Focal Lesion of the Motor Cortex

History. You examine an 11-year-old female boxer dog. Her vaccination history is current. She had an adeno­carcinoma of the mammary gland removed 6 months before your examination.

The owner states that over the past few days the dog has become progressively weaker in the left front and left rear legs and occasionally stands with the left front paw flexed such that the dorsal surface touches the ground. On the previous day the dog had a seizure.

Clinical Examination. On physical examination of the patient, you find several routine old-age changes and the results of the mammary surgery. You find also that the dog seems drowsy and is weak on the left front and left rear legs. She has a proprioceptive positioning reaction deficit of both the left front and the left rear leg. Radiographic study of the chest reveals metastatic, neoplastic lesions in the lungs.

Comment. The proprioceptive positioning reaction (or response) is tested by flexing the animal's paw, dorsal side down, while gently supporting her weight. A normal dog senses (conscious proprioception) that the paw is upside down and returns it to the normal pads-down posture (motor response). This is called a “response” (or reaction), rather than a reflex, because it involves a degree of conscious control. This particular response requires normal f unction of skin and joint receptors and the peripheral nerve in the tested leg and of the sensory neuron tracts that ascend toward the brain along the ipsilateral (same) side of the spinal cord. Traveling along a multisynaptic pathway, the sensory information crosses to the contralateral (opposite) side of the brain in the brainstem and reaches the contralateral (with respect to the side of the original stimulus) cerebral cortex. As the animal becomes consciously aware that the paw is in an unusual position, action potentials are sent back, down the corticospinal tract, to the lower motor neurons of the muscles of the leg, causing the paw to return to the normal position.

With the wiring diagram of this response in mind, you can see that a deficit in the proprioceptive positioning reaction of the left front and left rear legs could be caused by a lesion of the left cervical spinal cord, the right motor cortex, or supraspinal portions of the right corticospinal tract. This dog's seizure (a manifestation of brain disease) at about the same time suggests that the lesion is in the right cerebral cortex. The brain is a common site for metastasis, and the radio­graphic lung lesions suggest that the mammary tumor has spread to both the lung and the right side of the brain. The lung contains the first capillary bed that a metastatic cancer cell is likely to encounter when it enters the venous system of the mammary gland. Some cells stop here and grow.

Treatment. Dogs with metastatic mammary carcinomas are usually not treated except to make them more comfortable.