Assessing Central Nervous System Symptoms

The major value and primary application of clinical neurophysiology is in the assessment and characterization of neurologic disease. Selection of appropriate studies for the problem of an individual patient requires a careful clinical evaluation to determine possible causes of the patient’s symptoms. The nature of the symptoms and the conclusions of the clinical evaluation are the best guides to appropriate use of clinical neurophysiologic testing.

Movement Disorders

Surface EMG, EEG, and elicited response results provide a simple and noninvasive means of studying movement disorders. These techniques are particularly helpful in classifying involuntary movements such as tremor and myoclonus. In addition, EMG can assist with designing and performing botulinum toxin injections.

Long Latency Reflexes and the Silent Period

LLRs and the silent period are EMG phenomena that reflect the complex interplay of spinal, brain stem, and cortical influences in motor control. These techniques have been applied to the study of disorders of motor control such as Parkinson’s disease, Huntington’s disease, and dystonia. Abnormalities of these reflexes may help to detect lesions of the central nervous system.

Assessing the Motor Unit with Needle Electromyography

Virtually all primary neuromuscular diseases result in changes in the electric activity recorded from muscle fibers. These changes can best be depicted using fine needle electrodes inserted into the muscle to record spontaneous and voluntary EMG. Thus, EMG can be used to distinguish among lower motor neuron, peripheral nerve, neuromuscular junction, and muscle disease with great sensitivity and some specificity. The sensitivity is usually greater than clinical measures; specificity in identifying the cause of the disease often requires muscle biopsy or other clinical measures. Although EMG is somewhat uncomfortable for patients because needles need to be inserted into the muscles, it generally is well tolerated by patients and provides a rapid, efficient means of testing the motor unit.

Motor Evoked Potentials

MEP recordings are a safe and effective means of assessing conduction along the central and peripheral motor pathways in a variety of clinical settings. MEP is proven to be effective in monitoring central motor pathways that are at risk during surgical procedures and is finding usefulness in the diagnosis and prognosis of several central nervous system disorders such as multiple sclerosis, stroke, and spinal cord injury. The responses can be elicited by a variety of stimulation techniques using either magnetic or electrical stimulation to the cortex, spinal cord, or peripheral nerve. Recordings can be obtained from several structures including the spinal cord, peripheral nerve, or muscle dependent on the clinical application. The clinical neurophysiologist should have a basic understanding of the techniques as well as the potential physiologic and technical factors that need to be accounted for in the interpretation of these studies.

Brain Stem Auditory Evoked Potentials in Central Disorders

BAEPs are performed primarily in patients with suspected neurologic disorders to determine whether there is evidence of a brain stem lesion. BAEPs are highly sensitive to auditory conduction defects, but the findings are not pathologically specific. BAEPs provide data that are highly reproducible and objective and lend themselves to sequential studies for comparison. BAEPs are noninvasive and can be performed not only in the clinical neurophysiology laboratory but also in a hospital room or the intensive care unit. Patient cooperation is not critical, and BAEP waveforms are resistant to the effects of drugs or anesthesia. Important factors that need to be considered for accurate interpretation of BAEPs include the patient’s age, sex, and auditory acuity. The diagnostic yield of BAEPs has been confirmed in patients with acoustic neuromas, MS, or intra-axial brain stem lesions involving the auditory pathways. BAEPs appear to be complementary to structural neuroimaging studies such as MRI.

Electroencephalography in the Surgical Evaluation of Epilepsy

As technology advances, the availability and use of minute microprocessors may change the way we evaluate the patient with epilepsy. Currently trials are ongoing using permanently implanted electronic devices that are capable of recording limited but chronic EEG from depth or grid leads.65 The device has been shown to be able to analyze ongoing EEG and detect the onset of seizures by various algorithms and in response provide a small electrical shock in an attempt to abort the ictal activity, while other permanent devices stimulate at scheduled intervals.66 Trending data, brief samples, and seizures may be easily downloaded for analysis via the Internet. The ability to safely record and detect seizures on a chronic basis may change future treatment strategies. The current study and use of such devices is limited to patients felt not to be resective surgical candidates and are able to be participants in ongoing clinical studies.

Electroencephalographic Special Studies

This chapter reviews several quantitative analysis techniques that may be applied to digitized EEG data. The technique of MEG is also discussed. Many of these techniques were primarily used as research tools; but as they have become more widely available, they are having an increasing effect on EEG interpretation and diagnosis.

Adult EEG: Abnormal Nonepileptiform Activity

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Epileptiform Activity

EEG continues to be an important test to functionally evaluate people with suspected seizures. Although few specific EEG patterns exist for specific diseases, the presence of epileptiform discharges and patterns on the EEG may help identify certain syndromes. Clinicians using EEG in the evaluation and management of people with suspected epilepsy should be familiar with the different epileptiform discharges and their associated clinical significance.