|Fig 9A||Fig 9B|
MRI in animals is safe, however, light 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 with considerably less image resolution. Each scan includes 2-3 views of the head and neck and should include scans of the entire spine to be considered a complete study (Fig 9). The protocol for MRI scans performed at The Canine Chiari Institute at LIVS includes the assessment of CSF flow dynamics. “Screening” MRI studies are abbreviated studies designed to save on costs and anesthetic time. Screening studies encompass the back of the brain and upper neck only so the true extent of syringomyelia (i.e. how much of the spinal cord is involved) will not be ascertained.
|Fig 10||Fig 11||Fig 12|
More importantly, other diseases which could explain the clinical signs may be missed, e.g. a herniated disc however low cost screening studies are available to 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.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 or “water on the brain”, 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) or “glue ear”, cervical vertebra 1 (C1) subluxation or “cranial settling” (Fig 13), and dorsal compression between C1 and C2 (Fig 14).
|Fig 13A||Fig 13B|
Cervical vertebrae 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 vertebrae 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 compresses the cord in that region. Several recent reports have drawn attention to dogs without abnormal skull shape or reduced size in the back of the skull 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 neck region is well documented in both human and veterinary patients since the neck region is easily incorporated in the typical 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.
|Fig 15A||Fig 15B|
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.
CT/Spiral or Helical CT/3D Reconstruction
Computed tomography (CT) has also been used with MRI to diagnose CLM. In helical or spiral CT, the table moves the patient smoothly through the scanner with the X-ray source attached to a freely rotating gantry. The major advantage of helical scanning compared to the traditional CT scanning is speed; a large volume can be scanned quickly. Although brain compression in the back portion of the skull has been the focus of many studies, evaluation of the entire skull shape and size utilizing helical CT technology with 3D reconstruction is currently underway to identify additional mechanisms of syrinx formation. Patients with “normal” skull volumes are thought to have a loss of integrity of the brain’s suspensory appartus allowing for cerebellar slouching into the foramen magnum. This advanced imaging technique will be able to identify volume or conformational abnormalities in affected patients.
|Fig 16||Fig 17|
Radiographs (X-Rays) are two dimensional assessments of the body. In the diagnosis of CLM, radiographs can be used to assess the shape of the skull and the alignment of the vertebrae of the neck (Fig 16). Overlapping of the skull and the neck as seen with cranial settling (Fig 17), decreased size of the vertebrae, luxation of the first two vertebrae, and decreased size of the frontal sinus have been documented in dogs with CLM.
Brain Auditory Evoked Response (BAER)
Brain auditory evoked response (BAER) screens dogs for the ability to hear. In dogs with intact hearing, BAER can evaluate the integrity of brain-stem pathways which may become adversely affected when compressed by the CLM disorder. In a preliminary study of 38 CKCS, only one dog had a normal MRI and BAER. Twenty-three dogs without clinical signs had abnormal MRI findings with 16 of the 23 dogs (69.6%) also having abnormalities with BAER testing. Fourteen dogs with clinical signs had abnormal MRI findings, and 13 of the 14 dogs (92.8%) also had abnormal BAER tests. Therefore, BAER testing may play a more useful role in screening dogs with signs rather than dogs that are not yet showing signs of CLM.
Medical Infrared Imaging or Thermography
|Fig 18A||Fig 18B||Fig 18C||Fig 18D|
Another non-invasive imaging modality that may prove to be useful in screening dogs for CLM and SM is medical infrared imaging (digital infrared thermographic imaging or thermography). The infrared camera detects minute changes in surface skin temperature that are related to the cutaneous blood flow under the control of the autonomic nervous system. Medical infrared imaging (MII) requires no sedation or clipping of hair for a dog to be imaged by the infrared camera. Once the image is captured, computer software can be used to assess changes in the temperature and color pattern to detect an abnormality.An ongoing study at The Canine Chiari Institute of LIVS was established to formulate a thermographic imaging pattern for CLM in dogs. The complete analysis of thermal patterns is on-going, however, preliminary results revealed “cooler” thermographic patterns in dogs with abnormal MRI findings compared to the dog with a normal MRI (Fig 18). Magnetic resonance imaging findings classified as mild, moderate and severe CLM correlated with thermographic findings, 0%, 50%, and 100% of the time respectively. Based on these very preliminary findings, thermography may be a viable imaging modality to use as a screening tool to detect CLM in dogs.