Chapter 11 - Neurologic Tests
Many tests are used specifically for the evaluation of neurologic illnesses (Table 11-1). The general physician should be familiar with the diagnostic capabilities of all these tests and should be able to interpret the results of the lumbar puncture.
The technique of lumbar puncture is learned in the ward, so only a few points are made here. Spinal taps should not be done above the L2-3 interspace because the spinal cord may be present above this level. If the flow of spinal fluid is not brisk, the following check is simple (Figure 11-1): The manometer is tilted horizontally to increase the flow and then brought back to the vertical position so that the fluid falls to the true spinal fluid pressure. If this maneuver fails to increase the flow, the needle is not properly placed or a nerve root has blocked the needle. Fluid should not be aspirated because this may trap and damage nerve roots. If the initial CSF pressure is high (more than 180 mm H2O), the physician should try to let it fall without removing fluid; the patient should be encouraged to relax, and their neck and legs should, if necessary, be unflexed (thus reducing venous pressure). S/he should not be asked to hyperventilate (this causes cerebral vasoconstriction and lowers even truly high CSF pressures). S/he should be reassured, made comfortable, and allowed to rest for one minute (not much more). Even if the pressure is high, enough CSF should be removed for the performance of routine tests (about 6-9 cc; 2-3 cc in each tube).
Cell counts should be made in the first and last tubes. The fluid is then examined for color (compare with a tube of water against a white background) and then analyzed for protein and sugar. The whole CSF or the sediment can be stained for microorganisms (in appropriate cases). Analysis for bacterial or fungal antigens or polymerase chain reaction for viruses or tuberculous bacilli should be employed depending on the clinical situation. A separate tube (usually tube 2) should be sent for cultures. Fungal and AFB cultures should be requested when indicated. CSF protein electrophoresis and measurement of IgG synthesis (which requires comparison of spinal fluid with a blood sample) can give hints to the presence of an immune/inflammatory condition. Myelin basic protein concentration can confirm damage to CNS myelin.
Although elevated CSF pressure should raise the question of an intracranial mass lesion, intracranial pressure may also increase when there is an obstruction to the flow of CSF or when the intracranial venous pressure rises (Table 11-2). Reduced CSF pressure can be artifactual (needle not properly placed, or blocked by a nerve root - check flow) or caused by CSF block (as by a spinal tumor or cerebellar herniation through the foramen magnum), general dehydration, prior lumbar punctures (the result of CSF leakage through puncture holes in the arachnoid and dura), or a posttraumatic or spontaneous CSF leak.
This is a yellowish discoloration of the fluid that is usually the result of hemolyzed blood and takes two to six hours to develop after bleeding into the CSF. Initially, the supernatant looks pink; the hemoglobin pigments are then transformed into bilirubin, so that after a day the supernatant looks yellow. The supernatant from a traumatic tap should be clear and colorless if centrifuged (since the red cells have not broken down). Other causes of xanthochromia are given in Table 11-3.
Cells (Table 11-4)
Traumatic taps are common, and it is very important to be able to identify them the first time (i.e., without having to repeat the lumbar puncture). The criteria given in Table 11-5 are used. The presence of red blood cells (that are not due to a traumatic tap) indicates bleeding into the CSF due to subarachnoid hemorrhage or extension of cerebral hemorrhage to the subarachnoid space.
White cells are the result of inflammatory or infectious processes. High PMNs are usually indicative of bacterial infections (although they can be seen in early, acute viral processes). High lymphocytes are usually a sign of viral or chronic processes, although lymphoma should be considered.
