branch of the median nerve to a skin surface elec-trode site over the median nerve just proximal to the wrist. A delayed median nerve sensory latency suggests compression of the nerve at the carpal tunnel.
NEUROMUSCULAR JUNCTION FUNCTION
Information about the function of the neuromuscu-lar junction can be obtained from repetitive nerve stimulation studies. Placement of a skin recording electrode over the belly of a muscle and stimulating the motor nerve produces a compound muscle action potential (CMAP). If the nerve stimulation is repeated, the CMAPs appear identical on the oscillo-scope. In diseases of the neuromuscular junction, the amplitude of the CMAPs may decrease or increase.
In myasthenia gravis and botulism, repetitive nerve stimulation produces a decremental response in the CMAP. The test is safe, inexpensive, somewhat uncomfortable, and takes about 15 minutes.
SENSORY EVOKED POTENTIALS
Occasionally there are indications to evaluate the integrity of central conduction along major sen-sory pathways (visual, auditory, and peripheral sensory system); these are called evoked potentials.
As noted above, actual conduction velocities can-not be obtained, but central modality-specific latencies can. Evoked potential tests record com-puter averages of the EEG that are time locked to repeated (100–500 trials) specific sensory stimuli such as sound, light, or electrical stimulation of the peripheral nerve. The computer averaging reduces background EEG electrical activity to 0 while enhancing the time-locked stimulus signal.
Abnormalities are characterized by a delay for the time-locked signal average to reach its destination or distortions (usually a prolongation of the wave-form and loss of signal amplitude). Sensory evoked potentials are safe, inexpensive, and com-fortable. The major indication is the evaluation of possible diseases that cause central nervous system (CNS) demyelination of these sensory pathways.
lins (MW 150 kd). In addition, some proteins are made by the choroid plexus (transthyretin) and secreted into CSF. Finally, complex transport sys-tems exist in blood vessels of the brain and the CSF pathways to remove ions or proteins (such as potassium) or deliver molecules (glucose) to the CSF. These transporter systems may be active (requiring energy such as potassium–sodium transporter from mitochondria) or passive (no
energy requirement, such as the glucose trans-porter) and generally maintain their respective molecules within narrow concentrations. For the above reasons, the CSF-to-plasma concentration ratios vary greatly between molecules.
Approximately 2/3 of CSF is produced by the choroid plexuses located in the lateral and fourth ventricles (Figure 3-5). The source of the remain-ing CSF is unclear. Choroid plexus CSF travels 30 FUNDAMENTALS OF NEUROLOGIC DISEASE
Arachnoid Granulations Superior Sagittal Sinus
Subarachnoid Choroid Plexus of
3rd Ventricle Choroid Plexus of
Lateral Ventricle
Central Canal of Spinal Cord
Filum Terminale
Dura Mater
Choroid Plexus of 4th Ventricle
Lumbar Cistern Lateral Aperture
Figure 3-5 Cerebrospinal fluid flow in the central nervous system.
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from the lateral ventricle into the third ventricle, and along the aqueduct of Sylvius to reach the fourth ventricle. From the fourth ventricle, CSF passes via the foramina of Luschka and Magendie to exit the cerebellum into the subarachnoid space.
Blockage of CSF pathways up to this point pro-duces obstructive hydrocephalus. In the subarach-noid space, CSF travels up through the tentorium opening and over the cerebral convexities to reach the superior sagittal sinus. Blockage of CSF path-ways in the subarachnoid spaces is usually called communicating hydrocephalus since air introduced into the lumbar subarachnoid space can reach the lateral ventricle. At the superior sagittal sinus, CSF passes through arachnoid villi or pacchionian bodies to reach the sinus. Thus, CSF forms from blood and returns to blood.
In adults, the total CSF volume is approxi-mately 140 mL. The ventricles contain 25 mL, the spinal cord subarachnoid space 30 mL, and the remaining 85 mL are in the subarachnoid spaces around the brain. CSF is produced at a rate of 20 to 25 mL/h or 500 to 600 mL/d. Thus, CSF turns over about 4 times a day. CSF production is inde-pendent of CSF pressure (until a pressure of 450 mm CSF), but CSF absorption is dependent on CSF pressure in a linear fashion.
