Diagnostic Procedures
Diagnostic procedures may be needed to confirm a diagnosis suggested by the medical history and physical examination.
Spinal Tap
For a spinal tap (lumbar puncture), a sample of the fluid that surrounds the brain and spinal cord (cerebrospinal fluid) is withdrawn with a needle and sent to a laboratory for examination.
Examination of the cerebrospinal fluid can detect evidence of infections, injuries, tumors, and bleeding in the brain and spinal cord. These disorders may change the appearance and content of the cerebrospinal fluid, which is normally clear and colorless. For example, white blood cells in the cerebrospinal fluid make it appear cloudy and suggest a bacterial infection of the layers of tissue covering the brain and spinal cord (bacterial meningitis).
High protein levels in the fluid indicate damage to the brain, the spinal cord, or a spinal nerve root, which is the part of a spinal nerve next to the spinal cord, but they do not indicate the cause of the damage. Abnormal antibodies in the fluid suggest multiple sclerosis. Low sugar (glucose) levels suggest meningitis or cancer. Blood in the fluid suggests a brain hemorrhage. Many disorders, including brain tumors and meningitis, can increase the fluid's pressure.
Before performing a spinal tap, doctors use an ophthalmoscope to examine the optic nerve (see Section 20, Chapter 225), which bulges when the pressure within the skull is increased. If the pressure is increased because of a mass (such as a tumor) within the skull, performing a spinal tap may result in herniation of the brain (see Section 6, Chapter 87), a potentially fatal complication. Results of the neurologic examination may help doctors determine whether a mass is present. If doctors are still unsure, computed tomography (CT) of the head is performed.
 See the figure How a Spinal Tap Is Performed.
For a spinal tap, a needle is inserted between two vertebrae in the lower spine below the end of the spinal cord. During a spinal tap, doctors can directly measure the pressure within the skull. Pressure is measured by attaching a gauge (manometer) to the needle used for the spinal tap and noting the height of the cerebrospinal fluid in the gauge.
A spinal tap usually takes no more than 15 minutes and is usually performed at the person's bedside. A local anesthetic is used to numb the insertion site.
About 1 of 10 people develop a headache when they stand up after a spinal tap. The headache usually disappears after a few days to weeks. Other problems are very rare.
Computed Tomography
Computed tomography (CT) is a computer-enhanced scanning technique for analyzing a series of x-rays taken from different angles. A computer generates two-dimensional, high-resolution images that resemble anatomic slices of the organ being imaged.
CT can precisely detect a wide range of brain and spinal cord disorders, such as hydrocephalus, birth defects, tumors, areas of dead brain tissue due to stroke, and a ruptured or herniated disk. CT is used not only to diagnose neurologic disorders but also to monitor the effectiveness of treatment--for example, treatment of a brain abscess with antibiotics and treatment of brain cancer with radiation therapy. CT provides clearer images of abnormalities affecting the skull and spine and of bleeding due to hemorrhagic stroke during the first 24 hours afterward than does magnetic resonance imaging (MRI).
A person must lie still during the procedure, so that the image will not be blurred. The procedure may take 15 minutes to 1 hour, depending on the part of the body being scanned and the degree of resolution needed. For example, after an injury, fast CT with low resolution may be performed to obtain results quickly.
For spiral (helical) CT, the scanner rapidly rotates around the person and takes many x-rays in sequence. This procedure can provide images of blood vessels that are almost as clear as those obtained with magnetic resonance angiography (see Section 6, Chapter 77).
A radiopaque dye, which can be seen on x-rays, may be injected intravenously to enhance abnormalities in the images. When the dye is injected, the person may feel a warm sensation throughout the body. A few people have an allergic reaction to the dye.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) of the brain or spinal cord uses a magnetic field and very high frequency radio waves to produce highly detailed anatomic images. MRI detects most neurologic disorders (including previous strokes, most brain tumors, abnormalities of the brain stem and cerebellum, and multiple sclerosis) better than CT.
For the procedure, the person is placed in a tubular electromagnetic chamber and pulsed with radio waves, causing tissues in the body to emit radio signals back. These signals are converted to images. In about one fourth of the procedures, a substance that is weakly attracted by strong magnetic fields (paramagnetic contrast agent) is injected intravenously to enhance the images. No x-rays are involved, and MRI is usually very safe. As during CT, the person must lie still. A complete scan may take from 10 to 90 minutes, depending on the part of the body being scanned.
MRI can be used in different ways to investigate the brain. The same machine is used for all, but different software is used. Functional MRI can produce images of areas in the brain that are active in performing a task, whether it is reading, writing, remembering, calculating, or moving a limb. Another use involves identifying certain chemicals in small areas of the brain so that a brain tumor can be distinguished from a brain abscess. Perfusion MRI can be used to estimate blood flow in a particular area. Diffusion MRI can be used to detect the sudden accumulation of fluid (edema).
