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Neurological Diagnostic Tests

Neurosurgeons use a variety of diagnostic tests to help identify the specific nature of neurological diseases, conditions, or injuries. The results of these tests can help in planning an appropriate course of treatment. The following are descriptions of some of the most frequently performed diagnostic tests used by neurosurgeons.

Brainstem Auditory Evoked Response (BAER) Test

This test is used to diagnose hearing ability and may indicate the presence of brain stem tumors and multiple sclerosis. Electrodes are placed on the scalp and earlobes. Auditory stimuli, such as clicking noises and tones, are delivered to one or both ears through headphones. This test takes 30 to 45 minutes to complete.

Carotid duplex (also called carotid ultrasound)

In this procedure, ultrasound is used to help detect plaque, blood clots, or other problems with blood flow in the carotid arteries. A water-soluble gel is placed on the skin where the transducer (a handheld device that directs the high-frequency sound waves to the arteries being tested) is to be placed. The gel helps transmit the sound to the skin surface. The ultrasound is turned on and images of the carotid arteries and pulse wave forms are obtained. There are no known risks and this test is noninvasive and painless. This procedure takes 15 to 30 minutes to complete.

Cerebral angiography

(also called vertebral angiogram, carotid angiogram)

An angiogram provides accurate information that cannot be obtained from other tests. It can help detect the degree of narrowing or obstruction of an artery or blood vessel in the brain, head, or neck. It can also be used to detect the location and size of aneurysms and vascular malformations.

Arteries are not normally seen in an x-ray, so contrast dye is utilized. A local anesthetic is administered, the artery is punctured, usually in the leg, and a needle is inserted into the artery. A catheter (a long, narrow, flexible tube) is inserted through the needle and into the artery. It is then threaded through the main vessels of the abdomen and chest until it is properly placed in the arteries of the neck. This procedure is monitored by a fluoroscope (a special x-ray that projects the images on a TV monitor). The contrast dye is then injected into the neck area through the catheter, and x-ray pictures are taken. This procedure can take from one to two hours to complete.

Computed tomography (CT or CAT scan)

A diagnostic image is created after a computer reads x-rays. In some cases, a medication will be injected through a vein to help highlight brain structures. A CT scan can clearly show the smallest bones of the body as well as surrounding muscle and blood vessels. This makes it invaluable in diagnosing and treating cranial and spinal problems. A CT scan may be used for the following diagnostic purposes:

  • Locating brain damage in patients with head injuries
  • Detecting blood clots or bleeding in patients with stroke
  • Detecting some brain tumors
  • Illuminating enlarged brain ventricles (cavities) in patients with hydrocephalus
  • Assisting in planning radiation therapy for brain cancer
  • Detecting bleeding in a patients with ruptured aneurysms
  • Detecting spinal stenosis and herniated discs

A CT scan is a useful diagnostic test for hemorrhagic strokes because blood can easily be seen. However, damage from an ischemic stroke may not be revealed on a CT scan for several hours or days and the individual arteries in the brain cannot be seen. CT angiography (CTA) allows clinicians to see blood vessels of the head and neck and is increasingly being used instead of an invasive angiogram.

CT scanning is fast, painless, and simple and involves little radiation exposure. If contrast material is injected, a warm, flushed sensation may be experienced, as well as a metallic taste in the mouth for a few minutes. The CT scanner is a large, square machine with a donut-like hole in the center. The patient lies on a table that can move up or down and slides into and out of the center of the hold. Pillows may be used to help support the body and keep the patient still and in the proper position during the scanning process. Inside the machine, an x-ray tube on a rotating gantry moves around the body to produce the images, making clicking and whirring sounds as the gantry moves. A CT scan usually takes 10-15 minutes to complete.