An elevated level of protein in the CSF is non-specific and has many causes (Table 11-6). Protein electrophoresis may reveal other abnormalities; increased proportions of gamma globulins may be seen in persons with multiple sclerosis, SSPE, or neurosyphilis. The CSF IgG: albumin ratio (using an electroimmunodiffusion method) and the demonstration of oligoclonal IgG bands (using agarose electrophoresis of concentrated CSF) provide evidence of antibody production within the CNS, and are helpful in the diagnosis of multiple sclerosis (but are also positive in chronic CNS infection and other chronic inflammatory disease). The presence of myelin basic protein in the spinal fluid is supportive evidence for the diagnosis of multiple sclerosis and other demyelinating diseases (although it is a non-specific finding in other causes of damage to CNS myelin).
The CSF glucose is normally one half to two thirds of the serum value. For this to hold, ideally the spinal tap should be done under fasting conditions. Changes in CSF glucose lag from 30 minutes to two hours behind changes in the serum glucose. The causes of low CSF glucose are reviewed in Chapter 17. Infectious (particularly bacterial), or neoplastic conditions are the most common cause of low values. Chronic bacterial or fungal infections give rise to the lowest values. Severe inflammation of the meningies can also decrease the level.
When infection is suspected, stains for acid-fast bacilli and gram stains should be obtained on the spun down sediment, even if there are no WBC's in the CSF (e.g., alcohol, when present, will suppress white cell migration and the CSF may have bacteria on staining and no white cells until the alcohol is metabolized by the liver). There are rapid antigen tests for several common bacteria and fungi (particularly cryptococcus).
This is a rapid test for the presence of DNA. It is particularly useful for viral infections (Herpes simplex is the classic example) and for mycobacterium tuberculosis.
While most conventional bacteria will show growth in 1-3 days, note that tubercle bacilli and fungal cultures may take four to eight weeks to grow out.
Viral cultures may be appropriate in some cases. Cytology on the cells can be used to detect neoplastic cells. However, if there is a suspicion of neoplasia, a large volume of fluid should be collected. There are also some chemical markers (for certain types of tumors) that can be measured in the CSF in neoplasia. Flow cytometry can be used to determine whether lymphocytes are monoclonal (such as occurs in lymphoma).
A lumbar puncture must be done if a treatable infection is suspected. Not only can it determine the presence of infection but also provides information on the nature of the infection and how to treat it. Furthermore, since there are other conditions that can present similarly to infection, LP can narrow the differential.
LP can be hazardous in the setting of a mass lesion (such as abscess). The process of draining fluid below such a lesion may result in shift of the brain. Therefore, if a CT scanner is available, imaging should be done prior to the LP. If there will be any significant delay in obtaining the scan, empiric antibiotics can be given up to several hours prior to performance of the LP without seriously compromising the diagnostic value of the LP. A scan may also be diagnostic and (such as in the case of intracranial bleeding) so that lumbar puncture may not be necessary.
In addition to infection, lumbar puncture may be critical for the diagnosis of subarachnoid hemorrhage. It is included in the evaluation of many neurologic conditions (such as stroke and seizures) because abnormalities may point to various possible etiologies. There is usually an indication for doing a lumbar puncture; however, this should be weighed against the possible complications. Often (as in the diagnosis of tumor or abscess) better and less risky tests are available and should be done first.
Infection overlying the site of lumbar puncture is a relative contraindication. The puncture should be shifted to another level. A spinal tap should be contraindicated by the reasonable suspicion of an intracerebral mass, unless the suspicion of acute infection is stronger and time does not allow for CT scanning within a couple of hours to exclude the possibility of a mass lesion. This should be a very rare situation in modern emergency medicine. This is a particular issue with cerebral abscess, where both mass and infection are present. Other contraindications or cautions are listed in Table 11-7.
Complications (Table 11-8)
Post-tap headaches are fairly common. Their incidence may be reduced by: (1) using a small needle (20 or 22 gauge) and forcing fluids after the tap. It is presumed that post-tap headaches are caused by leakage of CSF from the puncture in the dura in excess of CSF production, so that a low-volume, low-pressure environment is created in the intracranial space. The headache is typically present only when the patient is in the sitting or erect position, indicating that it is caused by the brain settling against basal structures with stretching of pain-sensitive surface blood vessels and meningies. Rarely, the puncture hole may remain open for long periods (weeks or months) and injection of homologous blood into the site (a "blood patch") or even surgery may be necessary to stem the leak and alleviate the debilitating headaches.