MRI cannot be used for people who have an artificial pacemaker, magnetic metallic clips (used to treat aneurysms), or other movable, magnetic devices in their body because the magnetic field may cause movement, overheating, or other dysfunction of the device. Other metal devices, such as an artificial hip and the rods used to straighten the spine, are not affected by MRI. CT is used when MRI cannot be. People who are dependent on ventilators can be attached to special ventilators that have no magnetic parts, or a technician can stay with them and manually supply them with oxygen using a bag and mask.
Giving people who are prone to severe claustrophobia a sedative may help. Alternatively, an MRI scanner that has one or more open sides (open MRI) can be used. This procedure eliminates the need for a person to be placed in a narrow tube. Open MRI is also useful for people who are overweight and cannot fit into the MRI tube. However, the images are somewhat less clear and less detailed than those produced by standard closed MRI.
Magnetic Resonance Angiography
Magnetic resonance angiography (MRA) is magnetic resonance imaging (MRI) that is used to produce images of blood vessels of the head and neck. MRA is often used with MRI to evaluate people who have had a stroke. MRA is also useful when the risks of cerebral angiography are too high for a particular person or when a person refuses cerebral angiography.
Unlike cerebral angiography, MRA is not an invasive procedure, does not require insertion of a catheter into an artery, and may not require an intravenous injection of a paramagnetic contrast agent. However, cerebral angiography provides more accurate images of blood vessels than MRA.
Echoencephalography
Echoencephalography uses ultrasound waves to produce an image of the brain. This simple, painless, and relatively inexpensive procedure is used mainly for children younger than 2 because their skull is thin enough for ultrasound waves to pass through. It can be performed quickly at the bedside to detect hydrocephalus (commonly called water on the brain) or bleeding. CT and MRI have largely replaced echoencephalography because they produce much better images, especially in older children and adults.
Positron Emission Tomography
In positron emission tomography (PET), a substance necessary for brain function (such as oxygen or sugar) is labeled with a radioactive molecule (radionuclide) that gives off positively charged signals (positrons) for a very short time. PET can provide information about seizure disorders, brain tumors, and strokes. However, functional MRI, which is less invasive and does not involve radioactivity, has replaced PET. PET is used mainly in research.
For the procedure, the labeled substance, called a tracer, is injected into a vein. It is distributed throughout the brain in about 1 minute. The person's head is placed within a ring-shaped PET scanner, which detects radiation from different angles and records sites of high activity. The more metabolically active an area of the brain, the larger the amount of tracer it takes up and the more radiation given off. The resulting scan shows the different levels of activity in different colors. For example, the procedure may show which part of the brain is most active during mathematical calculations. A computer can be used to construct a three-dimensional image of the area. The radioactivity is low level, does not injure the body, and disappears within hours.
Single Photon Emission Computed Tomography
Single photon emission computed tomography (SPECT) uses radionuclides to produce images of blood flow to the brain. The radionuclides are injected intravenously, reaching the brain through the bloodstream. How much of the radionuclide brain tissue takes up provides an estimate of how much blood is flowing through it. A rotating camera detects the energy (gamma-ray photons) given off by the radionuclide. A computer analyzes this information and constructs a cross-sectional or three-dimensional image. The procedure is not very accurate, and it is not as specific as positron emission tomography. It has been largely replaced by perfusion MRI.
Cerebral Angiography
Cerebral angiography (arteriography) is an invasive procedure used to detect abnormalities in the blood vessels of the brain. Cerebral angiography can detect a bulge in the weakened wall of an artery (aneurysm), inflammation of arteries (arteritis), a blood vessel (arteriovenous) malformation, or a blocked blood vessel, which can cause a stroke.
For the procedure, a catheter is inserted through an incision into an artery, usually in the groin. A local anesthetic is given to numb the insertion site. The catheter is then threaded through the aorta to an artery in the neck. After the catheter is in place, a radiopaque dye is injected through the catheter into the artery. The radiopaque dye outlines the blood vessels so that blood flow patterns in the brain can be seen on x-rays. The images of blood vessels produced by cerebral angiography are more detailed than those of MRA.
Color Doppler Ultrasonography
Color Doppler ultrasonography shows different rates of blood flow in different colors (see Section 3, Chapter 21). It is used mainly to measure blood flow through the arteries in the neck (carotid arteries) or through the arteries at the base of the brain (vertebral arteries, basilar arteries, circle of Willis, and middle cerebral arteries) and to identify and evaluate narrowing or blockage of these arteries. Thus, color Doppler ultrasonography can help assess the risk of stroke. The procedure is useful for evaluating people who have had a transient ischemic attack and people who have risk factors for atherosclerosis but no symptoms.
For this painless procedure, a handheld recording probe (transducer) that emits high-frequency (ultrasound) waves is used. The waves bounce off structures in the body and produce a moving image. After placing gel on the person's neck, the examiner moves the probe over that area. The probe is connected to a monitor that displays the image. Color Doppler ultrasonography can be performed at the person's bedside or in the doctor's office, is relatively inexpensive, and does not use x-rays.
