NEUROMUSCULAR CASE OF THE MONTH - MARCH 1999

Progressive weakness, muscle atrophy and fasciculations
in a 13 year old female spayed DSH cat
Contributed by Dr. Robert Kroll
Oregon Veterinary Specialty Clinic
Portland, Oregon

Clinical History
The cat presented with an eight month history of progressive weakness beginning in the front limbs and gradually involving all four limbs. The owner reported that the cat began having a problem in her right thoracic limb that initially resolved with a single steroid injection. Weakness reoccurred two months later and progressed since that time. The owner indicated there was no further improvement with additional steroid therapy. Four weeks prior to presentation the weakness progressed to inability to ambulate associated with a flaccid tail.

Physical Examination
Physical and neurologic examination was difficult due to the fractiousness and irritability of the cat. The cat was non-ambulatory and had marked proprioceptive deficits in all four legs. Cranial nerves were considered normal. Forelimb spinal reflexes were markedly depressed with the left side worse than the right. Hind limb spinal reflexes were depressed with the right side worse than the left. There was decreased muscle tone and moderate muscle atrophy in all limbs with muscle fasciculations. A flaccid tail was also present.  On sensory evaluation, superficial pain perception was considered to be present in all limbs with a generally increased sensitivity to touch over the entire body. The neurolocalization was spinal roots, peripheral nerve and/or muscle with differentials including infectious disease (FIV, Felv, FIP, Toxoplasmosis, or other), inflammatory, neoplastic, or metabolic disease.

Diagnostic Tests Routine laboratory tests CBC and serum chemistry profile - no significant abnormalities Serum CK - 564 U/L (50-300) Electrolytes - K+ 3.8 meq/l (4.0-5.8) T4 - 1.6 µg/dl (1.8 - 4.5) Felv/FIV, FIP - negative Toxoplasma - positive to a dilution of  >1:2064 by latex agglutination Electromyography - Fibrillation potentials were present in most limb muscles evaluated by EMG (Fig. 1) and there was marked slowing of the motor nerve conduction velocities (proximal 29 m/sec, distal 35 m/sec) with very low amplitude (Fig. 2).

Figure 1. Electromyogram of right cranial tibial muscle. Figure 2. MCV from the left tibial nerve

Muscle and nerve biopsies - A fresh frozen biopsy from the left biceps femoris muscle showed multifocal areas of mononuclear cell infiltration (lymphocytes and macrophages) having an endomysial distribution (Fig. 3A; H&E stain). A large intramuscular nerve branch showed moderate depletion of myelinated nerve fibers (arrows),  myelin ovoids (asterisks), and mononuclear cell infiltration (Fig. 3B; Gomori modified trichrome stain). Formalin fixed biopsies from the peroneal nerve were plastic embedded and evaluated in 1 µm sections. Concentric masses of myelin debris referrable to chronic, myelinated nerve fiber degeneration were present and there was subperineurial edema (Fig. 3C). A swollen myelinated nerve profile filled with granular debris was present consistent with acute Wallerian degeneration (Fig 3D). In the upper portion of the section, a nerve fiber contains darkly staining axoplasm consistent with axonal degeneration. No identifiable infectious agents were found within either the muscle or nerve biopsies.

Figure 3A. Figure 3B. Figure 3C. Figure 3D.

Conclusion
Chronic inflammatory myopathy (myositis) and axonal degeneration. Toxoplasmosis was suspected and treatment with clindamycin was initiated at a dosage of 100 mg twice daily PO. While initial improvement was reported, the cat continued to decline and appeared to be getting more painful. The owner elected euthanasia. A necropsy was not performed. It cannot be said with certainty that Toxoplasmosis was the etiology of the neuromuscular dysfunction since the positive titer may be a result of past exposure rather than active infection. Alternatively, this may have been related to a paraneoplastic process. In humans, sensorimotor neuropathy is a frequent complication of malignant disease and may precede the symptoms of the neoplasm by periods of up to 5 years. Polymyositis may also be associated with a paraneoplastic process. Neoplasia was not identified in this cat.

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