Abstract
BackgroundA classic sign of canine syringomyelia (SM) is scratching towards one shoulder. Using magnetic resonance imaging (MRI) we investigate the spinal cord lesion relating to this phenomenon which has characteristics similar to fictive scratch secondary to spinal cord transection.Medical records were searched for Cavalier King Charles spaniels with a clinical and MRI diagnosis of symptomatic SM associated with Chiari-like malformation (CM). The cohort was divided into SM with phantom scratching (19 dogs) and SM but no phantom scratching (18 dogs). MRI files were anonymised, randomised and viewed in EFILM ™. For each transverse image, the maximum perpendicular dimensions of the syrinx in the dorsal spinal cord quadrants were determined. Visual assessment was made as to whether the syrinx extended to the superficial dorsal horn (SDH).ResultsWe showed that phantom scratching appears associated with a large dorsolateral syrinx that extends to the SDH in the C3-C6 spinal cord segments (corresponding to C2-C5 vertebrae). Estimated dorsal quadrant syrinx sizes based on the perpendicular diameters were between 2.5 and 9.5 times larger in dogs with phantom scratching, with the largest mean difference p-value being 0.009.ConclusionSM associated phantom scratching appears associated with MRI findings of a large syrinx extending into the mid cervical SDH. We hypothesise that damage in this region might influence the lumbosacral scratching central pattern generator (CPG). If a scratching SM affected dog does not have a large dorsolateral cervical syrinx with SDH involvement then alternative explanations for scratching should be investigated.
Highlights
A classic sign of canine syringomyelia (SM) is scratching towards one shoulder
The presence of phantom scratching was defined as a repetitive scratching action towards the neck region which could be induced by stimulation of a receptive field in the cervical region and/or induced by excitement or exercise in the absence of generalised pruritus and when clinical examination did not suggest skin and external ear disease and when the scratching behaviour had been observed by an European College of Veterinary Neurology (ECVN) diplomate or resident
Inter-rater reliability of syrinx identification based on the small sample of dogs was found to be good with significant kappa values in excess of 0.75 and the ICC results for the slice measurement within each quadrant were all significant and in excess of 0.8
Summary
A classic sign of canine syringomyelia (SM) is scratching towards one shoulder. Using magnetic resonance imaging (MRI) we investigate the spinal cord lesion relating to this phenomenon which has characteristics similar to fictive scratch secondary to spinal cord transection. Sherrington showed that approximately three months after transection of the caudal cervical spinal cord in dogs, stimulation of the skin in the scapular region caudal to the transection level induced a rhythmic and non-purposeful scratching action of the ipsilateral limb scratching towards, but not making contact with, the skin. He further described that there was a curvature of the body and neck with the “head partially turned back for the foot more readily to reach it” [13]. The similarity of phantom scratch to fictive scratch in a spinalised animal offers a hypothesis that phantom scratching may be a consequence of a hyperactive pathway involving the lumbosacral scratching central pattern generator and that it is a phenomenon that is distinct from neuropathic pain i.e. dogs with phantom scratching may not have behavioural signs of pain and vice versa
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