Meningitis and significant hemorrhage, while possible, is exceedingly rare.
Occasionally the sixth nerve(s) will be stretched enough by the settling to cause a partial or complete palsy.
We presented this information in some detail because the lumbar puncture is an important test that is frequently performed, often by a nonspecialist. Failure to measure pressure, count cells properly, note the presence of xanthochromia, or request the appropriate tests from the laboratory results in inadequate or frankly misleading information. Unfortunately, such failures are common, even in good hospitals. It is therefore important to master the preceding information.
This is an exceedingly rare procedure. When CSF cannot be obtained from the lumbar space (and when its analysis is considered critical to treatment), a cisternal tap may be required. The needle is placed in the midline, passing just under the occipital bone, into the (usually large) cisterna magna (Figure 11-2). This is technically fairly easy; however, if the needle is advanced too far it can enter the medulla, sometimes causing sudden respiratory arrest and death. The test should therefore be carried out only by experienced physicians (usually neurosurgeons or neurologists). An alternative route that may be used by neurosurgeons and neuroradiologists is lateral to C-1 with penetration through the large C-1 intervertebral hiatus.
The cisternal tap may be used in myelography when the upper margin of a spinal block needs to be defined, however, magnetic resonance imaging (MRI), has become the procedure of choice for defining the upper and lower limits of spinal cord or spinal cord compressing lesions. It is necessary at times in the intrathecal administration of irritating medications, such as amphotericin B. Medications are diluted more rapidly in the larger and more rapidly circulating volume of cisterna magna than in the smaller lumbar sac.
Routine skull x-rays are often of little clinical value because the skull is not affected by most intracranial events. Skull x-rays may be useful under certain circumstances, particularly trauma (Tables 11-9 and 11-10). However, the role of radiographs in detecting conditions such as fracture or sinusitis has largely be supplanted by CT scans.
X-rays of the cervical, thoracic, and lumbosacral spine may detect misalignment, fractures, metastatic lesions, degenerative changes, and occasionally enlargement of a foramen or of the walls of the spinal canal from tumors. Considerable discrepancy may exist between the x-rays and the clinical picture; severe degenerative changes in the cervical and lumbar spine may be almost asymptomatic, whereas a protruded intervertebral disk may produce considerable pain or focal neurologic deficit without causing any abnormality on the plain x-ray.
Oblique views of the cervical spine demonstrate the neural foramina. The lateral cervical spine x-ray demonstrates the AP diameter of the spinal canal (Figure 11-3). Persons with congenitally narrow canals (AP diameter of 12 mm or less (normal range is 14-22) are more likely to develop spinal cord compression with trauma or with the degenerative changes that occur with age (cervical spondylosis).
Positron emission tomography (PET) and less expensive single photon emission computerized tomography (SPECT) are imaging techniques using radioactive isotopes, which reflect brain metabolism and can be useful in helping determine or confirm epileptic seizure foci, in particular in patients with intractable epilepsy who may be candidates for neurosurgical intervention to remove the focus. They also can help differentiate the various forms of dementia, which affect specific areas of cerebral cortex (e.g., Alzheimer's parietal-temporal preference and frontal lobe dementia). Such scans may be helpful in distinguishing neoplastic from other mass lesions. In addition to these clinical uses, PET has been used for studying normal brain functions but will likely be superceded by functional MRI (see below). PET is expensive and not likely to be of routine use (outside of examination for cancer) and SPECT will likely continue to have limited application.
