Myelography is a contrast iodinated radiographic study used to highlight the spinal cord within the vertebral canal. In the preceding discussion it was noted that survey vertebral column radiographs do not allow visualization of the spinal cord or the meninges.
Myelography with the use of radiographs as described above allows visualization of the outline of the spinal cord.
The procedure is technical and should only be done by veterinarians experienced with the procedure. The procedure is carried out under anesthesia. A spinal needle is introduced into the spinal canal as for a spinal tap. For myelography of the entire spine usually lumbar puncture at L4-5 or L5-6 is performed (low back area). After sterile surgical preparation of the skin, the needle is inserted into the subarachnoid space. This space lies between the outer meninges (fibrous covering of the spinal cord) and the spinal cord itself. This space is filled with spinal fluid. The contrast agent is slowly and gently instilled with care until the entire spinal cord can be visualized into the high neck region.
Myelography for many years was the only way to further image the spinal cord. Although it does not allow for imaging the spinal cord itself, because the contrast agent surrounds and thus highlights the spinal cord, it can provide information on diseases causing compression from the outside the cord but within the canal (extra-dural), within the meninges (intra-dural, extramedullary) and also within the spinal cord (intramedullary).
Examples of extradural diseases include: intervertebral disc extrusion, tumors, cysts, infection, hemorrhage, compression from malformation, fracture or instability and other. Examples of intradural-extramedullary diseases include tumors or infection of the meninges/nerve roots, and cysts or dilations or adhesions of the meningeal elements. Intramedullary diseases include tumors of the spinal cord itself, ischemic stroke, hemorrhage, infection, inflammation, cavities (syrinx), traumatic high velocity intervertebral disc rupture and others.
Limitations of myelography are that it is an invasive procedure and requires a high level of expertise and experience. The images cannot be acquired in slices or multi-planar thus limiting the detail of the lesions imaged. Myelography does not image directly the spinal cord or lesion (focal disease abnormality) and thus further details may not be apparent. Although complications are rare they can occur and include chemical meningitis (rare with new agents), worsening of clinical signs (particularly in chronic or infectious/inflammatory diseases of the spinal cord or meninges (meningitis)), and seizures (brief for up to 12 hours but do not persist or become recurrent). Clinically significant direct trauma to the spinal cord, hemorrhage, or introduction of infection are rare complications. Absolute care should be taken to use the correct contrast agent. ONLY agents made to be used intrathecally should be used. Severe complications including death and paralysis can result due to use of other contrast agents.
Advantages are that myelography can be rapidly performed and requires only the same equipment as that for general radiography, except the correct contrast agent and spinal needle. For diseases such as intervertebral disc extrusion (rupture) it is very useful and is still used widely when such diseases are suspected.