An approximately 8 year old, castrated male Domestic Shorthair cat was adopted by the owner as an adult 4 years previously (thus the age is an estimation). Over the course of the past year the cat has had three episodes of sudden inability to use the pelvic limbs. Clinical signs during the first two episodes resolved within 3-4 days of onset. The most recent episode began approximately 2.5 months prior to presentation and clinical signs have remained static since the time of onset. The cat cannot stand, but will drag itself using the thoracic limbs. The owner reports that she is holding the cat over the litter box twice daily, and voiding is normal at these times. Radiographs of the thorax and abdomen were taken and were unremarkable.
Upon presentation the cat was bright and alert but overweight (body condition score 7/9). Otherwise the physical examination was unremarkable. Mentation was normal on neurological examination and cranial nerve examination did not reveal any abnormalities. The cat was nonambulatory and tetraparetic with significantly more strength in the thoracic limbs than the pelvic limbs. It was able to position itself in sternal recumbency, but could not support full weight on any of the limbs. Hopping was delayed in all limbs, but more so in the pelvic limbs compared to the thoracic limbs. Patellar reflexes were absent bilaterally. Upon testing withdrawal reflexes in all limbs, there was weak flexion of the proximal joints (shoulders and hips) and no flexion of the distal limbs. Spinal pain was not elicited. The neuroanatomic localization was to the neuromuscular system.
CBC – within normal limits
Chemistry profile – Mildly elevated creatine kinase – 567 U/L (64-440 U/L), otherwise unremarkable
Toxoplasmosis IgG and IgM IFA – negative
Electromyography – Electrodiagnostics were performed with the cat under general anesthesia. Increased insertional activity, fibrillation potentials and positive sharp waves were found in all muscles tested (thoracic limb, pelvic limb, epaxial muscles). There were no notable differences in proximal versus distal muscles or in thoracic versus pelvic limb muscles. All muscles were moderately to severely affected.
Motor nerve conduction velocity (NCV) – The right tibial nerve was tested. Amplitudes of the compound muscle action potential (CMAP) were markedly decreased proximally and distally (hock – 7.9 mV, reference >31.9; stifle – 2.6 mV; reference >26.6;
hip – 3.2 mV, reference >27.0). Motor NCV was also decreased proximally and distally (hock to stifle – 31 m/sec, stifle to hip – 25 m/sec, reference >60). Results of electrodiagnostic testing were consistent with a generalized polyneuropathy. Muscle and nerve biopsies were collected.
Muscle and nerve biopsies
A biopsy from the left lateral head of the gastrocnemius muscle was collected by an open biopsy procedure under general anesthesia and evaluated in frozen sections. A neurogenic pattern of muscle fiber atrophy was observed with intramuscular nerve branches practically devoid of myelinated fibers (arrow, Fig. A, modified Gomori trichrome stain). Fixed biopsies from the left tibial nerve were also collected and evaluated in 1 µm plastic sections (Fig. B, toludine blue stain). The density of myelinated fibers was moderately decreased with several fibers in each fascicle inappropriately thinly myelinated (arrows, Fig. B). Most fascicles contained several onion bulb formations. Marked endoneurial edema widely separated remaining nerve fibers.
Diagnosis and Treatment
A chronic relapsing form of polyneuropathy was confirmed. As this form of polyneuropathy may have an inflammatory cause, the cat was placed on 5 mg prednisone once daily. A short course of tramadol was given for any pain experienced at the biopsy site.
Although the owner has been unable to bring the cat back to the referral hospital for follow-up neurological examination, she is following up with the family veterinarian. Two months after initiation of treatment that cat was clinically doing very well. Neurological signs are controlled as long as the cat is on prednisone.