Discography

This test is used to determine if intervertebral discs in the spinal column are a source of pain. CT and MRI scans only illustrate anatomy and cannot determine the source of pain. In some cases, discs may appear abnormal on MRI or CT scans, but are not the source of pain. A discogram can help identify discs that cause pain and can be a helpful diagnostic tool in planning a course of treatment, including surgery. A negative discogram can help prevent unnecessary surgery that may not be beneficial. Because of the nature of this test, discography is usually done only if the pain is significant enough to consider surgery.

During this procedure, an IV with antibiotics and relaxation medication is administered. The patient lies on his or her back for discography of the cervical spine and on the side for discography of the thoracic and lumbar spine. The physician numbs the skin, and then directs a small needle using x-ray guidance, into the space of the suspected disc. The procedure may be performed at more than one disc level at the same time. After the needle is properly positioned, a small amount of contrast dye is injected into each disc. Immediately thereafter, the patient is taken to the CT scan machine where additional images are taken. The entire procedure usually takes about one hour to complete.

Ice packs and over-the-counter and/or prescription pain medication can be used to help manage muscle soreness that may occur in the first few days after the procedure.

Doppler ultrasound

This test is used to help detect blood clots and blocked or narrowed blood vessels in nearly any part of the body, especially in the neck, arms, and legs. A water-soluble gel is placed on the transducer (a handheld device that directs the high-frequency sound waves to the artery or vein being tested) and the skin over the veins or arteries of the extremity being tested. There is a "swishing" sound on the Doppler if the venous system is normal. There are no known risks and this test is noninvasive and painless. This procedure takes 30 to 60 minutes to complete.

Electroencephalogram (EEG)

This test is used to help diagnose certain seizure disorders, brain tumors, brain damage related to head injuries, inflammation of the brain and/or spinal cord, alcoholism, specific psychiatric disorders, and metabolic and degenerative disorders that impact the brain. It is also used to confirm brain death in patients on life support.

About 16-20 electrodes are attached to the patient’s scalp, either with temporary, conducting glue or with extremely fine needles. The electrodes (also called leads) are small devices that are attached to wires and carry the electrical energy of the brain to a machine that reads them. A very low electrical current is sent through the electrodes and the baseline brain energy is recorded. Patients are then exposed to a variety of external stimuli, such as bright or flashing lights, noises or certain drugs. The patient is asked to open and close his or her eyes, or to change breathing patterns. The electrodes transmit the resulting changes in brain wave patterns. Since movement and nervousness can change brain wave patterns, patients usually recline in a chair or on a bed during the test. This test can take up to one hour, however, for certain disorders the patient must be asleep, and then the test will take a minimum of three hours.

Electromyography (EMG)

This test is used to determine more about the functioning of peripheral nerves in the arms and legs. It can show if a nerve is pinched, and estimate the severity and location of the pinched nerve. EMG tests for the electrical impulse coming from the brain and/or spinal cord to the affected area. If this impulse is blocked somewhere along the spinal pathway, it may be delayed or reduced en route to its final destination (skin, muscle, finger tips, etc.) Abnormal function may imply that there is nerve injury or muscle dysfunction.

Muscles receive constant electrical signals from properly functioning nerves, and in turn they broadcast their own electrical signals. During an EMG, the electrical activity in muscles is measured. The neurosurgeon places very thin needles (like those used in acupuncture) into the muscles to record the electrical signals from the various leg or arm muscles. If a muscle is not receiving adequate impulses from its nerve, it broadcasts signals that indicate the muscle is confused.

The results of this test are often correlated with the results from the Nerve Conduction Study, which is customarily done first. This test takes 15 to 45 minutes to complete, depending on how many areas are being studied.

Intrathecal Contrast Enhanced CAT Scan

This test is similar to a myelogram, which utilizes contrast or x-ray dye to better visualize the spinal canal and nerve roots in the spine. This test can be performed to help diagnose problems in the cervical, thoracic and lumbar spine. Following application of a topical anesthetic, a small sample of spinal fluid is removed by lumbar puncture. Then, using x-ray guidance, the sample is mixed with a very low dose of intrathecal contrast dye and is injected into the spinal sac in the lower back. The patient is then asked to move to a position that will allow the contrast fluid to travel to the area being studied. The dye enables the spinal canal and nerve roots to be visualized more clearly on a CT scan. The scan takes 45 to 60 minutes to complete. Following the test, some patients may experience discomfort and/or a headache, caused by removal of the spinal fluid.

