Before the advent of MRI, the diagnosis of CLM and SM were commonly confused with that of intervertebral disk disease, allergic dermatologic disorders, epilepsy, and ear infections. Once Chiari malformation was reported in human patients, preliminary research provided the foundation to identify its analogue in the animal population.
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). The protocol for MRI scans performed at The Canine Chiari Institute at LIVS includes a T2W Sagittal sequence from the frontal sinus to the sacrum. Imaging of the brain consists of a T2W Sagittal, T2W Axial and a T1W Flair Axial sequence followed by a T2W Axial sequence of the cervical, thoracic and lumbar regions. Additionally, Phase Contrast Cine Sagittal sequences and Quantitative Flow Axial sequences are performed at the levels of the foramen magnum, C3 and C5. These sequences are then analyzed with ROI placement to determine flow velocity measurements in the assessment of CSF flow dynamics. “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 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. 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 vertebrae 1 (C1) subluxation (Fig 13), and dorsal compression between C1 and C2 (Fig 14). 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 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.
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. Additionally, the data obtained from spiral CT is often well suited for 3D imaging, which led to the rapid rise of helical CT as the most popular type of CT technology. The major benefit of helical CT scanning is the increased speed of volume coverage. Although encroachment on the cerebello-medullary region by portions of the occipital bone 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.
Radiographs 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 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. Lateral (side) views of the skull and neck are recommended in conjunction with MRI and CT to assess the extent of CLM in dogs.
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. These pathways may become adversely affected when compressed by the CLM disorder. There are some theories that dogs with CLM and SM have diminished hearing and this test may help in the screening process. 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 clinical/symptomatic dogs rather than asymptomatic dogs.
Medical Infrared Imaging or Thermography
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. Changes in the thermographic pattern in dogs being screened for CLM may help determine which dogs should be further evaluated with MRI. An ongoing study at The Canine Chiari Institute of LIVS was established to formulate a thermographic imaging protocol for dogs suspected of having CLM/SM, to identify thermal imaging patterns with normal dogs and those with CLM/SM, and to evaluate changes in thermal pattern and compare the results to those of brain auditory evoked response tests (BAER) and MRI findings, considered the standard 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.