In adults the spinal cord descends to about T12–L1, but in small children the spinal cord may descend as low as L2. Below that level, nerve roots travel to exit appropriate neural foramina. It is at the level of the nerve roots that it is safe to perform a lumbar puncture.
TECHNIQUE OF LP
Written permission following informed consent is highly recommended and often required. An explanation of what will transpire will often reas-sure a patient and make the procedure more com-fortable. Occasionally, a mild sedative is helpful in the anxious patient. Whenever possible the LP should be performed in the lateral recumbent position as this allows an accurate measure of the opening pressure. The patient, lying on a firm sur-face that does not sag, should be placed on the side with the knees curled toward the chin. The spinous processes should be in a horizontal line with the two iliac crests forming a perpendicular line. The intersection is usually the L3–L4 intervertebral space (Figure 3-6).
The LP needle is usually inserted in the L3–L4 space or the L4–L5 space. The skin over these areas should be thoroughly cleaned with an antiseptic solution such as betadine or alcohol. Wear sterile gloves during the procedure. Lidocaine may be injected intradermally and subcutaneously at the anticipated LP needle entry site. Normally a 20-gauge needle is used as this needle does not bend during insertion and allows accurate measurement of CSF pressure. Occasionally, smaller needles are used, but they may not allow accurate CSF pres-sure meapres-surement. The LP needle is inserted bevel up through the skin and then angled slightly cephalad toward the umbilicus. It is important to keep the needle horizontal with the patient during insertion. There is usually a “pop” sensation as the
CHAPTER 3—Common Neurologic Tests 31
L3 L4
Figure 3-6 Patient placement for lumbar puncture.
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needle passes through the dura into the subarach-noid space. One can stop the procedure at any step and remove the stylet to see if CSF returns. If blood is encountered, the needle should be with-drawn and the patient repositioned before the next try, often at the next higher interspace.
Once CSF is encountered, attachment of a three-way stopcock and a manometer (which usu-ally comes with a commercial CSF kit) allows measurement of the CSF pressure. If the pressure is elevated, relaxation of the patient and slight uncoiling the legs often reduce the pressure back to normal levels. CSF is then collected sequentially, using 4 to 5 tubes. In adults, 10 to 35 mL are usu-ally collected, depending on the tests to be ordered.
In small children, 3 to 5 mL are sufficient for stan-dard tests in hospitals that have microchemistry facilities. Figure 3-7 lists the commonly ordered CSF tests and the tube number the tests is often ordered from. Tube 1 is the most likely to have a skin bacterial contaminant and exogenous red blood cells (RBCs) from the LP needle puncture, which may produce misleading reports if this tube is used for bacterial cultures or cell counts. It is
advisable to collect an extra tube containing sev-eral ml of CSF and mark “save” on the tube in the event that additional tests are needed. In many lab-oratories, the “save” CSF tube is kept frozen for at least 1 month.
The CSF should promptly be taken to the clini-cal laboratory, since white blood cells begin to degenerate and lyse after 1/2hour and glucose lev-els may fall due to metabolism by white blood cells (WBCs). A procedure note should immediately be recorded in the patient’s chart that includes indica-tions for the LP, the location of the puncture, whether or not the spinal tap was traumatic, open-ing pressure, amount of fluid obtained, appearance of the fluid, and a list of tests ordered on the CSF.
NORMAL CSF VALUES
Table 3-2 lists common normal findings in adult CSF. Neonates transiently have more cells in their CSF and higher protein levels. In general for adults, the upper limit of the CSF protein level equals the patient’s age. Determination of normal CSF glucose level is difficult when blood glucose is markedly elevated because high blood glucose sat-32 FUNDAMENTALS OF NEUROLOGIC DISEASE
Figure 3-7 Outline of lumbar puncture algorithm.
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urates the blood–CSF glucose transporter. CSF polymerase chain reaction assays are increasingly being used to diagnose infections of the CNS even when the infectious agent cannot be isolated from CSF (see Chapter 13, “Central Nervous System Infections”).
