Canine Chiari Institute at Long Island Veterinary Specialists

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Definition

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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 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.

 

Fig 1A
Fig 1B

It is the most common cause of foramen magnum (FM) obstruction and syringomyelia (SM) in the dog. The exact cause is not completely understood, however it is believed that due to an abnormal skull shape or reduced skull size in the caudal occipital region, 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”. Abnormal or absent frontal sinuses have been reported to be associated with syrinx formation as well.  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 have a high heritability and statistical analysis suggests that it is epistatic (i.e. genes at two or more loci interact to produce disease).

Fig 3A Fig 3B
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. Chiari-like malformation results in a variety of signs in affected patients as reported by their owners 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 cats. Based on the results of intensive studies performed at Stone Lion Veterinary Hospital in London, in the CKCS breed, Chiari-like malformation and syringomyelia have a high heritability.

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.

Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 What is Chiari-like malformation (also known as COMS)?   Chiari-like malformation (CLM), formerly known as Caudal Occipital Malformation Syndrome (COMS) and occipital bone hypoplasia, is a condition in which 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. It is the most common cause of foramen magnum (FM) obstruction and syringomyelia (SM) in the dog. The exact cause is not completely understood; however, it is believed that due to an abnormal skull shape or reduced skull size in the caudal occipital region, 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 may result in an abnormal accumulation of fluid within the substance of the cervical spinal cord called SM or a “syrinx”. Abnormal or absent frontal sinuses have been reported to be associated with syrinx formation as well.  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) 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 have a high heritability and statistical analysis suggests that it is epistatic (i.e. genes at two or more loci interact to produce disease).

Although it had been discussed 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 ligamentum flavum at the junction of the skull and cervical vertebra 1 (atlantoccipital) and or cervical vertebrae 1&2 (atlantoaxial) can result in significant compression (Fig 3). This soft tissue compression can be found in addition to the osseous compression and can alter CSF dynamics 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 intracranial contents (brain, blood, CSF). The most obvious abnormality seen on MR imaging in most patients is in the caudal region 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 with 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.  This has led to the hypothesis that the condition may be due to insufficiency of the bone and/or craniosynostosis of the lambdoid (occipitoparietal) and cranial base sutures.  However the condition does not appear to be confined to a simple reduction in skull volume.  Investigations comparing skull volumes and brain volumes between SM affected and unaffected dogs have found that caudal fossa volume is significantly smaller for CKCS with early onset SM (less than 2 years old) compared to CKCS that are MRI clear of SM at 5 years or greater.  However in addition the parenchyma (i.e. brain) within the caudal fossa is significantly greater for CKCS with SM especially in CKCS with early onset SM (Driver and others 2010).  Driver and others (2010) also demonstrated that increased parenchyma within the caudal fossa was positively correlated with syrinx size and that ventricle dimensions are positively correlated with syrinx dimensions i. E. dogs with SM tended to have ventriculomegaly and dogs with wide syrinxes tended to have greater ventriculomegaly. This shortening results in a compensatory increase in the size of some of the other skull bones, meaning that the forebrain is adequately accommodated. However, there is no compensatory increase in size of the back of the skull, meaning that there is not enough room for the cerebellum and brain stem. Other possibilities, including a loss of the collagenous suspensory mechanism of the cerebellum, may play a role in the cerebellar herniation.

 

Last Updated ( Thursday, 21 October 2010 13:24 )  

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

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