Numerous techniques have been developed to help detect stenosis or occlusion of the carotid arteries. Turbulent and accelerated flow in a narrowed carotid artery can be detected using Doppler imaging techniques; B-mode ultrasonography can visualize carotid stenosis and may be able to detect ulcerations in the wall of the carotid artery; M-mode can detect accelerated flow in the stenotic region. While ultrasound is rapid, convenient and non-invasive, it is not as accurate as some of the imaging techniques and should be considered a screening modality. Magnetic resonance angiography and CT angiography are newer techniques visualizing the extra- and intracranial vasculature and, although not as accurate as conventional angiography, are gaining acceptance as they become technically more sophisticated and reliable. In the case of CT angiography, images can be obtained that are nearly as accurate as conventional angiography.
In this procedure, water-soluble contrast is introduced into the lumbar subarachnoid space (via lumbar puncture). Because the contrast used is heavier than spinal fluid, it can be moved up and down the spinal canal by tilting the patient appropriately. Normally, one sees the outline of the spinal canal, the spinal cord, and the proximal part of the nerve roots. The following abnormalities also may be seen (Figure 11-6):
- Extradural defects (from herniated disks, tumors, etc.), which indent the thecal sac.
- Intradural extramedullary defects (as from meningiomas or neurofibromas), which appear between the edge of the thecal sac and the shadow of the spinal cord.
- Intramedullary masses (syringomyelia, ependymoma, glioma, etc.), which expand the shadow of the spinal cord.
- A-V malformations and fistulas on the surface of the cord may be seen as serpiginous filling defects.
Complications are the same as those for lumbar puncture. Contrast use occasionally is associated with seizures and encephalopathy if it is allowed to spill into the intracranial subarachnoid space, but because of its water solubility it is absorbed rapidly and arachnoiditis is an unlikely complication. Standard myelography is being replaced today by low-volume dye injection into the subarachnoid space followed by a delayed CT scan which gives a much more detailed picture of the spinal canal contents and by spinal MRI which delineates soft tissue pathology better than CT myelography (CT myelography shows the bone structure).
Arteriography (angiography) *
Contrast material can be injected through a catheter passed into the aorta (from the femoral artery usually). It is possible to visualize both carotids and both vertebrals simultaneously (arch aortogram, four-vessel study) or one may selectively catheterize each of the four vessels.
Angiography is the most accurate method for visualizing vascular pathology. This includes abnormalities in the extracranial cerebral circulation (the carotid and vertebral arteries and their origins) and intracranial vascular abnormalities (aneurysms, AV malformations, vasculitis and occlusive disease). In addition, displacement of arteries or veins can be seen with tumors or other mass lesions, or with ventricular enlargement. Some neoplasms (for example meningiomas and metastatic hypernephromas) are heavily vascularized and will show as a blush on arteriography. Extracerebral mass lesions such as subdural hematomas can be clearly delineated. In addition to defining lesions, an increasing number of intracranial conditions can be treated by endovascular techniques, requiring angiography.
Although the CT scan and MRI (see below) can study blood vessels without the morbidity of angiography, they are not as accurate as angiography in defining vascular anatomy. However, improvements in CT angiography have made it nearly as good for defining vascular lesions. Angiography is still better at defining the vascular anatomy of a lesion (such as a tumor) that is to be treated surgically. The new technique of magnetic resonance angiography (MRA) is attractive in that it is not invasive and, therefore, not associated with risk. Additionally, MRA is often coupled with imaging of the brain structure to provide a more complete picture of intracranial disease. The advent of very fast CT scanners has promoted the development of CT angiographic procedures (CTA) that include the relatively low-risk administration of intravenous contrast. These less invasive techniques are still in development, although they will likely supersede conventional diagnostic angiography to a large degree in the future. On the other hand, conventional, catheter angiography will continue as a way to treat certain conditions.
