MRI in animals is safe, however, general anesthesia is required to ensure that the patient remains motionless during the scan. At large veterinary MRI centers, patients are monitored for heart rate and rhythm, respiratory rate, body temperature, and blood pressure with specialized “MRI compatible” equipment. Scan time varies with the type of MRI used. Large magnet scanners (3 Tesla) can scan patients in 20 minutes while
smaller magnets or older machines can have scan times exceeding 1 hour. Each scan includes 2-3 views of the head and neck (axial, sagittal and sometimes dorsal images), and should include scans of the entire spine (cervical, thoracic, and lumbar) to be considered a complete study (Fig 9). “Screening” MRI studies are abbreviated studies designed to save on costs and anesthetic time. Screening studies are of the back of the brain and upper neck only and so the true extent of the syringomyelia (i.e. how much of the spinal cord is involved) will not be ascertained. More importantly other diseases which could explain the clinical signs may be missed, e.g. a herniated intervertebral disc. However low cost screening studies are available for breeders because although it will not reveal the extent of the disease it is a sensitive and specific test for syringomyelia as it is rare to have a syrinx related to CLM in the lower spinal cord without also having one in the upper cervical cord. Since syringomyelia can occur anywhere in the spine, full spinal studies are recommended when possible.
Once the scans are finished, they can be viewed and adjusted on a computer for evaluation. Typical findings on a MRI are: cerebellar herniation and compression, kinking of the medulla, attenuation of CSF, ventriculomegaly/hydrocephalus, and hydromyelia/syringomyelia (Fig 10). The essence of the CLM condition is an indentation of the cerebellum called cerebellar compression, which causes a secondary herniation of the cerebellum beneath the bone at the back of the skull (occipital bone) onto the spinal cord. The medulla is part of the brainstem, which resides beneath the cerebellum. The malformation can cause crowding at the back of the skull, which can cause the kinking of the medulla that is sometimes noted.
Ventriculomegaly (Fig 11) is a more general term for hydrocephalus, which is an excessive accumulation of fluid in the ventricles of the brain. Some newer findings associated with CLM are so called primary secretory otitis media (PSOM)
(Fig 12), cervical vertebra 1 (C1) subluxation (Fig 13), and dorsal compression between C1 and C2 (Fig 14). Cervical vertebra 1 subluxation is a newer finding that is also described as cranial settling, basilar invagination, or basilar impression in human medicine. This condition happens when the first cervical vertebra moves forward under the skull. Another more recent finding, C1 and C2 dorsal compression, is found when exuberant tissue is found between the two vertebrae and compress the cord in that region.
Several recent reports have drawn attention to dogs without abnormal skull shape or reduced size in the caudal occipital region also exhibiting syrinx formation. Abnormal or absent frontal sinuses (Fig 15) have been reported to be associated with syrinx formation as well. Syrinx formation in the cervical region is well documented in both human and veterinary patients since the cervical region is easily incorporated in the cervicomedullary MRI study used clinically; however, little is known about the incidence in other spinal regions. Because of this observation, the entire central nervous system is now imaged when patients are evaluated as part of the CLM screening program at The Canine Chiari Institute at LIVS. Nineteen dogs had complete MRI studies from the brain to the sacral spinal cord. Of the 19 dogs, 19 (100%) had cervical, 9 (47.4%) had cervical / thoracic, 5 (26.3%) had cervical / thoracic / lumbar syrinx formation. These preliminary results suggest we need to look further to gain a better understanding of this complex disease process.
A new concept in MRI captures spinal fluid flow as a mini movie known as cine MRI or “CSF flow study”. This diagnostic capability enhances visualization of the way CSF flows from the brain to the spinal cord. Studies in humans indicate clinical signs are related to the amount of decreased CSF flow and not the amount of cerebellar herniation. Cine MRI has been able to document the abnormal fluid flow, and recheck MRIs after surgical decompression of the region reveal restoration of CSF flow. Preliminary studies in animals indicate that there are abnormalities in CSF flow in dogs that have CLM and SM. Currently there are studies using cine MRI to analyze dogs with CLM and SM to assess CSF fluid flow post-surgical decompression.