Neurological disease in horses (MMEP-test, TMS, EMG)

Symptoms of neurological diseases can be very variable (i.e. subtle ataxia, paresis, head-tilt, epilepsy, total paralysis,....).

It is very important to start with a clinical-neurological exam (turning short circles, stepping backwards, inspection of the eyelids, position of the ears,....). However, sometimes an advanced exam such as radiography of the vertebrae, myelography, analysis of the cerebrospinal fluid, an EMG or a MMEP-test, is necessary to make a diagnosis.

A radiographic exam may show causes of compression of the spinal cord due to fracture, OCD, lipping, arthritis of the vertebra. However in some cases a myelogram is necessary to find the exact location of spinal cord compression. A myelogram is performed under general anaesthesia. During this procedure a needle is placed under ultrasonographic guidance to inject contrast fluid into the spinal canal. Subsequently radiographs of the spinal cord are taken. During this procedure cerebrospinal fluid is collected for further research.

In case of muscle disease or atrophy an EMG can be performed. EMG records the electrical activity produced by the skeletal muscle. Signals may change with specific muscle disease.

The Magnetic Motor Evoked Potential test (MMEP-test) or Transcranial Magnetic Stimulation (TMS) allows to visualize the function of the spinal cord in an objective manner. This procedure is especially advised in ataxic horses or in horses where the lameness is not caused by a leg problem but by a problem of the motor nervous system.

During the MMEP-test, a painless test which can be performed on the sedated standing horse, a brief magnetic field through the skull produces a motoric signal in the brains that travels through the spinal cord to the muscles in the legs. The impulse that arrives in the muscles is captured with a registration needle. The time lag between magnetic stimulation and capture of the stimulus in each of the legs and the amplitude of the captured signal indicate whether intact conduction through the spinal cord occurs. The conduction time will increase if the integrity of the motor nervous system is damaged.