Upper Motor Neuron Disease

History. A 5-year-old male dachshund is brought to a local veterinary clinic. His vaccination history is current, and the dog has had no contributing past medical or surgical illnesses.

Clinical Examination. Physical examination abnormalities were limited to the nervous system. The dog was bright, alert, responsive, and able to bear weight normally on the front legs. However, he was weak and unsteady on the hind legs. No atrophy was apparent. All cranial nerve reflexes were normal, as were the spinal segmental reflexes of both front and hind legs. Intersegmental responses, including the proprioceptive positioning reaction, were normal in the front legs but absent in the hind legs (see Chapter 7). (Failure to exhibit the normal proprioceptive placing reaction indicates a lesion somewhere along the sensory or motor routing for this response. This routing includes the peripheral nerves for that limb, the spinal cord rostral to that limb on the same side, and the contralateral side of the brain.) Results of a complete blood cell count and serum chemistry analysis were within normal limits.

Comment. The absence of atrophy and the retention of segmental reflexes in the affected limbs indicate that the lower motor neurons, neuromuscular junction, and skeletal muscle are normal and that this is an upper motor neuron disease. Because only the hind limbs are affected by weakness and exhibit a deficit in proprioceptive placing, the cervical spinal cord and brain must be normal since motor commands to the front legs are transmitted reliably. Therefore the lesion must be between the front and hind limbs. This is a typical history and a typical clinical presentation for a dog with a herniated intervertebral disk.

Treatment. Treatment and prognosis depend on the severity of the spinal cord trauma. Medical management is aimed at reducing edema, vasospasm, inflammation, and other metabolic consequences of the disease that worsen the damage to the spinal cord. When surgery is indicated by the severity of the trauma, the goal is to relieve spinal cord compression. With appropriate medical and surgical management, many dogs recover useful spinal function.