Diagnostic procedures may be needed to confirm a diagnosis suggested by the medical history and physical examination.
Imaging tests such as computed tomography (CT—see Computed Tomography), magnetic resonance imaging (MRI—see Magnetic Resonance Imaging), angiography (see Angiography), positron emission tomography (PET—see Radionuclide Scanning), and Doppler ultrasonography (see Ultrasonography) are commonly used to diagnose neurologic disorders.
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Cerebrospinal fluid flows through a channel (the subarachnoid space) between the layers of tissue (meninges) that cover the brain and spinal cord. This fluid, which surrounds the brain and spinal cord, helps cushion them against sudden jarring and minor injury.
For a spinal tap (lumbar puncture), a sample of cerebrospinal fluid is withdrawn with a needle and sent to a laboratory for examination.
The cerebrospinal fluid is checked for evidence of infections, tumors, and bleeding in the brain and spinal cord. These disorders may change the content and appearance of the cerebrospinal fluid, which normally contains few red and white blood cells and is clear and colorless. For example, the following findings suggest certain disorders:
Doctors do not do a spinal tap when the pressure within the skull is increased, for example, when there is a mass (such as a tumor or abscess) in the brain. In such cases, a spinal tap may suddenly reduce pressure below the brain. As a result, the brain may shift and be pressed through one of the small openings in the relatively rigid tissues that separate the brain into compartments (called herniation—see Herniation: The Brain Under Pressure). Herniation puts pressure on the brain and may be fatal. The medical history and neurologic examination help doctors determine whether herniation is a risk. For example, doctors use an ophthalmoscope to examine the optic nerve (see What Is an Ophthalmoscope?), which bulges when the pressure within the skull is increased. As another precaution before a spinal tap is done, CT or MRI of the head is often done to check for masses.
For a spinal tap, people typically lie on their side in a bed and draw their knees to their chest. A local anesthetic is used to numb the insertion site. Then, a needle is inserted between two vertebrae in the lower spine below the end of the spinal cord.
During a spinal tap, doctors can 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.
About 1 of 10 people develops a headache when standing up after a spinal tap. The headache usually disappears after a few days to weeks. Other problems are very rare.
Echoencephalography uses ultrasound waves to produce an image of the brain. This simple, painless, and relatively inexpensive procedure can be used in children younger than 2 years because their skull is thin enough for ultrasound waves to pass through. It can be done 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.
In myelography, x-rays of the spinal cord are taken after a radiopaque dye is injected into the subarachnoid space via a spinal tap. Myelography has been largely replaced by MRI, which produces more detailed images and is simpler and safer to do. Myelography with computed tomography (CT) is used when doctors need more detail of the spinal cord and surrounding bone than MRI can provide. Myelography with CT is also used when MRI is not available or cannot be done safely (for example, when a person has a heart pacemaker).
Electroencephalography (EEG) is a simple, painless procedure in which the brain's electrical activity is recorded as wave patterns, printed on paper, and/or recorded in a computer (see Brain Activity During a Seizure). EEG can help identify seizure disorders, sleep disturbances, and certain metabolic or structural disorders of the brain. For example, EEG can help identify where a seizure originates and show changes in electrical activity associated with confusion, which may result from disorders such as liver failure (liver encephalopathy) or certain drugs.
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 (the EEG).
If a seizure disorder is suspected but the initial EEG is normal, another EEG is done after using a tactic that makes seizure activity more likely. For example, the person may be deprived of sleep, be asked to breathe deeply and rapidly (hyperventilate), or be exposed to a flashing light (stroboscope).
Sometimes (for example, when 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 people are monitored in the hospital by a video camera. The camera detects the seizure-like behavior, and by examining the EEG at that moment, doctors can determine whether brain activity indicates a seizure or is normal, indicating a psychiatric disorder.
Stimuli for sight, sound, and touch are used to activate specific areas of the brain, that is, to evoke responses. EEG is used to detect the response evoked by the stimuli. Based on these responses, doctors can tell how well those areas of the brain are working. 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.
Evoked responses are 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.
Evoked responses can also help predict the prognosis for people who are in a coma. If stimuli do not evoke typical brain activity, the prognosis is likely to be poor.
Electromyography and Nerve Conduction Studies
Electromyography and nerve conduction studies help doctors determine whether muscle weakness, sensory loss, or both result from injury to the following:
In electromyography (EMG), small needles are inserted into a muscle to record the electrical activity of the 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. The record produced by EMG is called the electromyogram. It is abnormal if muscle weakness results from a problem with a spinal nerve root, peripheral nerve, muscle, or neuromuscular junction. Each type of problem produces a distinctive pattern of abnormalities, which can be identified based on the person's symptoms and results of the examination and electromyography. Unlike CT or EEG, which can be done routinely by technicians, EMG requires the expertise of a neurologist, who chooses the appropriate nerves and muscles to test and interprets the results.
Nerve conduction studies measure the speed at which motor or sensory nerves conduct impulses. A small electrical current stimulates an impulse along the nerve being tested. The current may be delivered by several electrodes placed on the surface of the skin or by several needles inserted along the pathway of the nerve. 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 nerve conduction. The nerve may be stimulated once or several times (to determine how well the neuromuscular junction is functioning). Results are abnormal only if the symptom results from a problem with a nerve or neuromuscular junction. For example,
However, results may be normal if the affected nerves are small and do not have a myelin sheath (the outer layer of tissues that helps nerves conduct impulses faster). Results are also normal if the disorder involves only the brain, spinal cord, spinal nerve roots, or the muscle. Such disorders do not affect the speed of nerve conduction.
Muscle and nerve:
Occasionally, doctors cannot determine the cause of the nerve damage or muscle weakness based on results of blood tests, imaging tests, EMG, or nerve conduction studies. In such cases, doctors typically refer the person to a specialist, who may remove a small sample of nerve and usually muscle tissue and examine it under a microscope (biopsy). The sample is removed from an area of the body where symptoms occur. The sample is stained to help doctors identify the pattern of muscle or nerve damage and to determine whether white blood cells (which indicate inflammation) are present.
Often, the sensory nerve examination and EMG do not detect damage of the nerves that sense pain or that automatically regulate body processes (called autonomic nerves). Doctors may suspect such damage if people have less sensitivity to pain, have burning pain in their feet, feel dizzy or light-headed when they stand up, or sweat too much or too little. To check for this damage, doctors may use a small round cutter to remove a sample of skin (punch skin biopsy) and send it to a laboratory to be examined under a microscope. If the nerve endings in the skin sample have been destroyed, the cause may be a disorder (such as vasculitis—see Overview of Vasculitis) that affects small nerve fibers, including pain-sensing and autonomic nerves fibers (see Autonomic Nervous System Disorders).
Genetic abnormalities cause many neurologic disorders—particularly those that affect movement, including those that cause tremor or problems walking. Genetic tests are done if results of the examination and other tests suggest a genetic abnormality. When genetic testing is recommended, people are usually referred to a genetic counselor. If they are not, people may request an appointment with one.
Last full review/revision August 2012 by Michael C. Levin, MD