization process in some toy breed dogs went awry and the brain did not decrease in size in proportion to the skull and/or the skull is not big enough for the brain. Studies have suggested that the CKCS appears to have a brain more appropriate for a bigger dog. Other work performed by Dr. Rusbridge and others in the Griffon Bruxellois have suggested that CLM in this breed is characterized by a short skull base, however the condition does not appear to be confined to a simple reduction in skull volume.  Other possibilities, including a loss of the collagenous suspensory mechanism of the cerebellum, may play a role in cerebellar herniation.

A reference guide for veterinary professionals interested in Chiari -Like Malformation and Syringomyelia.