Intervertebral disc disease is one of the most common disorders in dogs (but rare in cats), and symptomatic disease can arise at any site where there is an intervertebral disc, but there are specific predilection sites. To perform proper treatment, especially surgical intervention, localising the lesion is important. The fundamental method to localise the spinal cord lesion including intervertebral disc disease is to localise the lesion into one of following spinal cord segments: cervical (C1–5), cervicothoracic (C6–T2), thoracolumbar (T3–L3) and lumbosacral (L4–S3). Using the information associated with clinical signs, postural reactions and LMN (lower motor neuron) and UMN (upper motor neuron) signs of the four limbs obtained by neurological examination, the examiner can localise the lesion.

Spinal Cord Dysfunction and Neurological Signs

Cervical Spinal Cord (C1–5)

A lesion in this spinal cord segment can cause hemiparesis, hemiplegia, tetraparesis or tetraplegia. However, some cases only show neck pain without proprioceptive or gait deficits. Clinical signs are typically ipsilateral to the lesion. Patients with lesions in this area may show decreased or absent postural reactions in four limbs. A C1–5 lesion might cause UMN signs to both the thoracic limbs and pelvic limbs but is not always apparent.

Cervicothoracic Spinal Cord (C6–T2)

Similar to C1–5 lesion, the patients with C6–T2 spinal cord lesion might show similar gait abnormalities with decreased or absent postural reactions on neurological examination. The absolute difference of neurological disorders between cervical and cervicothoracic lesions is the reflexes in the four limbs. A C6–T2 lesion typically causes LMN signs to the thoracic limbs (if the lesion involves the grey matter or nerve roots) and UMN signs to the pelvic limbs.

Thoracolumbar Spinal Cord (T3–L3)

Clinically the patients with this lesion typically show symmetric/asymmetric paraparesis or paraplegia. In neurological examination, the thoracic limbs have normal postural reactions and spinal reflexes, and the pelvic limbs have decreased or absent postural reaction with UMN signs of spinal reflex.

Lumbosacral Spinal Cord (L4–S3)

Lesions in this spinal cord segment cause signs of LMN dysfunction in pelvic limbs and/or bladder function with symmetric/asymmetric paraparesis or paraplegia. L4–6 lesions cause signs of LMN dysfunction in the pelvic limbs with decreased to absent patellar reflex and intact withdrawal reflex, because the neurons of the femoral nerve are in this area. L7–S3 lesions cause signs of LMN dysfunction in the area which innervates the sciatic nerve (decreased to absent withdrawal reflex and gastrocnemius reflex), the pudendal nerve (decrease to absent perineal reflex) and the pelvic nerve (LMN bladder dysfunction).

Spinal Radiography

Survey radiographs of vertebrae are often performed at the suspected area of the lesion during the diagnostic evaluation. However, the limitation is that the spinal cord and non-mineralised intervertebral disc material cannot be seen on these radiographs. Therefore spinal radiographs often cannot reveal any abnormal findings of soft tissue spinal compressive disease such as Hansen type I disc extrusion. The survey radiographic findings to suggest intervertebral disc extrusion are:

 Wedging or collapse of the intervertebral disc space

 Decrease in the articular facet joint space dimensions

 Thinning or alteration in the intervertebral foramen (shaped like horse's head)

 Radio-opaque disc material apparently within the spinal canal (Figure 1)

To evaluate these findings on spinal radiographs, it is important that the spine should be straight and the radiographic beam centred directly over the suspected area to avoid misinterpretation of vertebral alignment, intervertebral foraminal size and intervertebral disc space. In addition, the presence of these findings does not always provide accurate lesion and definitive diagnosis. Therefore an advanced imaging modality that can describe intervertebral disc protrusion/extrusion and compression of the spinal cord, such as myelography, computed tomography (CT) scanning and magnetic resonance imaging (MRI) must be performed for accurate localisation.

Figure 1. Survey spinal radiograph (lateral view). Mineralised intervertebral disc material is apparently seen within the spinal canal (arrow) with collapse of intervertebral disc space (arrowhead).

Myelography

During the past two decades, myelography has been most commonly used to diagnose and localise the lesion of intervertebral disc disease. Myelography provides a white parallel outline of the spinal cord through an injection of radio-opaque contrast medium into the subarachnoid space (Figure 2). This technique can be used to detect intervertebral disc material compressing the spinal cord by observing the absence of the contrast medium in the sub-arachnoid space. Herniated disc material is usually seen ventral or lateral to the spinal cord, but is sometimes found dorsally. The disadvantages of myelography are:

 Invasiveness because the spinal needle enters the spinal cord in close proximity to the filum terminale and the spinal cord (for lumbar myelography) to inject contrast medium into ventral arachnoid space.

 It is difficult to localise the lesion when the spinal cord swelling occurs.

Figure 2. Myelography (healthy dog).

Computed Tomography

CT is also useful to detect herniated/protruded intervertebral disc material in the spinal canal. The degenerated disc material is seen with a hyperdensity (white). The advantage of CT is that images can be reconstructed on computer and several views including sagittal, transverse and horizontal images can be made to evaluate the lesion site (Figure 3). However it is very difficult to assess the spinal cord itself because it is poorly distinguished from other soft tissue within the vertebral canal. In addition, it is difficult to distinguish herniated disc material and haemorrhage because both lesions show hyperdensity on CT images. Therefore CT myelography, CT with a subarachnoid injection of the contrast medium, might be needed for further evaluation.

Figure 3. Reconstructed CT images (sagittal view, top; transverse view, bottom). A hyperdense lesion can be seen on both the sagittal (arrow) and transverse (arrowhead) views.

Magnetic Resonance Imaging (MRI)

Because of its efficient ability to reveal the localisation of intervertebral disc disease and the condition of the spinal cord, MRI is rapidly becoming the gold standard imaging modality. MRI allows concise localisation of extruded or protruded disc material including the affected area (right or left, ventral or dorsal) and extent of extruded disc material by evaluating multiple planes. The MR images are usually collected at least in sagittal and transverse T1-weighted, T2-weighted and contrasted T1-weighted images. Sagittal planes are important for understanding anatomical structures of the affected region of the intervertebral disc and the spinal cord, and transverse images are important to know the exact location of any compressive material surrounding the spinal cord in a cross-sectional plane (Figure 4). Hence these two different views are important to recognise the location of the lesion especially if surgical intervention is to occur. Additional views such as dorsal (coronal) images might be helpful to understand the location of the lesion and anatomical variations. However, we would recommend other types of image using FE3D sequence (TR 45.9 ms, TE 6.8 ms) to reveal the nerve roots called a 'nerve root image', and MR myelography using fast spin echo single shot (TR 6000 ms, TE 1000 ms) to describe the line of cerebrospinal fluid (CSF) in preference to taking simple dorsal images. The nerve root image can show accurate localisation of herniated disc material in association with nerve root (Figure 5, arrow). MR myelography can describe the lesion by detecting disappearance of CSF line (Figure 5, arrowhead).

Figure 4. MR images (midsagittal T2-weighted image, above and transverse T2-weighted image, left). An extruded intervertebral disc material compresses the spinal cord (arrow and arrowhead respectively).

Figure 5. MR nerve root image (left) and MR myelography (right). Herniated disc material is compressing the spinal cord (white arrow) and the nerve root adjacent to the disc material is apparent (black arrow). Disappearance of CSF line can be seen on MR myelography (arrowhead).