COMPLICATIONS OF LUMBAR PUNCTURE
A traumatic lumbar puncture occurs in 10% to 20% of LPs. It most commonly occurs when the LP needle hits a tiny vein in Batson’s plexus,
located on the dorsal side of the spinal subarach-noid space. When this happens, fresh RBCs and serum proteins from the blood and CSF enter the needle. Often the number of RBCs rapidly decreases from tube 1 to tubes 3 or 4. However, this fresh blood may falsely elevate CSF WBC and protein levels. If the RBC and WBC counts and protein measurements are done on the same tube, one simple rule of thumb is to subtract 1–2 WBC/mm3and 1 mg/dL of protein for every 1,000 RBCs/mm3. Table 3-3 (analysis of bloody CSF)
CHAPTER 3—Common Neurologic Tests 33
Table 3-2 Normal Lumbar Cerebrospinal Fluid (CSF) Findings in Adults
Test Normal finding
Appearance Clear and colorless against a white background Opening Pressure 70–180 mm CSF in recumbent position Red Blood Cells (RBCs) < 5 RBC/mm3
White Blood Cells (WBCs) 5–10 WBC/mm3 Differential Mainly mononuclear cells
Total Protein <45–<60 mg/dL depending on assay technique (<30 mg/dL if cisternal CSF,
<25 mg/dL if ventricular CSF) Percent Immunoglobulins <15% of total protein
Oligoclonal Bands None or rarely one band
Glucose >40 mg/dL (usually >60% of blood glucose)
Gram Stain Negative
Cultures Sterile for bacteria, mycobacteria, fungi, and viruses
CSF-VDRL test Non reactive
Cytology No malignant cells
* VDRL = Venereal Disease Research Laboratory.
Table 3-3 Analysis of Bloody Cerebrospinal Fluid
CSF finding Traumatic Lumbar Puncture (LP) Subarachnoid Hemorrhage
Color Tube 1 pink to red All tubes uniform color
Tube 3 clearer
Red Blood Cell Count Higher in tube 1 than in tube 3 All tubes uniform
Color of Supernatant Fluid Nearly colorless Xanthochromic (yellow color)
Bilirubin Absent Present after first day
Clot May occur on standing Absent
Repeat LP at Higher Interspace Often clear or nearly clear Same as initial LP Head Computed Tomography No blood in subarachnoid space Blood may be seen in
subarachnoid spaces 023-038_Davis03 3/2/05 4:08 PM Page 33
gives a useful approach to distinguishing a trau-matic LP from a subarachnoid hemorrhage.
While not life threatening, post-LP headaches may be quite uncomfortable. The headache begins several hours after the LP and may last for several days. The headache is usually frontal and develops when the patient moves from a lying to a sitting or standing position. Returning to a lying position relieves the headache. The incidence of post-LP headache is highest in young adult women and is uncommon in children and the elderly. In young adults the incidence is about 10%. The risk of a post-LP headache increases when larger-size LP needles are used. There is little evidence that drinking large quantities of water or lying prone prevents a post-LP headache, but lying prone for a few hours may be of benefit. With simple bed rest, the headache usually disappears.
A brain herniation from lumbar puncture is the most-feared complication, but fortunately is quite rare (less than 2% even if the CSF pressure is ele-vated). If the patient has markedly elevated pres-sure, it is still important to collect at least 5 mL of CSF for diagnostic tests before withdrawing the LP needle. Once the needle is withdrawn, CSF begins to leak out the hole in the dura. If the CSF pressure is unexpectedly markedly elevated, there are sev-eral things that should be done immediately after the LP. The patient should be observed closely for signs of neurologic deterioration over the next 8 hours. Prompt neuroimaging (CT or MRI) often identifies the cause of elevated CSF pressure. A secure intravenous line may be established should mannitol administration be required. Notification of a neurosurgeon that a potential problem exists is helpful should a surgical cause of the increased CSF pressure be identified. If brain herniation begins, the patient should be given intravenous (IV) mannitol, intubated, and hyperventilated to lower intracranial pressure.