Overview of Neurodiagnostic Tests
Neurodiagnostic tests, also called neurodiagnostics, are done when a patient's illness or condition is thought to be based in the central nervous system (brain and spinal cord). Signs of nervous system disorders include the following:
- Chronic headaches
- Hearing and vision changes
- Numbness and tingling
There are two types of neurodiagnostic testing: imaging tests or scans (e.g., x-ray, CT scan, MRI scan, PET scan) and electrical impulse detection (e.g., EEG, EMG). Neurodiagnostic testing may be conducted as part of a neurological exam.
Computerized tomography (CT scan) uses x-ray technology to produce multiple cross-section images. In addition to providing images of the brain and nervous system, they can be used to identify broken bones, tumors, blood clots, heart disease, and internal bleeding.
Magnetic resonance imaging (MRI scan) uses a powerful magnet combined with radio waves to examine organs, soft tissues, and skeletal structures. MRI scans are especially valuable in finding brain and spinal cord abnormalities. They may also be used to help diagnose torn ligaments, tumors, circulation (blood flow) problems, inflammation (e.g., arthritis), and infection.
Positron emission tomography (PET scan or PET imagery) uses radiation from the emission of positrons (extremely small particles discharged from a radioactive substance) to create images that can help detect and evaluate cancer and the effects of cancer therapy. This test also can be used to diagnose a variety of neurological conditions, including memory disorders, tumors, and seizure disorders.
In a PET scan, a small amount of a radioactive substance is combined with a natural body compound (e.g., glucose, water, or ammonia) and is injected into the patient. Once the substance has had time to circulate through the body and accumulate in the tissues (generally 30 to 90 minutes), a PET scanner is used to detect the various levels of energy given off by the positrons and produce images. Different colors or levels of brightness indicate different levels of organ and tissue function. Cancer tissue, for example, uses more glucose than normal tissue and shows up brighter on the PET scan.
PET Scan Preparation
PET scans are outpatient procedures that generally take 1 to 2 1/2 hours to perform. Patients are advised to wear loose, comfortable clothing; avoid eating for 4 hours prior to the appointment; and drink water before the appointment. Because the PET scan relies on glucose level activity, patients who have diabetes should consult a physician about how to control glucose levels during the procedure to avoid false test results.
It is important to arrive on time for PET scan appointments. The radioactive substance that is used has a short lifespan and it must be prepared immediately before the procedure to remain effective during the scan. Once the substance is injected, patients are advised to rest and avoid significant movement, so that the radioactive material can be properly localized in the tissue. Scanning usually begins after 30-90 minutes.
PET Scan Procedure
The PET scanner is a large machine that is shaped like a doughnut. Inside the hole in the middle are several rings of detectors that record energy emission and send the information to computers, where the images are displayed. The patient lies on a padded examination table and is moved into the hole. The scan takes approximately 3045 minutes. During this time, the patient must remain as still as possible.
Most people do not experience discomfort during a PET scan. However, patients who are claustrophobic or uncomfortable holding still for a long period of time may experience anxiety. The radioactive substance does not cause any physical sensations. After the procedure, patients should drink plenty of fluids to flush out the radioactive substance. There are no restrictions on activity following a PET scan.
Benefits & Risks of PET Scan
The primary benefit of positron emission tomography is that it can detect minor changes in biochemical levels, even before these changes show up as tumors or other abnormalities. In addition, the radiation exposure is lower than x-rays or CT scans.
There are some risks, however, primarily to women who are pregnant or nursing, due to the injection of the radioactive substance into the body. The potential risks to the fetus must be weighed against the benefits of the PET scan. Women who may be pregnant should inform the PET technicians before the procedure.
An electroencephalogram (EEG or brain wave test) is a noninvasive procedure used to detect and record brain cell activity. Because normal brain waves have recognizable frequencies and amplitudes, variations or abnormalities in brain waves may suggest the following:
- Attention deficit disorders (e.g., ADHD)
- Seizure disorders (e.g., epilepsy)
- Tumors or abscesses in the brain
- Head injury
- Encephalitis and other inflammatory diseases
- Cerebral hemorrhages
- Cerebral infarct (stroke)
- Sleep disorders
- Alcohol or drug abuse
- Migraines (in some cases)
- Syncope (loss of consciousness)
An EEG also is used to confirm brain death in a patient who is comatose. This neurodiagnostic test is not used to measure intelligence or diagnose mental illness.
Patients usually are advised to wash their hair the night before the procedure, using no oils, sprays, or conditioners. Patients also should avoid caffeine for at least 8 hours prior to an EEG. If the test will be used to measure brain activity while sleeping, the patient way be asked to reduce or even eliminate their sleep time the night before. Some patients are advised to stop the use of medications before an EEG, but it is important not to do so unless directed by a physician.
In an electroencephalogram, small metal disks (electrodes) are attached to the scalp to detect and record brain wave activity. These disks are attached by wires to a machine that records and converts electrical signals from the brain into wavy lines on a moving sheet of graph paper.
Before the procedure begins, an EEG technician attaches 16 to 25 disks to the scalp using a sticky paste while the patient lies still on a table or reclining chair. In some cases, the patient is asked to perform a function, such as breathing deeply and rapidly, or looking at a blinking light. Generally, there is no discomfort associated with an EEG, although some patients may be bothered by the feeling of the disks.
This neurodiagnostic test normally lasts 1 to 2 hours and is performed on an outpatient basis. However, patients who have suspected or known seizure disorders may be given a 24-hour EEG.
In this procedure, the electrodes are attached to a portable lightweight recording box that is worn over the shoulder with a strap. During the test period, the patient or a family member pushes an event button every time a seizure occurs. After 24 hours, the box is returned to the EEG center for evaluation of the test results. In some cases, patients are admitted to a hospital for extended EEG testing.
Risks of EEG
In general, there are no risks associated with an EEG. However, in patients who have seizure disorders, a seizure may be triggered by the use of flashing lights or by a request to hyperventilate.
Electromyography (EMG, myogram, or nerve conduction test) is a neurodiagnostic test used to assess the health and function of the muscles and the nerves that control the muscles. It can be used to determine whether muscle weakness and loss of muscle strength are caused by a muscle injury or by an underlying neurological disorder.
EMG can be used to detect many disorders and conditions, including the following:
- Amyotrophic lateral sclerosis (ALS, Lou Gehrig's disease)
- Carpal tunnel syndrome
- Guillain-Barre syndrome
- Muscular dystrophy
- Myasthenia gravis
- Sciatic nerve dysfunction
There is no special preparation required for an EMG. Patients may be advised to avoid the use of creams and lotions on the day of the test.
EMG ProcedureWhen a muscle is at rest, it is electrically "silent." When a muscle is contracted, it generates a variety of signals known as “action potential.” An EMG measures this action potential.
During an EMG, a needle electrode is inserted through the skin into the muscle being tested and the patient is asked to contract the muscle in some way. The electrical activity produced during the test is displayed as waves on an oscilloscope, and may be heard on a speaker as loud pops. The presence, size, and shape of the waves indicate the ability of the muscle to respond to nerve stimulation and can help diagnose nerve or muscle damage. The test normally takes 30 to 60 minutes to perform.
During the test, there may be some discomfort when the needle is inserted into the muscle. Following an EMG, the area may feel tender or sore for a few days.