Permanent morbidity or death (usually from stroke) occurs in less than 0.5% of patients, depending on the experience and skill of the arteriographer and the condition of the patient. Persons with vascular disease (e.g., hypertensives, diabetics) have a higher incidence of complications than other patients. Patients must be selected with some care for this procedure; the benefits must be weighed against the risks. The use of computer assisted digital subtraction technique has decreased the amount of dye needed for angiography and therefore the risks.
A thin beam of x-rays is scanned across the patient's head in a large series of angles around the entire head and at sequential cross-sectional planes (termed "slices") (Figure 11-7). The results, instead of being developed on x-ray film, are fed into a computer, which solves simultaneous equations for each of many thousand points. The computer thereby determines the radiographic density for each of these points and results can be printed out as an image or numbers (in Hounsfield units - after the Nobel prize winning inventor of the CT scan) can be determined for any point. Differences in density that are too small to be detected by plain x-rays are demonstrated. The resulting picture consists of a section through the brain in which the subarachnoid spaces, brain, ventricles, and other details of brain anatomy are seen. Tumors, hemorrhages, cysts, hydrocephalus, and other conditions can be identified. The dose of radiation is similar to that of a routine skull series. Intravenous injection of positive contrast material may increase the yield of the procedure by increasing the density of lesions that have a defective blood-brain barrier (some neoplasms, chronic subdural hematomas, abscesses, infarcts, some demyelinating plaques, etc.).
Because computerized axial tomography is noninvasive, it has become an important screening test. It is useful in the diagnosis of infarction, tumor, trauma, hemorrhage, hydrocephalus, atrophy, and other processes. It makes the diagnosis of cerebellar hemorrhage, heretofore difficult, relatively easy. It makes it possible to follow ventricular size (and thereby to detect progressive hydrocephalus) without subjecting the patient to discomfort or risk. Water-soluble contrast (metrizamide) injected into the lumbar subarachnoid space can provide adequate contrast so that the shape of the spinal cord and of lesions impinging on the subarachnoid space can be visualized on the CT scan. Scans of the lumbosacral spine, reconstructed in crossection, without contrast, give good resolution of herniated and bulging discs, but are especially good for demonstrating bony spinal stenosis, usually the result of chronic osteoarthritic changes in the facet joints.
CT scanning has become an essential evaluation tool in acute trauma not only because it can define fractures and dislocations very accurately, but also due to the fact that extravasated blood is quite dense and readily visible.
Some intracranial lesions may be missed. For example, subdural hemorrhages may be missed because they lie close to the skull, and both blood and bone appear dense. In fact, some chronic subdural hematomas may be of the same radiographic density as the surrounding brain (isodense) and appear only as a shift of brain substance to the opposite side. Vascular lesions, such as aneurysms and AV malformations, are better diagnosed by arteriography (or CT angiography). Small irregularities in the ventricular system and some pituitary fossa abnormalities are better detected by magnetic resonance imaging (MRI), because MRI resolution is greater. Lesions in the posterior fossa and near the base of the skull are difficult to observe on CT scans due to distortion of the x-rays by the very thick bone in the skull base. Lesions in these locations are much better visualized by MRI scanning, as are lesions within the brain paraenchyma (i.e., strokes, demyelinating plaques, etc.).
This newer system follows on the heels of CT scanning, which was hailed as the greatest neurodiagnostic advance of the century. MRI is even more impressive, however, it has not replaced CT scanning which continues, for the time being, to be more available, less expensive, quicker, and better at defining some things (e.g., it images bone and recent hemorrhage better than MRI).
MRI, as its name implies, uses electromagnetic waves (not ionizing radiation) to form reconstructions of tissue and is especially suitable for imaging the intracranial and intraspinal contents. A powerful magnetic field is used to align hydrogen-containing dipoles (almost exclusively water molecules). A radiofrequency pulse is delivered to the body, adding energy to the molecules and moving them away from their position of alignment. Following the pulse the water molecules return to their resting (aligned) state, releasing quanta of energy (or "echos") which can be detected. This energy release is proportional to the concentration of water molecules present in the tissue and the timing of release is dependent on the freedom of the water molecules to move in the environment (which is dependent on the chemical environment in which the molecule exists). Computer analysis of the emissions allows construction of an image with excellent soft tissue contrast resolution which can be displayed in virtually any plane and which is free from bony artifact (there is no signal from compact bone).
