EEG/EMG (Neurophysiology)

Hoag Neurophysiology Department evaluates disorders of the Central and Peripheral Nervous Systems. To evaluate the brain, spinal cord and cranial nerves we use tests that record the movements of electricity along the Central Nervous System (CNS). These tests include various forms of Electroencephalogram (EEG), Evoked Potentials, and Electrodiagnostic testing.

Electroencephalogram (EEG) Hoag Neurodiagnostic Lab

The neurodiagnostic lab, also referred to as the EEG lab, at Hoag Hospital offers digital technology to record the measurement of electrical activity in your brain. In a comfortable setting, a registered electroneurodiagnostic technologist will conduct and monitor a noninvasive EEG, and answer any questions that you may have about the recording process. This outpatient procedure is safe and painless.

Types of EEG available at Hoag

For outpatients not admitted to the hospital:

  • Routine EEG testing – using recording wires with electrodes, brain waves are monitored for approximately 20 minutes to identify seizures or a risk for seizures.
  • Prolonged EEG testing – when routine testing is not adequate, a longer duration of testing may be necessary to identify seizures or a risk of seizures.

For patients scheduled to enter the hospital:

  • Comprehensive Epilepsy Monitoring Unit – we reserve this test for patients who may qualify for epilepsy surgery or for those whose diagnosis of seizures is unclear or debilitating. These patients are expected to stay in the hospital until enough events are captured to assure their physician that the correct diagnosis and treatment plan are in place. In most cases, patients remain in the epilepsy monitoring unit for three to five days.
  • Electrodiagnostic Testing – for disorders of the nerves, muscles or neuromuscular junctions.

Electromyography (EMG) and Nerve Conduction Study (NCS)

These tests can be used to differentiate a problem with the nerves (such as neuropathy) from a problem of the muscles (such as myopathy). Since these tests can be difficult to interpret, they are designated as physician-dependent procedures. Although the tests have technical limitations, they are very useful in detecting conditions that may benefit from specialized treatment, such as chronic inflammatory demyelinating polyradiculoneuropathy (CIDP).


Electromyography (EMG) is a diagnostic procedure to assess the health of muscles and the nerve cells that control them (motor neurons). Motor neurons transmit electrical signals that cause muscles to contract. An EMG translates these signals into graphs, sounds or numerical values that a specialist interprets.

An EMG uses tiny devices called electrodes to transmit or detect electrical signals. During a needle EMG, a needle electrode inserted directly into a muscle records the electrical activity in that muscle.

A nerve conduction study (NCS), another part of an EMG, uses surface electrodes — electrodes taped to the skin — to measure the speed and strength of signals traveling between two or more points. EMG results can reveal nerve dysfunction, muscle dysfunction or problems with nerve-to-muscle signal transmission.

Evoked Potentials

Types of Evoked Potentials available at Hoag:

  • Visual Evoked potentials – typically used to support a diagnosis of Multiple Sclerosis or Neuromyelitis Optica (Devic’s syndrome)
  • Auditory Evoked potentials – typically for patients with hearing loss from nerve damage.
  • Somatosensory Evoked Potentials – many uses to evaluate the CNS and Peripheral Nervous System

EEG Frequently Asked Questions

What is an EEG?

An electroencephalogram (EEG) records brainwave activity with the use of electrodes. This diagnostic tool translates the electrical brainwave activity into a series of wavy lines. This is a diagnostic test, not a treatment. The EEG does not measure intelligence, nor diagnose emotional or mental illness.

How long does the EEG last?

For a routine EEG recording, it takes approximately 20 minutes, but please allow one hour total for the application and removal of electrodes. For a sleep deprived EEG recording, it takes approximately 40-60 minutes. Allow two hours total for the application and removal of electrodes.

How do I prepare for my EEG?

Before the EEG, your physician may advise you to alter your medication(s) dosage(s). Continue with your normal meal intake. Do not use oils, hair spray, or conditioner on your hair the night before the EEG. Please arrive with clean, dry hair on the day of your EEG.If your physician recommends a sleep deprived EEG, be prepared to stay awake the night before the EEG and avoid caffeine intake the day of your EEG. For a pediatric routine EEG, it is recommended that the test be scheduled around nap time or limit your child’s sleep the night before the test.

What should I expect during the EEG?

First, the technologist will ask you a few questions about your medical history. Then, you will be asked to lie on a comfortable bed to begin the preparation for your EEG recording. Before the application of six small electrodes (discs), the technologist will apply a mild abrasive cleanser. This is followed with the use of a conductive cream to ensure a clearer recording for the physician to evaluate. An electrode cap is then placed on your scalp. Once each electrode cap site is also cleansed, you will continue to lie on the bed with your eyes closed and remain still while the technologist begins to record.

At this point, it is important to stay as alert as possible. The recording includes photic stimulation, a flashing-strobe light given in eight, ten second intervals. In addition, you may be required to conduct a breathing exercise referred as hyperventilation, which lasts three to five minutes. After the exercise, you may feel a slight dizziness or numbness in your hands or feet. These symptoms will lessen once you begin normal breathing. During part of the EEG recoding, the technologist may ask you to sleep.

What happens after the EEG?

The technologist will remove the cap and electrodes from your scalp. After these are removed, you may have cream and gel in some areas of your hair. These are water-soluble and can easily be removed with shampoo. Please feel free to bring hair accessories such as a hairbrush or comb, a hat or cap, a hair clip or ponytail holder.

When do I receive my EEG results?
Once the EEG recording is completed, the technologist will notify the neurologist at Hoag. After the neurologist has evaluated your recordings, the results will be sent to the ordering physician or referring physician. To obtain your results, please contact your ordering or referring physician.