Lumbar Puncture (Spinal Tap)

This is an invasive diagnostic test, usually done in the hospital, which uses a needle to remove a sample of cerebrospinal fluid from the space surrounding the spinal cord. The fluid is tested to detect bleeding or brain hemorrhages, diagnose infections of the brain and/or spinal cord, identify some cases of multiple sclerosis and other neurological conditions, and measure intracranial pressure.

The patient is asked to either lie on one side, in a ball position with knees close to the chest, or lean forward while sitting on a table or bed. A puncture site in the lower back is located, between two vertebrae, then the area is cleaned and a local anesthetic is administered. The patient may experience a slight stinging sensation from the injection. Once the anesthetic has taken effect, a special needle is inserted into the spinal sac and a small amount of fluid (usually about three teaspoons) is extracted.

There is usually a sensation of pressure when the needle is inserted; however, a common side effect is a headache, which can be alleviated by having the patient lie flat. Risk of nerve root injury or infection from the puncture can occur, but is rare. This procedure takes about 45 minutes to complete.

Magnetic Resonance Imaging (MRI)

This is a diagnostic test that produces three-dimensional images of body structures using magnetic fields and computer technology. The images result from different water concentrations of the various tissues. MRI is an important tool in planning surgery, radiation therapy, treatment for stroke, or other interventions for brain disorders. MRI may be used for the following diagnostic purposes:

  • Detecting brain and spinal cord tumors
  • Diagnosing nervous system disorders such as multiple sclerosis
  • Identifying diseases of the blood vessels including stroke
  • Diagnosing pituitary diseases
  • Detecting spinal stenosis and herniated discs

The conventional MRI machine consists of a closed cylindrical magnet in which the patient must lie totally still for short periods of time. MRI causes no pain, but some patients find it uncomfortable and claustrophobic, due to the close quarters and the range of sounds evoked by the scanning. "Open" MRI machines are now being used with increasing frequency at some facilities. MRI is a noninvasive procedure and can take from 15 to 60 minutes to complete, depending on the part of the body being imaged.

Magnetic Resonance Angiogram (MRA)/Magnetic Resonance Venogram (MRV)

This is a noninvasive study which is conducted in a magnetic resonance imager (MRI). The magnetic images are assembled by a computer to provide an image of the arteries in the head and neck. The MRA shows the actual blood vessels in the neck and brain and can help detect blockage and aneurysms. MRA is a noninvasive procedure and can take from 10 to 20 minutes to complete. A magnetic resonance venogram (MRV) images the veins using the same technology.

Myelogram

This is an older, invasive test that is still used in certain cases to examine the spinal canal and spinal cord. Following application of a topical anesthetic, a small sample of spinal fluid is removed by lumbar puncture. A contrast dye is injected into the spinal sac which mixes with the spinal fluid so that it will show up on x-rays. Anything that is pushing into the nerves will show up as an indentation pushing into the spinal sac. This indentation may indicate a herniated disc, lesions, tumors, or injury to the spinal nerve roots. The myelogram may be combined with a CT scan to get a better cross-section view of the spine.

Before the existence of CT and MRI, a myelogram was the best test to determine the cause of pressure on the spinal cord or spinal nerves. It is still used in patients who have metal plates and screws in their spine, which prevents them from undergoing MRI, and can make a CT scan difficult. This takes 45 to 60 minutes to complete. Following the test, some patients may experience discomfort and/or a headache, caused by removal of the spinal fluid.

Nerve Conduction Study

This is a sensitive test usually done in conjunction with an EMG. It utilizes electrical stimulation of a specific nerve or nerves and records the nerve’s ability to transmit an impulse. This study can determine if a nerve is functioning normally.

