In EEG, electrodes attached to areas of the patient's scalp record the brain's electrical activity and transmit this in
formation to an electroencephalograph, which records the resulting brain waves on recording paper. The procedure may be performed in a special laboratory or by a portable unit at the bedside.
- To determine the presence and type of seizure disorder
- To aid diagnosis of intracranial Iesions, such as abscesses and tumors
- To evaluate the brain's electrical activity in metabolic disease, head injury, meningitis, encephalitis, mental retardation, and psychological disorders
- To evaluate brain death.
- Explain to the patient that this test records the brain's electrical activity.
- Describe the procedure to the patient and family members, and answer all questions.
- Tell him that he must forgo caffeine before the test; other than this, there are no food or fluid restrictions.
- Thoroughly wash and dry his hair to remove hair sprays, creams, and oils.
- Explain that during the test, he'll relax in a reclining chair or lie on a bed and that electrodes will be attached to his scalp with a special paste. Assure him that the electrodes won't shock him.
- If needle electrodes are used, explain that he'll feel a pricking sensation as they're inserted; however, flat electrodes are more commonly used.
- Do your best to allay the patient's fears because nervousness can affect brain wave patterns.
- Check his medication history for drugs that may interfere with test results, for example, anticonvulsants, antianxiety agents, sedativehypnotics, and antidepressants. Withhold these medications for 24 to 48 hours before the test, as ordered by the doctor
- A patient with a seizure disorder may require a "sleep EEG." In this case, keep the patient awake the night before the test, and administer a sedative (such as chloral hydrate) to help him sleep during the test.
- If the test is performed to confirm brain death, provide family members with emotional support.
Procedure And Posttest Care
- Position the patient on the bed or reclining chair. Reassure him as the electrodes are attached to his scalp.
- Before the recording procedure begins, instruct the patient to close his eyes, relax, and remain still.
- During the recording, observe the patient carefully; note blinking, swallowing, talking, or other movements, and record these imdings on the tracing. These activities may cause artifacts on the tracing and be misinterpreted as abnormal tracing.
- The recording may be stopped at intervals to let the patient rest or reposition himself. This is important because restlessness and fatigue can alter brain wave patterns.
- After an initial baseline recording, the patient may be tested under various stress-producing conditions to elicit patterns not observable while he's at rest. For example, he may be asked to breathe deeply and rapidly for 3 minutes (hyperventilation), which may elicit brain wave patterns typical of seizure disorders or other abnormalities. This technique is commonly used to detect absence seizures. Also, photic stimulation tests central cerebral activity in response to bright light, accentuating abnormal activity in absence or myoclonic seizures.
- Review carefully the reinstatement of anticonvulsant medication or other drugs withheld before the test.
- Carefully observe the patient for seizure activity and provide a safe environment.
- Help the patient remove electrode paste from his hair.
- If the patient received a sedative before the test, take safety precautions, such as raising the bed's side rails.
- Observe the patient carefully for seizure activity.
- If seizure activity occurs, record seizure patterns and be prepared to provide assistance. Have suction equipment and diazepam or lorazepam for I.V. injection readily available.
EEG records a portion of the brain's electrical activity as waves; some are irregular, whereas others demonstrate frequent patterns. Among the basic waveforms are the alpha, beta, theta, and delta rhythms.
Alpha waves occur at a frequency of 8 to 11 cycles/second in a regular rhythm. They're present only in the waking state when the patient's eyes are closed but he's mentally alert; usually, they disappear with visual activity or mental concentration. Beta waves (13 to 30 cycles/second) - generally associated with anxiety, depression, and use of sedatives - are seen most readily in the frontal and central regions of the brain. Theta waves (4 to 7 cycles/second) are most common in children and young adults and appear in the frontal and temporal regions.
Delta waves (0.5 to 3.5 cycles/second) normally occur only in young children and during sleep.
Usually, about 100 pages of recordings are evaluated, with particular attention paid to basic waveforms, symmetry of
cerebral activity, transient discharges, and responses to stimulation. A specific diagnosis depends on the patient's clinical status.
In seizure disorders, EEG patterns may identify the specific disorder. In absence seizures, the EEG shows spikes and waves at a frequency of 3 cycles/second. In generalized tonicclonic seizures, it generally shows multiple, high-voltage, spiked waves in both hemispheres. In complex partial seizures, the EEG usually shows spiked waves in the affected temporal region. And in patients with focal seizures, it usually shows localized, spiked discharges.
In patients with intracranial lesions, such as tumors or abscesses, the EEG may show slow waves (usually delta waves but possibly unilateral beta waves). Vascular lesions, such as cerebral infarcts and intracranial hemorrhages, generally produce focal abnormalities in the injured area.
Generally, any condition that causes a diminishing level of consciousness alters the EEG pattern in proportion to the degree of consciousness lost. For example, in a patient with a metabolic disorder, an inflammatory process (such as meningitis or encephalitis), or increased intracranial pressure, the EEG shows generalized, diffuse, and slow brain wavess.
- Interference from extraneous electrical activity; head, body, eye, or tongue movement; or muscle contractions (possible production of excessive artifact)
- Noise in the environment (possible patient reactions)
- Hair spray and gel (possible faulty electrode contact with skin)
- Caffeine, anticonvulsants, barbiturates, antidepressants, hypnotics, and tranquilizers and other sedatives (possible masking of seizure activity)
- Acute drug intoxication or severe hypothermia resulting in loss of consciousness (flat EEG )