CHAPTER 131 Diagnostic Imaging in Neurologic Disease
Diagnostic imaging of horses with neurologic disease can be both rewarding and challenging. Although they are very useful for horses with traumatic injury or congenital malformations, conventional radiographic studies lack specificity and sensitivity for detecting one of the more common equine neurologic diseases, cervical stenotic myelopathy (CSM). Computed tomography (CT) and magnetic resonance imaging (MRI) are advanced imaging methods that have become the standard of care in smaller veterinary patients and in human medicine, but their use in horses is limited because of patient size. Nuclear medicine is very sensitive in detecting areas of abnormal bone physiology, but the modality is nonspecific and rarely yields a definitive diagnosis. Ultrasound has limited applicability in the diagnosis of neurologic disease in horses, but it has been used to assist in cerebrospinal fluid (CSF) sampling and for injection of corticosteroids into the cervical vertebral articulations.
Determination of the anatomic location of the neurologic disorder impacts the choices of potential imaging modalities, as does equipment availability. Generally speaking, the skull and cervical portion of the vertebral column are the regions most amenable to diagnostic imaging in adult horses. In foals, it is often possible to image the thoracolumbar segment of the vertebral column and pelvis, whereas in adult horses evaluation of these areas is limited.
Localization of neurologic disease to the skull is typically straightforward because of the clinical presentation. Commonly encountered diseases include traumatic injury, manifestation of stylotympanohyoid disease, and occasionally, brain disorders such as abscess or neoplasia.
With the head lowered from the effects of sedation, standard radiographic projections should include dorsoventral (DV), lateral, and orthogonal oblique views, which are dorsal 45-degree lateral to ventrolateral oblique projections. Use of a grid enhances image detail, but grid use is not generally recommended in the field because of (1) the difficulty in aligning the grid with the x-ray beam, (2) the resultant grid cutoff artifacts, and (3) the threefold or more increase in milliamperes -second (mAs) that is necessary and that necessitates longer exposure time and increased chance of motion artifact.
The complexity of skull anatomy is visually overwhelming, even for specialists. Use of skull symmetry to compare the right and left sides is essential when viewing DV and oblique views. It must be noted that even subtle obliquity of the DV view and small differences in degrees of obliquity between oblique radiographs can yield a visual picture that mimics disease; careful attention to radiographic positioning is essential. The lateral radiograph is best for identification of fluid-air interfaces in the sinuses, and the oblique and DV views can be used to differentiate the laterality of the fluid.
In horses with known traumatic injury, the base of the skull must be carefully evaluated for integrity and alignment of the basisphenoid and basioccipital bones. The lateral radiographic view is the most informative in assessment of ventral margin and symphyseal alignment. Because of the normal radiolucent symphysis between these two bones, care must be taken to avoid overinterpreting a fracture at this location. Additional clues suggesting traumatic injury are bony fragments and abnormal soft tissue opacity overlying the guttural pouches, base of the skull, and proximal cervical spine, which are representative of muscle avulsion and hemorrhage.
Diagnosis of cranial fractures can be challenging because of the presence of multiple thin, linear radiolucencies representing the sutures between multiple flat bones in the skull of immature horses. Depression of the bones of the cranium is a diagnostic finding for fracture, and the presence of subcranial gas indicates an open wound.
Stylotympanohyoid disease may manifest as a head tilt, facial-nerve paralysis or paresis, or both. The stylohyoid bones and their articulation with the temporal bone are best assessed on a perfectly straight DV view and on oblique views. In most instances, preexisting and chronic disease have manifested in bony proliferation of the stylohyoid bone or bones at or near the articulation with the tympanic bulla; if severe, bony proliferation can extend distally along the shaft of the stylohyoid bone. Fracture of the stylohyoid bone can occur. These changes are usually evident on the radiographs. In more subtle cases, slight asymmetry in the opacity of the petrous temporal bone and tympanic bullae may be the only radiographic abnormality. Mild radiographic obliquity can mimic disease by yielding an asymmetric appearance of the stylohyoid bones, so confirmation of suspicious lesions should be made or refuted by obtaining two orthogonal oblique radiographs. In this circumstance, oblique views may be made as previously noted, which results in the articulations projecting above and below each other, or rostral 25-degree lateral to caudolateral oblique images can be made that project the stylohyoid bones rostral and caudal to one another. Nuclear medicine can be used to identify active bone metabolism, a so-called hot spot, in horses with stylotympanohyoid disease.
Computed tomography of the equine skull is a very useful complement to skull radiography. The cross-sectional nature of CT images makes interpretation relatively easy because there are no interfering overlying structures as with radiographs. CT typically reveals radiographically identified lesions to be more extensive or reveals pathologic changes not seen on radiographs. CT is very useful for assessment of stylotympanohyoid disease and in evaluation of the middle ear. CT has also been used in the diagnosis of rarer conditions such as brain abscesses in foals, brain tumors, and nasal tumors that have invaded the cribriform plate. Relatively common incidental findings on CT examination in older horses are choroid plexus cholesteromas and pituitary adenomas. CT examination of horses with seizures has been unrewarding in my experience.
Magnetic Resonance Imaging
Magnetic resonance imaging is superior to CT in the differentiation of brain parenchymal lesions and can reveal those not visible on a CT examination. Unfortunately, the availability of MRI is limited. For suspected bony lesions such as those induced by trauma, assessment with CT is preferred. MRI will undoubtedly play a more important role in the diagnosis of equine neurologic disease in the future.
CERVICAL PORTION OF THE VERTEBRAL COLUMN
In practice, careful assessment of survey radiographic findings is important in the horse with neurologic signs and aids the examiner in deciding whether to proceed with myelography. The underwriter of insured horses may also dictate this decision. Nuclear medicine has been used to locate sites of abnormal cervical vertebral bony physiology, a finding that is usually referable to degenerative osteoproliferative disease of the articular processes.
Spinal radiography and myelography are the mainstays of equine cervical spinal imaging because they are widely available and the entire cervical portion of the vertebral column can be imaged. Although greater spatial and contrast resolution are available with CT and MRI, those imaging modalities are limited not only in their availability but also in their utility for evaluating the caudal cervical vertebrae because of patient size.
Survey radiographs to screen the cervical vertebrae are routinely taken in standing horses, and this has several advantages over radiographs taken with the horse anesthetized. First, it is often easier to obtain true lateral radiographs because spinal alignment relative to the x-ray beam and cassette or digital capture device is easier to achieve than in the recumbent horse. Second, the need for general anesthesia and associated risks is avoided. If myelography is performed later, anesthetic time is greatly reduced if survey films are obtained beforehand. Radiography in standing horses requires a high-capacity x-ray generator to produce quality radiographs without patient motion artifact, so general anesthesia may be necessary if a lower-output portable x-ray unit is used.