During this test, electrode patches are placed along the known course of the nerve. The nerve is then stimulated with a tiny electrical current at one point. The nerve transmits a signal along its course and an electrode placed further done the arm or leg can capture the signal as it passes. A normally functioning nerve will transmit the signal faster and stronger than a damaged nerve.

This procedure is usually done before the EMG if both tests are being done, and takes 15 minutes to one hour to complete, depending on how many areas are being studied.

Oculoplethysmography (OPG)

This is a noninvasive test used to detect the presence of blockage in the carotid arteries. The test indirectly measures the blood flow in the ophthalmic artery, which branches off the carotid artery and supplies blood to the eye. Suction cups are placed on the eyes and sensors are placed on the earlobes to record pulses.

If the pulse arrives at the ophthalmic arteries and the earlobes at the same time, this usually means that the carotid arteries are free of blockage. In contrast, if the pulse arrives at the eyes and the earlobes at different times, this may indicate that blood flow in these arteries is restricted. This test only estimates blockage, so if this is suspected, angiography may be ordered.

Positron Emission Tomography (PET) Scan

PET is a nuclear diagnostic test that can detect and stage most cancers. PET can also provide early information about heart disease and many neurological disorders, such as Alzheimer's disease. A PET scan examines the body's chemistry. A PET image can map the biological function of an organ, detect subtle metabolic changes, and may be used to determine if a tumor is benign or malignant.

The PET scan utilizes a machine called a cyclotron, which is an accelerator that propels charged particles using alternating voltage in a magnetic field to generate radioisotopes. The patient is injected with a radionuclide specific to the function or type of metabolism being tested for. The radionuclide will collect in that specific area of the body. The patient lies on a scanning table while a ring-shaped machine is positioned over the target area of the body. Detectors in the ring pick up gamma rays emitted from the body tissues. A computer analyzes the data and produces cross-sectional images on film and/or a video monitor. The test takes between one and two hours to complete.

Selective Nerve Root Block

Nerve roots exit the spinal cord and form nerves that travel into the arms or legs. These nerves allow movement of the arms, chest wall, and legs. These nerve roots may become inflamed and painful because of irritation from a bony spur or herniated disc in the spine. The block provides important information about which nerve is causing pain by temporarily numbing the specific nerve root.

The neurosurgeon directs a very small needle, using x-ray guidance near the specific nerve being tested. A small amount of contrast dye is injected to insure proper needle position. This may increase the normal level of pain for about 30 minutes. If pain is reduced after the injection, that nerve is most likely causing the pain. If the pain level is unchanged, that nerve is most likely not causing the pain. By confirming or eliminating the exact source of pain, this test allows the neurosurgeon to develop appropriate treatment, which may include further nerve blocks and/or surgery. The entire procedure takes two to three hours to complete.

Single Photon Emission Computed Tomography (SPECT) Scan

SPECT scan is a nuclear test that can provide information about blood flow to tissues and metabolic activities in the body. It is used frequently in patients with epilepsy to help pinpoint the area in the brain involved in producing seizures. It is also used to help identify certain types of tumors, and can provide early information about neurological disorders such as Alzheimer’s disease. It is useful in diagnosing certain spinal conditions including stress fractures, spondylosis, tumors and infections. A small amount of a radioactive drug is injected into a vein and a scanner is used to create detailed images of areas inside the body where the radioactive material is absorbed by the cells. The test takes between one and two hours to complete.

Somatosensory Evoked Response (SSER) Test

This test can detect problems with the fibers that transmit sensation from the body to the brain as they travel through the spinal cord, brainstem and cortex. Electrodes are attached to the wrist, the back of the knee, or other locations. A mild electrical stimulus is applied through the electrodes. Electrodes on the scalp then determine the amount of time it takes for the current to travel along the nerve to the brain, as well as its amplitude. This test takes about two hours to complete. It may be used during surgery to detect any injury to the brain and spinal cord.

Copyright, the American Association of Neurological Surgeons, June 2006.

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