CT Myelography – Indications, Expectations and Complications

The recent adaptation of computed tomography (CT) scanners to accommodate the entire cervical region of anaesthetised adult horses has significantly advanced imaging of the equine neck. CT myelography has become the gold standard method of assessing cervical pathology and spinal cord compression in horses, given that MRI is not yet feasible. It offers significant advantages over radiographic myelography in that it allows circumferential assessment of the myelographic dye column, rather than being limited to assessment in the sagittal plane. CT image reconstruction techniques can be applied to improve soft tissue detail, enabling delineation of soft tissue masses within the spinal canal, intervertebral disc protrusion and the joint capsule outline of articular process joints. At Liphook Equine Hospital, images are obtained to the C7/T1 articulation in the majority of cases, with imaging to T2 in smaller animals [1]. It should be noted that the neck is imaged in a neutral or slightly extended position. The neck cannot be flexed within the bore of the scanner and supplemental radiographs are required to achieve flexed views.

The decision to perform CT myelography must be made after consideration of the potential benefits of the procedure vs. the risks. It is clearly indicated in horses in which a cervical stabilisation procedure is being considered. It might be indicated in ataxic horses in which cervical stabilisation is not feasible, and it is being performed in some clinics in nonataxic horses with other signs of cervical dysfunction [2]. The pros and cons of these approaches will be discussed.

To date, parameters to guide the interpretation of CT myelography have not been described and CT comparison with post-mortem findings has not been reported. Interpretation relies on clinical experience, interpretation from first principles and application of criteria used for radiographic myelography. Rovel et al. (2021) provided the first quantitative data from CT myelography following imaging of 26 ataxic Warmblood horses [3]. The site(s) of extradural compression was identified in 22/26 horses, with a cause of ataxia unclear in the remaining 4 cases. Dorsolateral or lateral compression were identified in 50% of cases, which might not have been identified with radiographic myelography. Further work is required in this field.

A range of complications have been reported associated with CT myelography [1,2,4,5] and these have similarly been reported with radiographic myelography [6]. These are primarily related to recovery from general anaesthesia, exacerbation of pre-existing clinical signs, and complications associated with the myelography procedure. Anaesthetising an ataxic horse might increase risks related to the recovery phase. Exacerbation of the pre-existing cause of cervical dysfunction might occur as the horse is moved and the neck manipulated under conditions of decreased muscular tone during anaesthesia and imaging. Seizure activity can occur secondary to myelography and, in addition, some horses can display depressed mentation for several days thereafter. Ourselves and others have also experienced equipment malfunction, resulting in inability to complete a scan protocol following induction of anaesthesia.

At the time of writing, CT imaging of the neck has been performed under general anaesthesia in 107 equines at Liphook Equine Hospital, including myelography in 68 cases. Relevant features of these cases, findings and complications will be discussed, along with risk mitigation techniques.

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