Small ischemic, demyelinating and neoplastic lesions are much better shown then by CT scan. Additionally, newer procedures have been developed (diffusion- and perfusion-weighted images) that are capable of detecting strokes as they are happening and define their stages and the amount of brain that is at risk. Intravenous contrast materials have been developed, such as gadolinium (DTPA), which enhance the signal from any lesions that contain disrupted blood-brain barrier. Areas where bony artifact obscures CT images such as the temporal lobes, cerebellum and brainstem, the region of the foramen magnum, pituitary gland and acoustic nerve are more clearly delineated by MRI. The movement of blood leaves a void on the reconstruction because the protons do not remain available for emission, once flipped. This can be seen as a "flow void" on the scans and allows some analysis of the patency of the larger arteries at the base of the brain and in the neck. Several techniques have been developed to incorporate the information in the MR image into a detailed image of the blood vessels, processes that are generically called magnetic resonance angiography (MRA). This has become an improving and increasingly important technique for imaging aneurysms and arteriovenous malformations although, at least for aneurysms, it does not yet match conventional angiography.
Special MRI techniques are now available for measuring functional localization in the cortex (functional MRI - fMRI). The amount of deoxygenated hemoglobin in a brain region will affect the amount of magnetic signal that is obtained. Therefore, comparing the signal from a brain region before and during some stimulus, action or thought can detect a change in blood flow (blood oxygen level detection - BOLD imaging). This information can be superimposed on a highly detailed MR image of the brain. For example, attention-getting visual stimulation of either the left or right visual fields will increase metabolism in, and therefore blood-flow to, the appropriate calcarine cortex, which can be mapped.
Magnetic resonance spectroscopy can provide information on tissue chemical makeup of a brain region. This technique is becoming increasingly useful in differentiating various pathologies such as neoplasms, ischemic lesions, abscess and demyelinating plaques.
In addition to some limitations in availability of MRI scanners (and issues of cost), there are several practical and theoretical limitations. Because bone has a paucity of hydrogen ions it is poorly seen. Bony lesions such as fractures and osteoarthritic changes are best seen with CT. MRI scans are susceptible to many artifacts based on motion, or magnetic distortion (particularly by any metal in the region). The scanner itself may damage or reprogram the circuits of pacemakers, implanted stimulators or automated defibrillators. It also may traction ferromagnetic implants or clips. Some old aneurysm clips could be dislodged (more recent clips are made of titanium, which is not ferromagnetic). Usually larger implants (such as hip replacements, etc) are not a problem (although they may warm up a bit in the scanner). Very recent steel vascular clips may be dislodged.
There are quite severe issues of patient tolerance. The patient must be very still and the space is quite small (there are larger, "open" scanners, but they are less available and the image is not as good). Confused, young or uncooperative patients may not tolerate this procedure and patients with claustrophobia also have difficulty. General anesthesia can be used but the patient is difficult to monitor during this rather long procedure. This issue of difficult monitoring also complicates the use of MR scanning of patients who are acutely ill and makes it less helpful in trauma cases. Monitoring or therapeutic equipment that has any ferromagnetic components has to be kept out of the room with the scanner, providing dome difficulty in use.
The electric activity of the brain can be recorded from electrodes applied to the scalp. The pattern of the EEG varies with age and with the state of attention. In the normal adult, the EEG consists of low-voltage, fast activity when the patient is alert and attentive; alpha rhythm (8-13 cps) is more prominent when the eyes are closed and the patient is not concentrating. Slower activity is seen in persons who are drowsy, and specific patterns characterize the different stages of sleep.
