The causes of spinal cord injury (SCI) are mainly compressive or contusive and result from intervertebral disc diseases, vertebral fracture or subluxation/luxation (primary SCI), which are commonly seen in dogs and cats following traffic accidents. Some of these SCI cases can recover after adequate conventional treatments including surgical decompression and stabilisation, and medication, but others do not respond to the treatments. The important issue in treating SCI cases is to predict their outcome prior to treatment especially with regard to possible benefits of surgical intervention, although it is difficult to assess accurately. In this lecture I would like to introduce some hints of predicting prognosis following spinal cord trauma.

Thoracacolumbar Intervertebral Disc Extrusion

Prognosis for cases with acute thoracolumbar disc extrusion is largely predictable based on the results of neurological examination. Presence or absence of deep pain perception (DPP) is the most important prognostic indicator. It is widely understood that the prognosis for dogs with thoracolumbar intervertebral disc extrusion in which DPP remains intact is good following either conservative or surgical treatment. In contrast, loss of DPP is associated with a guarded or poor prognosis, perhaps particularly if DPP has been absent for > 48 hours. However, various investigators have reported that between 7% and 76% of paraplegic dogs without hindlimb DPP at the time of initial examination will recover, even when DPP has been absent for > 48 hours. Therefore further predictive factors would be helpful.

Apart from neurological examination several prognostic indicators have been reported. Peracute onset of clinical signs, severe spinal cord swelling and myelomalacia on myelographic images have also been mentioned as poor to hopeless prognostic indicators in paraplegic dogs. A few reports have suggested that visual assessment of the spinal cord at the time of surgery is a useful means to determine the prognosis for dogs without DPP, with intramedullary haemorrhage and evidence of myelomalacia after durotomy being interpreted as poor prognostic signs.

In human patients with SCI, magnetic resonance imaging (MRI) has been established as the imaging modality of choice because it enables identification of spinal cord oedema, haemorrhage or swelling, which can aid in formulating a prognosis. Similar to human patients, we have reported that MRI findings of the spinal cord of dogs with intervertebral disc extrusion could be used as a new prognostic factor. Around 70% of dogs with intervertebral disc extrusion which did not have DPP and showed abnormal MRI findings specifically hyperintensity on T2-weighted images (Figure 1), failed to regain the ability to walk after hemilaminectomy. In contrast, all of those cases without DPP which did not show any abnormal intensity in spinal cord on MRI, notably T2-weighted images, had a successful outcome after surgery. Furthermore, only 10% of dogs with an extended area of hyperintensity (> 3 times the length of the L2 vertebral body) on T2-weighted images that also had lost DPP had a successful outcome. In the cases that had DPP, the extended area of abnormal hyper-intensity in the spinal cord on T2-weighted images might also correlate to outcome. Results of our study have shown that regardless of whether DPP was or was not preserved, the success rate following surgery in dogs without areas of hyperintensity of the spinal cord on T2-weighted MR images was 100%, although some dogs had persistent mild neurological deficits. We concluded that predictive power was enhanced by combining MRI findings with the information on DPP status.

Figure 1. Midsagittal T2-weighted MR image from a dog with paraplegia cause by L3–4 disc extrusion. An area of hyperintensity can be seen on the image (arrow).

Fracture and Subluxation/Luxation

Generally, loss of deep pain sensation to the pelvic limbs is always associated with a poor prognosis; in association with fracture-luxations, it immediately indicates a very poor or hopeless prognosis.

Ascending-Descending Myelomalacia

Ascending-descending myelomalacia is defined as progressive gross softening of the spinal cord resulting from haemorrhagic necrosis and most commonly occurs as a sequel to acute SCI associated with (usually peracute) intervertebral disc extrusion. Almost all cases in which progressive myelomalacia has occurred are fatal within several days due to respiratory failure. Although the precise pathophysiological mechanisms of progressing myelomalacia are not known, it might develop following primary (contusive and compressive lesion) and secondary (ischaemia, oedema and inflammatory) SCI. It has been previously reported that myelomalacia could be diagnosed on myelography - by observing entry of contrast medium into the spinal cord. In addition, previous reports of single cases and our own studies have indicated that ascending-descending myelomalacia could be diagnosed by MRI. In dogs with ascending-descending myelomalacia, MRI of the spinal cord typically reveals hyperintensity on T2-weighted images. Overall, our findings suggest that T2-weighted detection of an extensive area of abnormal hyperintensity with focal compression in the spinal cord (Figure 2) as well as xanthochromia in cerebrospinal fluid samples may have diagnostic value. Such characteristics may allow a presumptive diagnosis of progressive myelomalacia to be made before lower motor neuron deficits appear in the pelvic limbs, before extinction of the cutaneous trunci reflex occurs, before the extent of analgesia in the caudal aspect of affected dogs begins to migrate cranially, and before any surgical intervention is performed.

Figure 2. Midsagittal T2-weighted MR image from a dog with progressive myelomalacia. A diffuse area of hyperintensity can be seen in the spinal cord (black arrows) with isointensity in the caudal part of the spinal cord (white arrowhead).

References

1.  Ito D, Matsunaga S, et al. Prognostic value of magnetic resonance imaging in dogs with paraplegia caused by thoracolumbar intervertebral disk extrusion: 77 cases (2000–2003). Journal of the American Veterinary Medical Association 2005;227:1454–1460.

2.  Okada M, Kitagawa M, et al. Magnetic resonance imaging features and clinical signs associated with presumptive and confirmed progressive myelomalacia in dogs: 12 cases (1997–2008). Journal of the American Veterinary Medical Association 2010;237:1160–1165.