9 CHAPTER IXMETHODS OF INVESTIGATIONS IN NEUROLOGY
Lumbar puncture and the examination of the cerebrospinal fluid. Diagnostic lumbar punctures (LP) are performed for the purpose of examining either the fluid itself or the hydrodynamics of the fluid. The test enables to determine the composition of cerebrospinal fluid (CSF), cerebrospinal fluid pressure, to investigate abnormalities in pressure in such conditions as spinal subarachnoid block. Therapeutic lumbar punctures make possible to increase intracranial pressure. The indications for lumbar puncture: meningitis, encephalitis, acute inflammatory polyneuritis (Guillain-Barre syndrome), acute demyelinating disorders, seizure, stroke, polyneuropathy; dementia; altered level of consciousness, therapeutic administration of antibiotics or antineoplastic agents, subarachnoid hemorrhage. Contraindications to lumbar puncture: clinical signs of impending uncal, central transtentorial, or cerebellar herniation, and the situation, when the needle must pass through an area of infection that could result in infection in the subarachnoid space.
Technique of lumbar puncture. In adults, the LP is best performed with the patient in the lateral decubitus position with the head and spine parallel to the floor. This positioning is particularly important for the accurate measurement of the opening and closing pressures. After positioning the patient, certain landmarks should be palpated, beginning with the iliac crest. An imaginary line that runs between the crests will intersect approximately the L3-L4 interspace. Once this area is located, the spinal processes of L3 and L4 should be palpated and between them the interspace may be felt. In infants and children the inter-space between the fourth and fifth lumbar spinous processes is the one of choice. This procedure is easily accomplished in thin patients but may be difficult in obese or edematous individuals. When the landmarks are located, the patient should pull the knees into the chest and assume the so-called fetal position. This maneuver helps widen the vertebral interspace. The clinician should appropriately prepare the skin surface using a sterilizing agent and then place the sterile draping. One to two percent lidocaine should be injected intradermally and then is infiltrated into the deeper tissues. Once the patient is anesthetized, a spinal needle with a stylet should be inserted into the skin toward the interspace. The patient should be comfortable, warm, and relaxed, and reassured of the simplicity and relative painlessness of the test. If the patient is tense or nervous, some sedation by mouth may be administered before the puncture is to be done. When fluid appears at the needle hub, a threeway stopcock and manometer are attached. The fluid is allowed to rise slowly in the manometer. The upper limit of normal CSF opening pressure with a patient in the lateral recumbent position is 110 mm H2O in young infants, 150 mm H2O in children, 180 mm H2O in adults.
Myelography: X-rays are taken after a radiopaque agent is injected into the subarachnoid space via lumbar puncture. Magnetic resonance imaging (MRI) has replaced myelography for evaluation of intraspinal abnormalities, but computed tomography (CT) myelography is still done when MRI is unavailable. Contraindications are the same as those for lumbar puncture. Myelography may exacerbate the effects of spinal cord compression, especially if too much fluid is removed too rapidly.
Table 10.1. Cerebrospinal Fluid Abnormalities in Various Disorders (Merck Manual)
Condition
Pressure
Wbc/?L
Predominant Cell Type
Glucose
Protein
Normal
100–200 mm H2O
0–3
L
50–100 mg/dL (2.78–5.55 mmol/L)
20–45 mg/dL
Acute bacterial meningitis
?
100–10,000
PMN
?
> 100 mg/dL*
Acute syphilitic meningitis
N or ?
25–2000
L
N
?
Lyme disease of CNS
N or ?
0–500
L
N
N or ?
Brain abscess or tumor
N or ?
0–1000
L
N
?
Viral infections
N or ?
100–2000
L
N
N or ?
Cerebral hemorrhage
?
Bloody
RBCs
N
?
Cerebral thrombosis
N or ?
0–100
L
N
N or ?
Spinal cord tumor
N
0–50
L
N
N or ?
Guillain-Barr? syndrome
N
0–100
L
N
> 100 mg/dL
L = lymphocyte; N = normal; PMN = polymorphonuclear leukocyte; ? = increased; ? = decreased.
Electroencephalograpy. The electroencephalogram (EEG) remains the primary test for identifying and characterizing many physiological disturbances of the brain, particularly in the evaluation of seizures, encephalopathies, coma, and focal cerebral dysfunction. EEG is generated by synchronized synaptic activity on the dendrites of cortical pyramidal neurons. The EEG results from a myriad of such processes, each of which individually generates a signal too feeble to be detected at the scalp. EEG is an irreplaceable test in the evaluation of possible seizures. Significant epileptiform abnormalities are a useful and reliable confirmation that a patient's spells are epileptic seizures, and allow them to be classified as partial or generalized in onset. The EEG is often used to evaluate patients with encephalopathy or coma, when it may give valuable information on the location and the severity of the process and on the neurological prognosis. The EEG may be diagnostic in cases of Creutzfeldt-Jakob disease.