Pathologic processes may produce EEG abnormalities (Table 11-11). Minor abnormalities are seen in a high percentage of brain-damaged persons, but since they are also seen in as many as 10-15% of normal persons, they are of little diagnostic value. Conversely, a normal EEG does not rule out serious organic disease affecting the hemispheres and certainly does not "rule out" epilepsy (about 40% of patients with documented epilepsy have normal EEGs in between seizures).
Special "activating" techniques may be used to bring out abnormalities, especially in the diagnosis of seizure disorders. Hyperventilation is used routinely. Photic stimulation (flashing lights) may evoke synchronous discharges in normal persons ("photic driving") and may provoke seizures in susceptible individuals. Sleep is useful in bringing out seizure discharges in many persons.
Special placement of electrodes may help diagnose conditions that are not apparent on the routine EEG. Nasopharyngeal electrodes (inserted through the nose, Figure 11-4) record activity from the orbitofrontal and mesial temporal regions remote from electrodes placed on the scalp. These are used principally in the diagnosis of limbic (temporal lobe) complex partial epilepsy.
Increasingly, prolonged, continuous monitoring of the EEG is useful in the assessment of "spells" that may be seizures. This is often accompanied by video monitoring of the patients behavior. This may be done with scalp electrodes, however, the EEG is attenuated and diffused by the meningies, skull and scalp. Intracranial electrodes, either applied to brain surface by positioning in the subdural space (electrocorticogram), or from electrodes in the substance of the brain (depth electrodes) may increase accuracy in detecting and localizing abnormal activity. These measures are sometimes used in the evaluation of refractory seizure disorders, especially prior to surgery.
Electrical responses to sensory stimuli are usually too small to be detected in the routine EEG. By computer averaging responses to repeated stimuli, however, clear-cut potentials can be demonstrated to visual, auditory and somatosensory stimuli. By changing the placement of the recording electrodes, the evoked potential can be made to reflect cortical (as in the case of the standard visual evoked response), brain stem (as in the brain stem auditory evoked response) or spinal cord (as with somatosensory evoked responses) electrical events. Changes in latency or amplitude of specific components of the evoked response reflect abnormalities in conduction in specific sensory pathways, and thus can help to document the presence and location of lesions. This may be used to particularly good effect in patients with suspected demyelinating disease. Since areas of demyelination markedly slow conduction velocity, evoked responses have proved sensitive in detecting subclinical lesions in persons with multiple sclerosis. The visual evoked response (VER) has been shown to be the most useful in providing evidence for multifocality of lesions, a criterion for diagnosing multiple sclerosis (see Chapter 14).
Motor and sensory nerve conductions can be determined from peripheral nerves superficial enough to be stimulated transcutaneously. The technique for determining motor nerve conductions is illustrated in Figure 11-5. The muscle action potential is recorded from C, the median nerve is stimulated at B, and the latency (time from stimulus to response) is recorded. Similarly the latency from stimulating the nerve at A is determined. Latency A minus latency B represents the time it takes for the nerve impulse to travel from A to B. The distance from A to B divided by this time is the conduction velocity. The nerve conduction velocity is normal (about 40-70 m/second) as long as there are some fast-conducting fibers left in the nerve. A normal nerve conduction does not, therefore, rule out a peripheral neuropathy. Demyelinating neuropathies (see Chapter 21 ) produce marked slowing of the nerve conductions. Axonal neuropathies may produce some increase in distal latency, but usually the nerve conductions are normal or only slightly reduced. Focal slowing may be detected in cases of nerve compression which as a rule initially cause a demyelinating lesion.
Sensory nerve conductions are determined by stimulating the skin and recording from the appropriate nerve. The potential recorded is very small, and consequently very mild nerve injuries alter or abolish it.
