181 DIAGNOSTIC METHODS IN NEUROLOGY
Harrison’s Manual of Medicine
DIAGNOSTIC METHODS IN NEUROLOGY
Electromyography (EMG)/Nerve Conduction Studies (NCS)
A dramatic increase in the role of imaging in diagnosis of neurologic disorders occurred with the development of computed tomography (CT) in the early 1970s and of magnetic resonance imaging (MRI) in the 1980s. In general, MRI is more sensitive than CT for the evaluation of most lesions affecting the CNS, particularly those in the spinal cord, cranial nerves, and posterior fossa. CT is more sensitive than MRI for visualizing fine osseous detail, such as temporal bone anatomy and fractures. Recent developments, such as helical CT, CT angiography, MR angiography, positron emission tomography, Doppler ultrasound, and interventional angiography, have continued to advance diagnosis and guide therapy. Conventional angiography is reserved for cases in which small-vessel detail is essential for diagnosis (Table 181-1).
Table 181-1 Guidelines for the Use of CT, Ultrasound, and MRI
An EEG records the electrical activity of the brain from multiple electrodes placed on the scalp. It is used primarily in the investigation and management of epilepsy. It is also useful in the evaluation of altered mental status, coma, sleep disorders, and brain death.
NORMAL EEG RHYTHMS IN ADULTS In an awake pt lying quietly with eyes closed, an 8- to 12-Hz alpha rhythm should be present over the posterior head regions and attenuate with eye opening. There may be beta activity (>13 Hz) as well, which is more pronounced with drowsiness and certain drugs. Theta (4–7 Hz) activity is present predominantly over the temporal regions with drowsiness and light sleep. Normal sleep activity includes delta activity (<4 Hz), sleep spindles, and vertex waves.
ABNORMAL EEG RHYTHMS IN ADULTS The EEG in a patient with epilepsy may show ictal, interictal, or normal activity. An electrographic seizure consists of abnormal, repetitive, rhythmic activity having abrupt onset and termination. Interictal epileptiform activity in correlation with an appropriate history strongly suggests epilepsy and includes bursts of abnormal spike and sharp wave discharges. Generalized 3-Hz spike-and-wave activity is present in absence seizures. Focal interictal activity makes partial complex seizures more likely. A normal EEG does not exclude the diagnosis of epilepsy. Continuous inpatient or portable monitoring increases the likelihood of capturing a spell in difficult cases.
In pts with altered mental status, there may be generalized or focal slowing; triphasic waves suggest a metabolic cause. Reactivity of the record correlates with a better prognosis. Loss of reactivity or a burst-suppression pattern is present with severe coma. Electrocerebral silence in the absence of drug overdose or hypothermia implies that useful cognitive recovery will not occur. Periodic lateralized epileptiform discharges (PLEDs) can be seen with herpes simplex encephalitis. Generalized periodic complexes in the presence of a dementing disorder are consistent with Creutzfeld-Jakob disease. The EEG does not reliably distinguish between dementia and pseudodementia.
Evoked potentials (EPs) are averaged time-locked cortical potentials following stimulation of visual, auditory, or somatosensory afferents. Prolongation of the latency of these potentials reflects a lesion in the specific pathway being tested but is nonspecific. They are most often used in the evaluation of possible multiple sclerosis (MS).
VISUAL EVOKED POTENTIALS (VEPS) Elicited by monocular stimulation with a reversing checkerboard pattern and recorded from the occipital region of the scalp. A P100 response is recorded at a latency of approximately 100 ms. Latency, amplitude, and symmetry of the response are measured. VEPs are most commonly abnormal with optic neuritis (active or residual) but can be abnormal with other lesions of the optic nerve such as compression or with glaucoma.
BRAINSTEM AUDITORY EVOKED POTENTIALS Using earphones, clicks presented to one ear produce seven wave forms (I–VII) recorded from the scalp, representing successive activation of the auditory nerve and brainstem auditory pathways. Applications include screening for acoustic neuroma, localization of level of the lesion in coma (usually normal in toxic/metabolic coma or bihemispheric disease and abnormal with brainstem pathology), and hearing evaluation of infants.
SOMATOSENSORY EVOKED POTENTIALS (SEPS) Generated by delivering multiple small electrical stimuli to large sensory nerve fibers of the limbs to produce afferent volleys recorded at many levels along the somatosensory pathway (proximal peripheral nerve trunks, spinal cord dorsal columns, and primary sensory cortex). The SEP may be abnormal in multiple disorders, including MS when there is a lesion in the cord, vitamin B12 deficiency, AIDS, cervical stenosis, or Lyme disease. The presence or absence of SEPs may have prognostic significance with coma and spinal cord injury. They can also be used intraoperatively during spine surgery or carotid endarterectomy to indicate possible iatrogenic injury to the cord or parietal cortex.
