THE MERCK VETERINARY MANUAL
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Electrodiagnosis in Neurologic Disease

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An electroencephalogram (EEG) is a recording of the electrical activity of the surface of the cerebral cortex, which is influenced by subcortical structures. The EEG is consistently abnormal in hydrocephalus, meningoencephalitis, head trauma, and cerebral neoplasia. An EEG may determine whether seizure discharges are focal or diffuse. The EEG is often normal in idiopathic epilepsy, unless seizures are not well controlled and interictal spikes are present.

An electromyogram (EMG) is a recording of the electrical activity of muscles and is used to evaluate the health of the motor unit. The motor unit consists of the LMN, the nerve root, the peripheral nerve, the neuromuscular junction, and skeletal muscle. The peripheral nerve can be stimulated, and motor and sensory nerve conduction velocities calculated and compared to known normal values to help define the neuropathy as an axonopathy or myelinopathy. Repetitive nerve stimulation may lead to a reduction of the evoked potential in myasthenia gravis. Abnormalities of late waves (F and H) may be associated with disorders of the nerve roots.

The brain-stem auditory evoked response (BAER) is a recording of electrical activity in the auditory pathway from the inner ear receptors through the brain stem to the cerebral cortex. The animal may or may not be awake for this test. No response or a diminished response is seen in auditory nerve disorders associated with hearing loss. Brain-stem disorders may also alter the BAER; it may be abnormal in the Chiari malformation of Cavalier King Charles Spaniels and may also be used as an indicator in determining brain death.

Spinal cord evoked potentials can be used to evaluate spinal cord integrity. These are elicited by stimulating a peripheral nerve and recording the potential as it ascends the spinal cord.

Last full review/revision July 2013 by Thomas Schubert, DVM, DACVIM, DABVP

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