A needle electrode is inserted in the muscle. Normally there is no muscle activity at rest, but with minimal voluntary contraction, individual motor unit potentials are seen. These represent the summation of the membrane action potentials of many muscle fibers, all innervated by the same anterior horn cell (the motor unit). With increasing contraction, more motor units are recruited, the firing rate increases, and a dense interference pattern is seen in which the baseline of the EMG is no longer visible.
When the muscle is partially denervated, there are changes in the excitability of muscle fibers, so that after about three weeks muscle fibers may fire at rest (fibrillations). If denervated fibers are reinnervated by neighboring healthy nerve axons (a process that takes place over months), motor units are formed that are larger than normal. As the number of motor units decreases, the interference pattern seen with maximal contraction becomes less dense, and large polyphasic cross innervation units are seen.
The number of motor units remains normal, and therefore the interference pattern is normal except in the very late stages. The individual units are smaller because some muscle fibers have dropped out. The motor unit potentials are, therefore, on the average, smaller in amplitude and shorter in duration. More units must be fired to produce a particular strength of contraction, so the full interference pattern is achieved earlier. Polyphasic potentials are also seen. The EMG picture of brief, small-amplitude, abundant, polyphasic potentials (BSAPP) is characteristic (although not pathognomonic) of myopathy. Although usually the muscle is silent at rest, fibrillations can occur, and are often seen in persons with polymyositis, perhaps a result of denervation caused by damage to the intramuscular nerves.
In normal persons muscle activity ceases abruptly with muscle relaxation. In patients with myotonia, repetitive afterdischarges are seen. This correlates with clinical myotonia (see Chapter 21 ) but is more sensitive. Cases of myotonic dystrophy can therefore be diagnosed by EMG before clinically obvious myotonia is present.
Repetitive stimulation of a nerve produces little attenuation of the summated muscle action potential at rates of 3 to 30 per second. In persons with myasthenia gravis, however, this attenuation is marked because neuromuscular transmission is marginal at the start. Conversely, in the "myasthenic" syndrome associated with carcinoma (the Eaton-Lambert syndrome), repetitive stimulation augments the muscle response.
Define the following terms:electroencephalogram; alpha/beta/theta/delta waves; electromyogram; motor unit; fasciculation; fibrillation; nerve conduction study; compound muscle action potential (CMAP); sensory nerve action potential (SNAP); lumbar puncture (spinal tap); oligoclonal bands; IgG synthesis; pleocytosis; opening pressure.
11-1. What is recorded in the electroencephalogram?
11-2. What are the normal brain waves when awake?
11-3. What is seen during sleep?
11-4. What kinds of waves are seen over damaged brain tissue?
11-5. What does diffuse slowing of brain waves indicate?
11-6. What findings are seen in epileptic patients?
11-7. How often are EEGs abnormal in patients with epilepsy?
11-8. What does electromyography refer to?
11-9. What does the electromyogram show in a muscle at rest?
11-10. What is seen in a muscle that is denervated?
11-11. What is recorded in the muscle when the muscle is voluntarily contracted?
11-12. What happens to motor units with time after partial denervation of a muscle?
11-13. What are motor and sensory nerve conduction studies?
11-14. How fast do nerves conduct in the limbs?
11-15. What are the advantages of CT scans?
11-16. What are the limitations of CT scans?
11-17. What are the advantages of MRI scans?
11-18. What are the limitations of MRI scans?
11-19. What is the role of IV contrast in MR and CT scans?
11-20. What are angiograms?
11-21. What is tested with a lumbar puncture?
11-22. What is indicated by and elevated protein level in the CSF?
11-23. What are oligoclonal bands and what do they signify?
11-24. What is a normal CSF glucose level and what do abnormalities indicate?
11-25. What is the significance of finding white blood cells in the spinal fluid?
11-26. What is the significance of finding red blood cells in the spinal fluid?
11-27. How are infections of the nervous system evaluated?
11-28. What conditions elevate CSF pressure?