Chapter 11 - Neurologic Tests
Questions
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.
An electroencephalogram is the electrical recording of brain waves in cerebral cortical neurons.
Brain waves fall in different frequencies. Delta waves are the slowest (<3 Hz); theta waves (4-7.5Hz); alpha waves (8-12Hz); beta waves (<12Hz).
Electromyography is the electrical recording made from muscle. The overall activity from the muscle can be recorded from the surface of the body (surface EMG), but a needle placed in the muscle is necessary if responses from individual muscle fibers are to be recorded or if the morphology of motor units is to be evaluated.
A motor unit is a motor neuron and all of the muscle fibers to which it is connected.
A fasciculation is the spontaneous activation of a motor nerve axon and all of the muscle fibers attached to it. There is usually an involuntary twitch of the muscle seen on the skin surface. Persistent fasciculations in a weak muscle are usually indicative of motor neuron disease though transient fasciculations can be seen in overworked or fatigued muscles. A fasciculation potential is the electrical activity recorded during a fasciculation.
A fibrillation is the involuntary contraction of an individual muscle fiber. Normal muscle fibers never contract without a nerve stimulus and, therefore, the entire motor unit response is seen. Denervated muscle fibers become spontaneously active and fibrillation potentials and positive sharp waves (the electrical signals from a single muscle fiber ) can be seen.
A nerve conduction study tests the speed and amplitude of conduction along a peripheral nerve. The speed of conduction is a reflection of the health of the fiber (particular the large, myelinated fibers) while the amplitude is reflective of the signal that is transmitted. Motor conduction studies and sensory conduction studies test motor and sensory fibers, respectively.
Compound muscle action potential is the electrical signal recorded from the skin surface following electrical stimulation of the nerve to the muscle.
Sensory nerve action potential is the electrical signal recorded from the skin surface directly over a sensory nerve following electrical stimulation of the nerve elsewhere along its course.
A lumbar puncture (spinal tap) is the insertion of a needle between the lamina of the vertebrae and into the thecal sac with the purpose of obtaining cerebrospinal fluid. This is done below the L2 level (usually between L3-4 or L4-5) in order not to injure the spinal cord.
Oligoclonal bands (of protein) are specific bands of IgG in spinal fluid seen with electrophoresis of the CSF.
IgG synthesis rate is the measure of how much immunoglobulin is being synthesized in and around the brain. This can also be expressed as the IgG synthesis ratio. It is elevated in infectious or inflammatory disease of the brain.
Pleocytosis is the presence of white blood cells in the cerebrospinal fluid.
Opening pressure is the pressure measured in the CSF (expressed in millimeters or centimeters of water) right after entering the thecal sac with the needle.
11-1. What is recorded in the electroencephalogram?
11-1. The electroencephalogram records brain waves, electrical rhythms arising from activity of neurons in the cerebral cortex
11-2. What are the normal brain waves when awake?
11-2. The normal waves when awake and relaxed, with the eyes closed are: alpha waves posteriorly and beta waves elsewhere else.
11-3. What is seen during sleep?
11-3. The specific findings in sleep stages include: delta waves in slow-wave sleep (stage 3 and 4 sleep); and vertex sharp waves and sleep spindles in stage 2 sleep.
11-4. What kinds of waves are seen over damaged brain tissue?
11-4. Slowing of waves. This is a nonspecific finding of brain dysfunction from any cause.
11-5. What does diffuse slowing of brain waves indicate?
11-5. If the slowing is diffuse, it usually indicates diffuse encephalopathy (often due to toxic, metabolic or drug-related effects on the brain).
11-6. What findings are seen in epileptic patients?
11-6. In between seizures, epileptiform discharges are often seen in areas that generate seizures. These are not necessarily associated with actual seizures although they are almost always seen during seizures. There are usually very fast frequencies seen during the actual seizure.
11-7. How often are EEGs abnormal in patients with epilepsy?
11-7. About 60% of patients with epilepsy have epileptiform discharges in the interictal period (between seizures) on a single EEG. If you do 3 EEGs, about 85% will show abnormalities,
11-8. What does electromyography refer to?
11-8. Electromyography is the electrical record of muscle activity. It can be recorded from the surface of the body if overall muscle function is to be examined. Needles placed in the muscle can record from individual muscle fibers.
11-9. What does the electromyogram show in a muscle at rest?
11-9. Normal muscle is electrically silent at rest. When muscles are severely damaged or when muscle fibers are disconnected from their nerve, they may become spontaneously active.
11-10. What is seen in a muscle that is denervated?
11-10. Muscle fibers that are separated from their motor nerve fiber show fibrillation potentials and positive sharp waves beginning about a week or so after damage of the nerve.
11-11. What is recorded in the muscle when the muscle is voluntarily contracted?
11-11. When muscles are contracted, motor units can be recorded. These are the electrical discharges of the motor nerve fiber and all of the muscles to which it is attached (the motor unit). As more force is exerted, the individual motor units fire faster and more motor units are recruited.
11-12. What happens to motor units with time after partial denervation of a muscle?
11-12. When nerves to a muscle have been damaged, they attempt to regenerate. Also, surviving nerve fibers in the muscle sprout in order to reinnerve denervated muscle fibers. The size of the motor unit increases (i.e., the number of muscle fibers attached to a given nerve fiber). This results in a giant motor unit potential, indicating a chronic process that has had time to attempt to recover (usually over months).
