Comparative Neuromuscular Laboratory

NEUROMUSCULAR CASE OF THE MONTH - MAY 2001

Progressive weakness and muscle atrophy in an
11 year old MC Italian saddle horse
Contributed by Dr. Gualtiero Gandini
Departimento Clinico Veterinario
Universita Degli Studi Di Bologna, Italy

Clinical History
The horse was referred because of progressive weakness that began a few months prior to presentation. The horse was used only for short rides and pleasure walks and was not considered an athletic animal. The owner said that the horse was progressively losing weight (Fig. 1, muscle atrophy evident) and was spending a lot of time lying on the floor of his box. Slight exercise resulted in a quick onset of fatigue and muscle tremors. Recently, the head was continuously maintained at the level of the floor and the horse was not able to raise it (Fig. 2). Appetite was good and all other functions such as micturition and defecation were normal. The horse, in the month prior to referral, was treated with dexamethasone for 2 weeks, then vitamin E and selenium for another 2 weeks without any clear improvement.

Physical Examination
The horse was in bad general condition and very thin with generalized muscle atrophy. Ribs were prominent (Fig. 1) and spinous processes of the thoracic vertebrae of the spine were easy to palpate. Muscle atrophy was pronounced in the shoulder and scapular region, as well as in the hind limbs. With the exception of a slight systolic left heart murmur (II/VI), no other abnormalities were found on physical examination.

Neurological Examination
Mental status was normal. Posture was characterized by the head continuously maintained at the level of the floor. Attempts to raise it resulted in muscle tremors of the neck and shoulders. Gait was slightly tetraparetic, with hypometria of all four limbs. Ataxia was not noticed even when the horse was walked with the head maintained elevated or in tight circles. Backing of the horse showed no gait abnormalities. The sway test was positive and it was easy to push the horse laterally. Proprioceptive deficits were not evident. Spinal reflexes showed a decreased flexor reflex on all four limbs. Cervicofacial and cutaneous trunci reflexes were considered normal. Cranial nerve examination showed a decreased palpebral reflex and decreased tongue strength. Neurolocalization was to the peripheral nervous system, the primary differential being equine motor neuron disease (EMND 1,2)

Figure 1.

Figure 2.

Diagnostic Testing
CBC and biochemical profile were normal. Plasma levels of Vitamin E were not measured because of previous supplementation. Electromyography was performed under general anesthesia and spontaneous activity was detected only in the muscles of the neck and spine. The motor nerve conduction velocity was not performed because of lack of abnormalities within the muscles of the limbs.

Muscle and Nerve Biopsies: Fresh and fixed biopsies were collected from the semimembranous, sacrococcygeus, and extensor carpi radialis muscles for evaluation. Pathological changes within the muscle biopsies included non-specific myofiber atrophy, mild mononuclear cell infiltration (semimembranous muscle), and increased capillary density within some muscle fascicles. A biopsy was also collected from the ventral branch of the spinal accessory nerve (3), plastic embedded, and evaluated in 1 um sections. Marked loss of large myelinated nerve fibers, endoneurial fibrosis and axonal degeneration consistent with "Wallerian-like" degeneration were observed (Fig. 3). Occasional clusters of regenerating nerve fibers were also present (Fig. 4). The finding of axonal degeneration within this nerve is consistent with previous reports of EMND (3).

Figure 3.

Figure 4.

Conclusion
The horse was euthanitized because of poor condition. The brain and spinal cord were evaluated by Dr. Rosmarie Fatzer, University of Bern. Pathological findings included degeneration of neurons in the ventral horns of all sections of the spinal cord, lipofuscin in neurons, and perivascular accumulations of macrophages. The most conspicuous change in the brain was a slight degeneration in the genu of the facial nerve. The histopathological changes supported the clinical diagnosis of EMND.

References
1. Divers TJ, deLahunta A, Hintz HF et al. Equine motor neuron disease. Equine Veterinary Education 2001;13:63-67.

2. Valentine BA, deLahunta A, George C et al. Acquired equine motor neuron disease. Vet path 1994;31:130-138.

3. Jackson CA, deLahunta A, Cummings JF et al. Spinal accessory nerve biopsy as an ante mortem diagnostic test for equine motor neuron disease. Equine Vet J 1996;28:215-219.