ELECTROMYOGRAPHY (EMG)/NERVE CONDUCTION STUDIES (NCS)
The EMG/NCS primarily assesses the peripheral nervous system (motor neuron, dorsal root ganglion, peripheral nerve, neuromuscular junction, and muscle). EMG/NCS can be used to localize or exclude a lesion in the peripheral nervous system and to identify characteristics of these lesions that help with diagnosis, treatment, and/or prognosis. For example, neuropathies can be categorized as focal, multifocal, or generalized; sensory or motor; axonal or demyelinating. EMG/NCS is useful to localize and characterize focal neuropathies (location along nerve, axonal or conduction block, complete or partial). The presence and type of spontaneous activity in a myopathy can direct diagnosis and treatment. Some CNS disorders can also be evaluated by EMG (tremor, ataxia, asterixis, myoclonus, and dystonia).
NERVE CONDUCTION STUDIES NCS involve electrical stimulation of large myelinated motor and sensory nerves, recording a response over the muscle and distal sensory nerves, respectively. Distal latency, duration, and conduction velocity of the response usually reflect the integrity of myelin. The amplitude reflects the integrity of axons. Late responses include F and H waves, which provide information about the proximal conduction in motor (F waves) or motor and sensory (H waves) nerves. F waves can be particularly important in the early diagnosis of inflammatory neuropathies such as Guillain-Barré syndrome. Blink reflexes evaluate the conduction in branches of the trigeminal and facial nerves.
REPETITIVE STIMULATION Repetitive stimulation of motor nerves can identify and characterize disorders of the neuromuscular junction. A train of supramaximal stimuli is delivered to a motor nerve, recording over the muscle at rest and after exercise. A decrement of the compound muscle action potential amplitudes is seen in myasthenia gravis and congenital myasthenia. An increment is seen with Lambert-Eaton myasthenic syndrome and botulism.
ELECTROMYOGRAPHY EMG involves placing a needle in skeletal muscle and recording the electrical activity of the muscle at rest (spontaneous activity) and with activation (motor unit action potentials, or MUAPs). Resting muscle does not show activity except at the motor endplate. Abnormal spontaneous activity, such as fibrillation potentials and positive waves, is seen with denervation and certain types of myopathy. MUAPs are of a characteristic duration and morphology in each muscle. Short-duration MUAPs suggest a myopathic process or disorder of the neuromuscular junction. Long-duration MUAPs are present with axonal neuropathies and motor neuron disease. The pattern of activation (reduced or rapid recruitment) reflects a neurogenic or myopathic process, respectively.
Single-Fiber EMG Single-fiber EMG is a sensitive but not specific technique for disorders of the neuromuscular junction. The firing of a pair of muscle fibers from the same motor unit is recorded in the muscle. The variation of the time interval between the two fibers is called jitter, and disappearance of the second muscle fiber action potential is called blocking. An increase in jitter and/ or blocking is seen in disorders of the neuromuscular junction.
Autonomic studies include determination of heart rate variation with respiration, Valsalva ratio, heart rate response to standing/tilting, blood pressure response to sustained hand grip, and a measure of sympathetic skin response. These tests can provide objective evidence of autonomic insufficiency (central or peripheral) and provide a measure of function for small unmyelinated peripheral axons.
INDICATIONS Lumbar puncture is used to obtain pressure measurements and secure a sample of CSF for cellular, chemical, immunologic, and bacteriologic examination in the evaluation of infections of the CNS, meningeal cancer, inflammatory neuropathies, acute demyelinating disorders, benign intracranial hypertension, and other unexplained neurologic disorders (see Chap. 203 for normal values). It is also used for adminstration of spinal anesthesia, antibiotics, or antitumor agents and to inject contrast agents for myelography.
CONTRAINDICATIONS These include thrombocytopenia or other disorders of blood coagulation, the presence of local skin or soft tissue infection along the needle tract, and increased intracranial pressure. If increased intracranial pressure is suspected, a head CT or MRI should be performed prior to the study to exclude a mass lesion. CSF should always be obtained with suspected meningitis. A fine-bore needle should be used. If the pressure is >400 mmHg, the minimal amount of fluid should be removed with administration of mannitol or dexamethasone if needed to prevent herniation.
COMPLICATIONS The most common complication is a positional headache due to persistent leakage of CSF. Treatment is discussed in Chap. 4. Seeding of the subarachnoid space with bacteria is rare.
For a more detailed discussion, see Martin JB, Hauser SL: Approach to the Patient with Neurologic Disease, Chap. 356, p. 2326; Aminoff MJ: Electrophysiologic Studies of the Central and Peripheral Nervous Systems, Chap. 357, p. 2331; Dillon WP: Neuroimaging in Neurologic Disorders, Chap. 358, p. 2337; and Engstrom JW, Martin JB: Disorders of the Autonomic Nervous System, Chap. 366, p. 2416, in HPIM-15.