Comparative Neuromuscular Laboratory

NEUROMUSCULAR CASE OF THE MONTH - FEBRUARY 1999

Chronic progressive pelvic limb paresis in a
1 year old male castrated DSH cat
Contributed by Drs. Michael Podell and Victoria Ochoa
Comparative Neurology Service
The Ohio State University
Columbus, Ohio

Clinical History
The cat presented for a chronic, progressive history of weakness in the pelvic limbs for approximately two to three months (See video segment). Over the past month, there seemed to be a more progressive weakness in the rear limbs, with the cat walking almost completely plantigrad. Generalized muscle fasciculations and tremors were observed when the cat stood. The front legs did not appear to be clinically affected.

Physical Examination
Physical examination was normal. Neurologic examination revealed a plantigrade stance and loss of proprioception in the pelvic limbs, both on the left and the right side. There was marked weakness more on the right hind than the left hind limb, and lower motor neuron reflexes to the pelvic limbs. The neurolocalization for this problem was spinal cord segments L4-S1 involving either the spinal cord, spinal roots and/or peripheral nerve.

Diagnostic Tests

Routine laboratory tests
CBC and serum chemistry profile - no abnormalities

Serum CK - 1036 U/L (100-850)

Electrolytes - no abnormalities

Felv/FIV, FIP, Toxoplasma - negative

Lumbar CSF analysis - mild protein elevation (33 mg/dl) with no cytologic abnormalities

Myelography - unremarkable spine with myelogram

Electromyography - Spontaneous activity including
fibrillations and positive sharp waves were present in muscles supplied by the femoral, peroneal, tibial and possibly the radial nerve (Fig. 1). Motor nerve conduction velocity revealed a decrease velocity of the peroneal nerve (Fig. 2) and decreased amplitude of all nerves.

Muscle and nerve biopsies - Fresh frozen muscle biopsies from the vastus lateralis and gastrocnemius muscles showed small and large groups of atrophic fibers with several necrotic fibers undergoing phagocytosis. A biopsy from the peroneal nerve was plastic embedded and evaluated in 1 um sections (Fig. 3). Nerve fibers were widely separated by endoneurial edema. Demyelinated axons and onion-bulb formations were observed showing concentrically arrayed supernumerary Schwann cells. As evaluated by electron microscopy (Fig. 4), several layers of supernumerary Schwann cells were present within the onion bulb formations.

Figure 1. Electromyogram of
cranial tibial muscle

Figure 2. MCV from peroneal
nerve

Figure 3. Toluidine blue stained plastic embedded section of peroneal nerve showing onion
bulb formations and
demyelinated nerve fibers.

Figure 4. Electron microscopic evaluation of onion bulb formation showing several layers of supernumerary Schwann cells
and a relatively thin myelin
sheath for the axon diameter

Conclusion
Chronic, demyelinating polyneuropathy with numerous onion bulb formations and edema. Onion bulb formations, indicative of repeated episodes of segmental demyelination followed by remyelination, may be a feature of chronic demyelinating polyneuropathies, diabetic or toxic neuropathies, and hereditary motor and sensory neuropathy. A guarded prognosis was given. The cat did poorly and was euthanitized. A necropsy was not performed.