Signs of Upper Motor Neuron Disease Differ from Signs of Lower Motor Neuron Disease
Lesions of upper motor neurons cause clinical signs that are significantly different from those produced by lower motor neuron disease, although paralysis/paresis may be seen in both scenarios.
1. Inappropriate movement. Lesions of upper motor neurons can cause a variety of movement disorders, depending on the location of the lesion. Spinal cord disease, affecting portions of upper motor neurons projecting to the cord, often causes various degrees of weakness below the lesion. Disease of the brain that affects upper motor neurons may cause seizures, rigidity, circling gaits, and other inappropriate movements. More specific examples of this general category are presented in Chapters 10, 11, and 12 on the central control of movement, the vestibular system, and the cerebellum, respectively.
2. No atrophy. Because the lower motor neuron is intact, the muscle does not atrophy. (Modest disuse atrophy may develop much later.)
3. Retained but exaggerated segmental reflexes. Because the neuronal circuitry of the segmental reflex arc (see Chapter 7) is not interrupted in upper motor neuron disease, reflexes such as the muscle stretch and toe-pinch withdrawal are retained, whereas in lower motor neuron disease reflexes are depressed or lost. Because upper motor neurons are normally capable of exerting significant inhibitory control over spinal reflexes, however, damage to these neurons can decrease this inhibition, resulting in exaggeration of the reflex response (hyperreflexia).
4. Normal electromyogram. Because the muscle is not atrophied and the lower motor neurons are intact, the electrical activity of the muscle appears normal.
The following clinical correlations illustrate common examples of lower and upper motor neuron disease. Before going to Chapter 10, the reader should understand these concepts and why these dogs have the clinical signs mentioned.