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 see Common Imaging Tests: Computed Tomography), magnetic resonance imaging (MRI—see see Common Imaging Tests: Magnetic Resonance Imaging), angiography (see see Common Imaging Tests: Angiography), positron emission tomography (PET—see see Common Imaging Tests: Radionuclide Scanning), and Doppler ultrasonography (see see Common Imaging Tests: Ultrasonography) are commonly used to diagnose neurologic disorders.
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For a spinal tap (lumbar puncture), a sample of cerebrospinal fluid is withdrawn with a needle and sent to a laboratory for examination.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:
Before doing a spinal tap, doctors use an ophthalmoscope to examine the optic nerve (see What Is an Ophthalmoscope?), which bulges when the pressure within the skull is increased. If the pressure is increased because of a mass (such as a tumor or abscess), a spinal tap is not done because it may suddenly reduce pressure below the brain. As a result, the brain may shift and be pressed through one of the small natural openings in the relatively rigid tissues that separate the brain into compartments (called herniation—see Head Injuries: Herniation: The Brain Under Pressure). Herniation puts pressure on the brain and is potentially fatal. The medical history and neurologic examination help doctors determine whether herniation is a risk. But CT or MRI of the head is usually more accurate and is often done as a precaution before a spinal tap is done.
For a spinal tap, people typically lie on their side 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 and is usually done at the person's bedside.
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 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 computed tomography (CT) is used when additional detail of the spinal canal and surrounding bone, which MRI cannot provide, is needed. 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 and printed on paper or recorded in a computer (see Seizure Disorders: 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 identify where a seizure originates and show the characteristic electrical activity associated with confusion due to liver failure (liver encephalopathy).
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 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 video 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. Video EEG is also used when preparing a person with epilepsy for surgery to see what type of seizure results from an abnormality in the particular brain area in which the seizure originates.
Stimuli for sight, sound, and touch are used to activate specific areas of the brain, that is, to evoke responses. 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. EEG is used to detect electrical activity evoked by the stimuli.
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.
Electromyography and Nerve Conduction Studies
Electromyography and nerve conduction studies help doctors determine whether muscle weakness, sensory loss, or both results 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 EMG is abnormal if muscle weakness results from a problem with a spinal nerve root, peripheral nerve, muscle, or neuromuscular junction. The EMG produces a distinctive pattern of abnormalities. 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 findings.
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,
Disorders that affect only the brain, spinal cord, spinal nerve roots, or the muscle do not affect the speed of nerve conduction.
Last full review/revision October 2007 by Michael Jacewicz, MD