Recording. The recording is performed with metal disk electrodes, filled with conductive gel or paste and attached on the scalp wiped with alcohol. Often an elastic cap that holds the entire electrode array in place is used. 21 electrodes are applied usually. The patient must be fully alert during a portion of the record, his eyes are normally closed. A number of activating procedures like hyperventilation, photic and sound stimulation, sleep and sleep deprivation are commonly used to bring out abnormalities, particularly in patients with suspected seizures. The signal is amplified, recorded and then undergoes computer analysis. Various kinds of quantitative measures of the EEG activity are available, the most important of them are frequency (Fourier) analysis with topographic mapping and event detection in the practice of long-term monitoring of epileptic patients.
Description of EEG activity. The EEG is usually described in the terms of amplitude and frequency, which are measured in ?V and hertz, respectively. Frequency may also be described in terms of four bands: delta, under 4 hertz; theta, from 4 to under 8 hertz; alpha, from 8 to 13 hertz; and beta, over 13 hertz. The polarity and localization of the signal is determined by bipolar and referential montages.
Figure 9.1. Normal EEG (L.R. Zenkov, 2004)
Other EEG-based techniques. Clinical polysomnography includes EEG, eye movements, the chin electromyography, and respirations monitoring. It provides additional information to determine sleep stages and is used to detect and quantitate various sleep disorders, most often respiratory problems (sleep apnea). Measurement of evoked responses (EP). Visual, auditory, or tactile stimuli are used to activate corresponding neuroanatomic tracts and relay stations, resulting in small cortical wave potentials. Ordinarily, these small potentials are lost in the background noise of the EEG, but computer averaging of a series of stimuli, time-locked to the EEG, cancels out the noise to reveal a waveform. The latency, duration, and amplitude of the evoked responses reflect the physiologic integrity of the tested sensory pathway. For example, visual evoked responses may reveal unsuspected optic nerve damage by multiple sclerosis. Somatosensory evoked responses may pinpoint the physiologic disturbance when multiple levels of the neuraxis are affected by structural disease (e.g., metastatic carcinoma that invades the plexus and spinal cord).
Clinical Electromyography. The pattern of electrical activity in muscle [i.e., the electromyogram (EMG)], both at rest and during activity, may be recorded from a needle electrode inserted into the muscle. When determining whether weakness is due to nerve, muscle, or neuromuscular junction disorder is clinically difficult, these studies can identify the affected nerves and muscles. In electromyography, a needle is inserted in a muscle, and electrical activity is recorded while the muscle is contracting and resting. Normally, resting muscle is electrically silent; with minimal contraction, action potentials of single motor units appear. As contraction increases, the number of potentials increases, forming an interference pattern. Denervated muscle fibers are recognized by increased activity with needle insertion and abnormal spontaneous activity (fibrillations and fasciculations); fewer motor units are recruited during contraction, producing a reduced interference pattern. Surviving axons branch to innervate adjacent muscle fibers, enlarging the motor unit and producing giant action potentials. In muscle disorders, individual fibers are affected without regard to their motor units; thus, amplitude of their potentials is diminished, but the interference pattern remains full. In nerve conduction velocity studies, a peripheral nerve is stimulated with electrical shocks at several points along its course to a muscle, and the time to initiation of contraction is recorded.
Ultrasonography. Diagnostic ultrasonography (US) uses sound waves above the audible level to generate diagnostic medical images. The patient is not exposed to ionizing radiation. US has various imaging applications, with specific uses in neurological disorders. Diagnostic US can be coupled with Doppler devices, with or without color, allowing flow measurements in vascular structures, such as commonly employed for carotid artery studies.
Echoencephalography: Ultrasonography can be used at the bedside (usually in the neonatal ICU) to detect hemorrhage and hydrocephalus in children < 2 yr. CT has replaced echoencephalography in older children and adults.
Duplex Doppler ultrasonography: This noninvasive procedure can assess dissection, stenosis, occlusion, and ulceration of the carotid bifurcation. It is safe and rapid, but it does not provide the detail of angiography. It is preferable to periorbital Doppler ultrasonography and oculoplethysmography for evaluating patients with carotid artery transient ischemic attacks and is useful for following an abnormality over time. Transcranial Doppler ultrasonography helps evaluate residual blood flow after brain death, vasospasm of the middle cerebral artery after subarachnoid hemorrhage, and vertebrobasilar stroke.
