Imaging of the Neurological Case

What are the indications?

Trauma is the most commonly reported cause of neurological disease in the horse. This is usually spinal cord rather than brain injury [1,2]. Other causes include degenerative, developmental and inflammatory.

Presentation may include: ataxia; spasticity; weakness; paresis; cranial nerve deficits; fracture; abnormal behaviour, posture or mental status; or seizures.

Cervical vertebral malformation (CVM) is the most common cause of ataxia in horses. Type 1 (developmental) and type 2 (degenerative) CVM both commonly occur and can cause dynamic or static spinal cord compression. In these cases, positive contrast computed tomography (CT) myelography is considered gold standard for antemortem diagnosis and is required prior to surgical treatment of disease.

A thorough history, including concurrent musculoskeletal disease, is required in all cases as CVM is associated with developmental orthopaedic disease such as osteochondrosis.

Prior to imaging, all horses must undergo a thorough neurological examination, including both resting and dynamic examinations [1-4], the goals of which are to determine if there is a neurological problem present and to determine neuroanatomical localisation of the lesion or lesions [5].

Imaging modalities

While radiography can identify gross osseous abnormalities such as fracture, osteomyelitis, arthropathy and malformation, it is not possible to assess the associated soft tissue structures with accuracy. CT and magnetic resonance imaging (MRI) both provide significantly more anatomical detail.

MRI has a much greater range of soft tissue contrast, depicts soft tissue anatomy in greater detail, and is more sensitive and specific for abnormalities within the brain itself compared with CT imaging [6]. MRI of the equine head requires general anaesthesia in most settings. MRI of the cervical spine in the live adult horse is still not achievable due to limitations in width of magnet design, but may be possible in foals and smaller patients.

CT is more widely available and cost-effective than MRI, requires shorter imaging time and is easier to perform. In humans, CT is the initial imaging modality of choice during the first 24 hours after brain injury [7,8]. CT is also superior in evaluating bones and detecting acute subarachnoid or acute parenchymal haemorrhage in human patients [9].

CT is now commonly used in order to assess disease within the head and neck in the horse. Most equine CT machines are adapted from those designed for humans and will image 70 cm+ length of patient in a short period of time. A standing sedated horse can undergo CT imaging of the head and cranial cervical spine. Contrast studies can be used to enhance soft tissue detail.

In order to image the entire cervical spine most CT designs will require the horse to undergo general anaesthesia. Both plain and positive contrast myelogram studies would then be performed in the neurological case. Imaging of the vertebral column caudal to the thorax is only possible in smaller patients due to design width limitations in most set-ups.

CT myelogram

It may be that gross abnormality is evident and compression of the spinal cord is without doubt. However, commonly in cases of low-grade ataxia, often with suspected CVM, this is not the case and objective measurements of the spinal canal are required in order to predict regions of spinal cord compression. Measurements including dural diameter, dorsal myelographic dye column width and cord area to subarachnoid space area ratios may be used [10,11].

Most of these measurements assess spinal canal width in the dorsoventral plane. Typically, in cases of cervical arthropathy, lateral–lateral compression may be present due to articular process joint enlargement. In addition, distension of the cervical articular process joints can also cause compression. A distinct advantage of 3D CT imaging versus 2D imaging is the ability to accurately determine the presence of spinal cord compression in any plane.

Outcome

Following a diagnosis then the appropriate treatment plan can be established. This may be conservative management, medical therapy or a surgical procedure [12,13].

[...]