Myelography
In myelography, x-rays of the spinal cord are taken after a radiopaque dye, usually iohexol, is injected into the cerebrospinal fluid via a spinal tap. Myelography has been largely replaced by MRI, which produces more detailed images, is simpler, and is safer. Myelography with CT is still used when additional detail of the spinal canal and surrounding bone, which MRI cannot provide, is needed. Myelography with CT is also used in emergencies when MRI is not available.
Electroencephalography
Electroencephalography (EEG) is a quick, simple, painless procedure in which the brain's electrical activity is recorded as wave patterns on a strip of moving paper or is collected in a database on a computer (see Section 6, Chapter 85). EEG can help identify seizure disorders, sleep disturbances, and certain metabolic or structural disorders of the brain. For example, EEG can show the characteristic patterns of electrical activity associated with confusion due to liver failure and the reduced electrical activity produced by damage to the brain (such as that due to stroke).
For the procedure, an examiner places small, round adhesive sensors (electrodes) on the person's scalp. The electrodes are connected by wires to a machine, which produces a record (tracing) of small changes in voltage detected by each electrode. These tracings constitute the electroencephalogram.
For people who have a seizure disorder, EEG is performed after a long period without sleep because sleep deprivation tends to increase seizure activity. EEG is also performed after the person is asked to breathe deeply and rapidly (hyperventilate) and is exposed to a flashing light, because both can trigger abnormal electrical activity.
Sometimes (for example, when a behavior that resembles a seizure is difficult to distinguish from a psychiatric disorder), the brain's electrical activity is recorded for 24 hours or longer while the person is monitored in the hospital by a television camera. The camera detects the seizure-like behavior, and examination of the EEG at that moment reveals either seizure activity or continued normal electrical activity, indicating a psychiatric disorder.
Evoked Responses
Stimuli for sight, sound, and touch are used to activate specific areas of the brain, that is, to evoke responses. For example, a flashing light stimulates the retina of the eye, the optic nerve, and the nerve pathway to the back part of the brain where vision is perceived and interpreted. Electroencephalography (EEG) is used to detect electrical activity evoked by the stimuli. Ordinarily, the brain's response to one stimulus is too slight to be detected by EEG. However, if many stimuli are used, the brain's responses to them can be averaged by a computer to produce a wave pattern.
Evoked responses provide information about how an area of the brain is functioning. This procedure is particularly useful in testing how well the senses are functioning in infants and children. For example, doctors can test an infant's hearing by checking for a response after a clicking sound is made at each ear. Evoked responses are also useful in identifying the effects of multiple sclerosis and other disorders on areas of the optic nerve, brain stem, and spinal cord. Such effects may or may not be detected by MRI.
Electromyography
In electromyography, small needles are inserted into a muscle to record the electrical activity of a muscle when the muscle is at rest and when it is contracting. Normally, resting muscle produces no electrical activity. A slight contraction produces some electrical activity, which increases as the contraction increases.
This procedure is used with nerve conduction studies to help doctors diagnose disorders of muscles, peripheral nerves, spinal nerve roots, and the neuromuscular junction. Disorders that weaken the nerve's connection to a muscle produce abnormal electrical activity in muscles. Examples of such disorders are carpal tunnel syndrome and diabetic neuropathy. Disorders of the muscle itself (the nerve is normal) produce a different kind of electrical activity. Polymyositis is an example of a muscle disorder.
Nerve Conduction Studies
Nerve conduction studies measure the speed at which motor or sensory nerves conduct impulses. Nerve conduction studies are used to determine whether symptoms such as muscle weakness are caused by a nerve disorder. If muscle weakness is caused by a nerve disorder (such as carpal tunnel syndrome, in which a nerve is pinched by ligaments in the wrist), the nerve conduction speed is usually slowed. If muscle weakness is caused by a muscle disorder, the nerve conduction speed remains normal. If muscle weakness is caused by a disorder of the brain or spinal cord, nerve conduction speed and electromyography results are normal. Weakness can also result when the connection between a normal nerve and a normal muscle (neuromuscular junction) malfunctions. Examples include myasthenia gravis, botulism, and diphtheria.
For the procedure, the nerve being tested is stimulated with a small charge of electricity to trigger an impulse. The charge may be delivered with several electrodes placed on the surface of the skin or with several needles inserted along the pathway of the nerve thought to be affected. The impulse moves along the nerve, eventually reaching the muscle and causing it to contract. By measuring the time the impulse takes to reach the muscle and the distance from the stimulating electrode or needle to the muscle, doctors can calculate the speed of the impulse.
A nerve may be stimulated repeatedly to determine how well the connection between the nerve and muscle is functioning. At this connection, the nerve impulse must cross from the nerve to the muscle. If this connection is malfunctioning (as in myasthenia gravis), nerve conduction studies using repeated stimulation of the nerve detect a progressively weaker response of the muscle.
| 
