Myelography is an imaging tool that involves injection of a radiopaque contrast agent into the subarachnoid space to “highlight” the edges of the spinal cord on radiographs, thus allowing the clinician to indirectly evaluate the spinal cord. Use of myelography has declined over the years due to increased availability of magnetic resonance (MR) and computed tomography (CT) imaging, but it is still the primary imaging modality for compressive spinal cord disease in many areas of the world. Myelography is often performed in conjunction with CT (so called CT-myelogram) to provide additional information.
The patient is placed under general anesthesia and survey spinal radiographs are obtained. The myelogram can be performed by injection via a cisternal or lumbar tap. Lumbar administration of contrast agent is safer for the patient, but more technically challenging. Ideally, CSF should be collected prior to administration of contrast agent as it can induce a mild inflammatory response that can last for 3-5 days. An iodinated, water-soluble, nonionic contrast agent (iohexol or iopamidol) is then injected into the subarachnoid space. The injection can be performed under fluoroscopy, if available, or a radiograph can be taken after injection of a small volume to be sure that contrast agent is in the subarachnoid space rather than the epidural space. A total volume of 0.3-0.45 mL/kg is given slowly over a couple of minutes. Radiographs are taken again and the contrast agent appears as a radiopaque line outlining the spinal cord.
There are several advantages to myelography, including:
- they are “easy” to perform and do not require specialized equipment
- information can be obtained about the entire spinal cord rapidly, and
- they are significantly less expensive than MR and CT imaging.
There are multiple disadvantages, including:
- they can be frustrating to perform at the lumbar space (easy to go epidural instead of subarachnoid),
- they are somewhat invasive,
- they provide no specific information about the spinal cord parenchyma,
- transient worsening occasionally occurs after myelography (especially patients with meningitis or Wobbler Syndrome),
- the contrast agent can induce seizures for up to 24 hours after the test, and
- in rare cases, cardiopulmonary arrest can occur.
Seizures are the most common adverse effect with myelography, occurring in 21% of dogs in one large study, more often in larger breed dogs (34.2% in dogs > 20 kg compared to 11.6% in small breed dogs). Fortunately, the seizures almost always respond to intravenous diazepam and do not persist after 24 hours once the body has cleared the contrast agent from the subarachnoid space.
There are four myelographic patterns that may be identified.
- Normal: There is continuous visibility of contrast agent throughout the length of the spinal cord. Note that there is normally enlargement of the spinal cord at the cervical (C6-T2) and lumbar (L4-S1) intumescences due to the increased number of lower motor neuron cells giving rise to the peripheral nerves of the limbs. Be careful not to misinterpret these regions as pathological spinal cord swelling.
- Extradural: The extradural pattern (fig. 1) occurs secondary to spinal cord compression outside the meninges, such as an intervertebral disk extrusion or vertebral neoplasm. Extradural pattern is characterized by thinning and/or deviation of the contrast column.
- Intradural/extramedullary: This myelographic pattern (fig. 2) occurs when there is a mass or other tissue within the meninges, but outside the spinal cord parenchyma, such as a meningioma or malignant peripheral nerve sheath tumor. The classic appearance is a contrast void at the lesion with a “golf tee” appearance of the contrast column on either side and thinning of the contralateral contrast column.
- Intramedullary: The final pattern (fig. 3) occurs when there is disease within the spinal cord parenchyma leading to enlargement of the spinal cord, such as spinal cord edema due to Fibrocartilaginous Embolism (FCE) or an expansile intramedullary mass. This spinal cord enlargement causes thinning of both contrast columns at the site of swelling (disease).