Diagnosis

A presumptive diagnosis of CVSM can often be made on the basis of history, signalment, and clinical signs and with the help of lateral radiographs of the cervical vertebral column. Even though identification of specific bony malformations (e.g., vertebral subluxation or osteoarthritis of articular processes) is suggestive of CVSM, their presence does not allow reliable discrimination between horses with and without the disease. An objective evaluation of the vertebral canal diameter by means of intravertebral and intervertebral sagittal ratio measurements is considered to be more accurate. Both techniques provide valuable information about the presence of vertebral canal stenosis, but neither method reliably identifies the actual site of spinal cord compression. Myelography is the standard imaging modality used to localize the compression site and to determine where vertebral interbody fusion should be performed. However, histologic evaluation of spinal cord samples from horses that underwent myelography revealed that this radiographic examination technique is frequently inaccurate, especially in the midcervical region. Similar limitations have been identified for contrast-enhanced computed tomography (CT) and, most recently, for magnetic resonance imaging (MRI). This dilemma leaves the equine veterinarian without a diagnostic tool for accurately localizing the site of spinal cord compression in horses with CVSM.

Vertebral Canal Endoscopy in Humans

The first peer-reviewed article on vertebral canal endoscopy was published in 1931 and described the examination of human cadaver spines with an arthroscope. Throughout the following decades, the procedure gained popularity as a diagnostic aid in patients with lower back pain. It allowed direct viewing of a variety of pathologic changes within the vertebral canal, including neuritis and stenosis of and tumors within the vertebral canal. More recently, CT and MRI have become the preferred diagnostic tools for conditions associated with the spinal cord in humans. However, in cases in which these techniques fail to provide a conclusive diagnosis, vertebral canal endoscopy can be used to provide additional diagnostic information and even facilitate treatment. Endoscopy of the epidural space, for example, is used to identify inflamed nerve roots in patients with chronic back pain and to treat these nerve roots with targeted corticosteroid injections at the same time. Subarachnoid endoscopy allows further evaluation and biopsy of subarachnoid masses and surgical removal of arachnoid cysts.

Anatomy

The cervical vertebral column of the horse consists of seven vertebrae. Together, the vertebral foramina of C1-C7 form the cervical vertebral canal that encloses and protects the spinal cord, its meninges, spinal nerves, blood vessels, fat, and connective tissue. Within the equine vertebral canal are two spaces that can be evaluated endoscopically: the epidural space and the subarachnoid space. The epidural space is located between the dura mater and the surrounding vertebrae. It contains mostly fat and lymphatic and blood vessels (including the ventral internal vertebral venous plexus). Additionally, the spinal nerve roots that emerge from the spinal cord pass through the epidural space to leave the vertebral canal through the intervertebral foramina. The subarachnoid or intrathecal space is the interval between the arachnoid mater and the pia mater. It contains the clear and colorless cerebrospinal fluid (CSF) that surrounds the brain and the spinal cord. Anatomic structures within the spinal subarachnoid space include the spinal cord, dorsal and ventral spinal nerve roots, blood vessels, trabeculations that connect the arachnoid mater to the pia mater, den­ticulate ligaments, and external branch of the accessory nerve.

Separation of the arachnoid mater from the dura mater, two meninges that are intimately fused in healthy horses, creates the subdural space. This space only forms in cadavers or secondary to pathologic processes and is therefore called an artificial or potential space.

Terminology

Cervical vertebral canal endoscopy (CVCE) refers to endoscopic examination of the cervical vertebral canal. It includes endoscopy of the subarachnoid space, called myeloscopy, and endoscopic examination of the epidural space, called epiduroscopy.

Similar to other surgical procedures, preparation for horses undergoing CVCE includes a thorough clinical examination, appropriate laboratory tests, administration of nonsteroidal antiinflammatory and antimicrobial drugs, and the withholding of feed for at least 6 hours. Additionally, the findings of a standardized neurologic examination should be documented. Chloramphenicol¹ (50 mg/kg, PO, every 6 hours) penetrates well into the central nervous system (CNS) and reaches high concentrations in the CSF. It is the recommended antimicrobial drug for this procedure, especially when the subarachnoid space is examined.