Sensory evoked potential (EP), or evoked responses, are tests that use conventional EEG recording techniques with specific electrode site placement to evaluate electrophysiologic integrity of the auditory, visual, and sensory pathways. These are brain responses "time locked" to some event. See Chart 16.1 for wave and standard deviation (SD) measurements.

Brainstem auditory evoked potential (BAEP): This study allows evaluation of suspected peripheral hearing loss, cerebellopontine angle lesions, brainstem tumors, infarcts, multiple sclerosis, and comatose states. Special stimulating techniques permit recording of signals generated by subcortical structures in the auditory pathway. Stimulation of either ear evokes potentials that can reveal lesions in the brainstem involving the auditory pathway without affecting hearing. EPs of this type are also used to evaluate hearing in newborns, infants, children, and adults through electrical response audiometry.

Visual evoked potential (VEP): This test of visual pathway function is valuable for diagnosing lesions involving the optic nerves and optic tracts, multiple sclerosis, and other disorders. Visual stimulation excites retinal pathways and initiates impulses that are conducted through the central visual path to the primary visual cortex. Fibers from this area project to the secondary visual cortical areas on the brain's occipital convexity. Through this path, a visual stimulus to the eyes evokes an electrical response in the occipital regions, which can be recorded with electrodes placed along the vertex and the occipital lobes. It is also used to assess development of blue-yellow pathway in infants.

Somatosensory evoked potential (SSEP): This test assesses spinal cord lesions, stroke, and numbness and weakness of the extremities. It studies impulse conduction through the somatosensory pathway. Electrical stimuli are applied to the median nerve in the wrist or peroneal nerve near the knee at a level near that which produces thumb or foot twitches. The milliseconds it takes for the current to travel along the nerve to the cortex of the brain is then measured. SSEPs can also be used to monitor sensory pathway conduction during surgery for scoliosis or spinal cord decompression and ischemia. Loss of the sensory potential can signal impending cord damage.

Procedures

1. Obtain BAEPs through electrodes placed on the vertex of the scalp and on each earlobe. Stimuli in the form of clicking noises or tone bursts are delivered to one ear through earphones. Because sound waves delivered to one ear can be heard by the opposite ear, a continuous masking noise is simultaneously delivered to the opposite ear.

2. Place electrodes used in VEP on the scalp along the vertex and occipital lobes. Ask the patient to watch a checkerboard pattern flash for several minutes, first with one eye and then with the other, while brain waves are recorded.

3. Record SSEPs through several pairs of electrodes. Apply electrical stimuli to the median nerve at the wrist or to the peroneal nerve at the knee. Scalp electrodes placed over the sensory cortex of the opposite hemisphere of the brain pick up the signals and measure, in milliseconds, the time it takes for the current to travel along the nerve to the cortex of the brain.

4. Follow guidelines in Chapter 1 for safe, effective, informed intratest care.

1. Abnormal BAEPs are associated with the following conditions:

   1. Acoustic neuroma

   2. Stroke

   3. Multiple sclerosis

   4. Lesions affecting any part of the auditory nerve or brainstem area

2. Abnormal VEPs are associated with the following conditions:

   1. Demyelinating disorders such as multiple sclerosis

   2. Lesions of the optic nerves and eye (prechiasmal defects)

   3. Lesions of the optic tract and visual cortex (postchiasmal defects)

   4. Abnormal VEPs may also be found in persons without a history of retrobulbar neuritis, optic atrophy, or visual field defects. However, many patients with proven damage to the postchiasmal visual path and known visual field defects may have normal VEPs.

3. Abnormal SSEPs are associated with the following conditions:

   1. Spinal cord lesions

   2. Strokes

   3. Multiple sclerosis

   4. Cervical myelopathy injury

Pretest Patient Care

1. Explain test purpose and procedure.

2. Have the patient wash and rinse hair before testing. Instruct the patient not to apply any other hair preparations.

3. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

1. Some difficulty in interpreting BAEPs may arise in persons with peripheral hearing defects that alter evoked potential results (i.e., subthreshold stimulation of peripheral nerves and inadequate skin preparation).

2. Maximum depolarization stimulation is divided into two protocols:

   1. Brachial plexus protocol involves stimulation of the median, ulnar, and superficial sensory radial nerves just proximal to the wrist.

   2. Lumbosacral (LS) protocol involves stimulating the posterior tibial and common peroneal nerves, which are the primary divisions of the LS plexus forming the sciatic nerve.