Seizure disorders. The EEG is a key test for the diagnosis and management of patients with seizure disorders. It should be emphasized that not all patients with clinically definite seizure disorders have abnormalities on EEG, and conversely, paroxysmal EEG abnormalities sometimes are seen in people without seizure disorders. During a tonic–clonic seizure, an EEG usually demonstrates widespread electrical discharges. Interictally (i.e., between seizures, not during seizures), patients with complex partial seizures may have focal spikes, well-defined waves that are sharply contoured and localized in one or more areas. During a complex partial seizure, they may show a focal buildup of rhythmic waves. Simple partial seizures may not show abnormalities on the surface EEG at all. Interictal EEGs in patients with seizure disorders are abnormal in approximately 70% of patients. Certain seizure disorders are classified according to EEG patterns:

Encephalopathy. Patients with metabolic encephalopathy of any cause have abnormal EEGs, consisting of nonfocal slowing of the EEG pattern or rhythmic bursts of symmetric frontal slowing. The EEG can be useful in identifying metabolic encephalopathies or in ruling out metabolic encephalopathies in patients with altered mental status. Certain patterns can be helpful (e.g., the “triphasic” waves of hepatic encephalopathy).

Coma. For patients in a coma, the EEG can assist with identifying severe injury, subclinical seizure activity, or major asymmetries and may provide good prognostic signs such as the presence of reactivity and sleep potentials.

Tumors. Depending on the location and size of a tumor, the EEG is often abnormal, with focal slowing or spike discharges. New onset of seizures in middle age is often the presenting symptom of tumor. However, the CT scan or magnetic resonance imaging (MRI) is the major test used to diagnose tumors or other space-occupying lesions.

Other diseases. Some disorders have characteristic EEG findings (e.g., Creutzfeldt-Jakob disease, herpes simplex encephalitis, subacute sclerosing panencephalitis). Psychiatric diseases (affective disorders, schizophrenia) usually have no effect on the EEG. Migraine headaches may be associated with focal slowing. The EEG is often used as an adjunct in the diagnosis of brain death (isoelectric EEG).

Patients with myopathy often show certain features: (i) lowamplitude, short-duration motor unit potentials; (ii) complex polyphasic motor unit potentials; and (iii) increased insertional activity (e.g., bizarre high-frequency discharges; usually seen with inflammatory myopathies). It is usually impossible to distinguish one myopathy from another by EMG. Some myopathies (e.g., polymyositis and muscular dystrophies) may show fibrillation potentials.

Myotonia presents a characteristic pattern of hyperexcitability with persistent waxing and waning and repetitive discharges, which sound like a “dive bomber.” The dive bomber pattern is not diagnostic of any single myotonic disorder.

Presynaptic neuromuscular junction (NMJ) disorders (e.g., botulism, Eaton-Lambert syndrome) may be characterized by progressive enhancement of motor unit action potentials evoked by repetitive stimulation of the motor nerve. Most also show normal amplitude miniature end-plate potentials. By contrast, postsynaptic NMJ disorders (e.g., myasthenia gravis) show a decremental response of the muscle action potential with repetitive nerve stimulation and subnormal amplitudes of the miniature endplate potentials.

Denervation produces increased polyphasic action potentials, bizarre high-frequency discharges, fibrillations, positive sharp waves, and sometimes fasciculations. Fibrillation potentials develop 3 to 4 weeks after the onset of nerve injury. Thus, someone with an acute root or nerve lesion may not show muscle fibrillation. Examination of muscle groups in the legs or arms may help to diagnose specific root lesions and whether denervated muscles are referable to a single root. Similarly, denervation can be used to help diagnose amyotrophic lateral sclerosis or other anterior horn cell diseases.