11-13. What are motor and sensory nerve conduction studies?
11-13. A motor nerve conduction study includes stimulating a motor nerve at several places along its course and recording from a muscle that is attached to the nerve. Sensory nerve conduction requires recording the electrical response directly in the nerve to stimulation of a sensory nerve. The distance between the various stimulation sites and the location of recording, along with the time until the response is seen (the latency) can be use to compute the speed of conduction.
11-14. How fast do nerves conduct in the limbs?
11-14. Conduction velocities are compared to normal and to those in the unaffected limb. Generally, sensory conduction is about 5 m/sec slower than motor conduction and upper limb conduction is about 5 m/sec faster than lower limb. In the upper limb, the lowest normal motor conduction is about 45 m/sec and lowest normal sensory conduction is about 40 m/sec.
11-15. What are the advantages of CT scans?
11-15. They are fairly quick to perform and relatively widely available. The create excellent images of bone lesions and are good for recent hemorrhage into tissues (such as in or around the brain). Therefore, they are very useful in trauma.
11-16. What are the limitations of CT scans?
11-16. There are several. CT scans use x-rays, with potential harmful effects of ionizing radiation. They can only be done in the axial (horizontal) plane (although they can be computer “reformatted” in other planes). They have trouble creating images of soft tissue, especially near thick bone. Therefore, lesions of brain parenchyma are not well seen (unless large).
11-17. What are the advantages of MRI scans?
11-17. MRI creates the best image of brain paraenchyma (infection, inflammation, stroke). Bone does not interfere with the image at all. MRI creates the most sensitive image for detecting early stroke (perfusion and diffusion-weighted images). Images can be made in any plane of interest. You can also make images of the blood vessels (MRAngiogram or MRVenogram).
11-18. What are the limitations of MRI scans?
11-18. Patients may be too claustrophobic or unable to stay still (and motion artifact is a major problem). It also takes quite a while to create an image and it is difficult to continue treatment and monitoring during the procedure. You cannot do MRI with certain implanted devices (defibrillator, pacemaker, certain aneurysm clips, cochlear implant, etc). Metal objects create a lot of artifact.
11-19. What is the role of IV contrast in MR and CT scans?
11-19. Contrast given IV during CT or MRI scan does not get into normal brain. Therefore, contrast entering a lesion (“enhancement” of a lesion) shows conditions that break down the blood-brain barrier. This highlights such lesions as: tumors, infections, severe inflammation, etc. Some patients may be allergic to CT dye (iodine) and a heavy dye load can impair renal function.
11-20. What are angiograms?
11-20. Conventional angiography involves taking x-ray images while introducing contrast into blood vessels. This creates the best image of blood vessels (arteries or veins, depending on how long you wait. It does involve heavy dye loads (which can be decreased somewhat by digital subtraction angiography. Recently, CT angiograms may be done by 2-D and 3-D reconstructions after intravenous contrast administration and MRA can also show blood vessels (although artifacts are more problematic with this technology).
11-21. What is tested with a lumbar puncture?
11-21. Lumbar puncture/spinal tap is performed below the L2 level (to avoid the spinal cord). Cerebrospinal fluid pressure is measured and collected fluid is tested for protein, glucose, cells, microorganisms and other specialized tests depending on the suspected clinical problem.
11-22. What is indicated by and elevated protein level in the CSF?
11-22. Protein in the spinal fluid is normally less than 45mg/dl. Elevation of protein level is nonspecific but indicates some problem with the blood-brain barrier (it is mostly seen in neoplasm, infection, inflammatory lesions and slight elevations are seen in diabetics).
11-23. What are oligoclonal bands and what do they signify?
11-23. Oligoclonal bands (of protein) are specific bands of IgG in spinal fluid seen with electrophoresis of the CSF. This finding indicates that there are very specific antibodies present in the CSF and indicates the presence of some inflammatory or infectious condition (multiple sclerosis or Lyme disease, for example).
11-24. What is a normal CSF glucose level and what do abnormalities indicate?
11-24. Glucose level should be more than 1/2 of the blood glucose level. It may be decreased in certain infections, severe inflammations and some neoplasms (especially carcinomatous meningitis).
11-25. What is the significance of finding white blood cells in the spinal fluid?
11-25. There are normally only a few WBCs in spinal fluid, and these are overwhelmingly lymphocytes. Increase suggests certain inflammations or infections (increased lymphocytes may suggest viral infection; neutrophils (PMNs; polys) are usually due to bacterial infections, for example).
11-26. What is the significance of finding red blood cells in the spinal fluid?
11-26. There should be no red cells in spinal fluid. A “traumatic tap” is suggested by many red cells in the first tube collected with quite a bit of clearing in subsequent tubes. However, hemorrhage should be considered if there are more than a few RBCs.
11-27. How are infections of the nervous system evaluated?
11-27. There are many types of cultures that can be done (depending on the organism) and there are some tests for bacterial or fungal antigens or PCR for certain viral agents (particular Herpes simplex).
11-28. What conditions elevate CSF pressure?
11-28. Pressure is elevated by anything that takes up space, inhibits resorption of CSF or blocks CSF flow (spinal taps should not be done if increased pressure is suspected until an images shows that the CSF pathways are all open).