Pitfalls In Diagnosis Of Epilepsy

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When epilepsy presents in a classical fashion, with recurrent complex partial or tonic-clonic seizures, accompanied by interictal epileptiform EEG patterns,

TABLE I-4 Pitfalls in Diagnosis of Epilepsy

Obtaining an inadequate history Overemphasizing the rare and obscure Leading the patient to an inaccurate history Mixed seizures and psychogenic seizures Over-reading the EEG Overinterpretation of a therapeutic trial Incorrect attribution of causation the diagnosis is easy. Unfortunately, the history may be incomplete, or other medical conditions may confound the clinical picture. In these circumstances, the diagnosis of epilepsy depends on the clinical judgment and experience of the practitioner. Several potential diagnostic pitfalls are to be avoided (Table I-4).

The cardinal error is obtaining an inadequate history. Observers of spells should be queried directly. "Dizzy spells" without loss of consciousness may be revealed by co-workers to be full tonic-clonic seizures. Diagnosticians should not train patients to give a textbook seizure history. By the time multiple physicians have asked a patient if they have ever experienced an odor "like burning rubber" at the start of their seizure, most patients have convinced themselves that they have.

The improper interpretation of an EEG can cause great harm. Many benign and normal variant patterns can be mistaken for epileptiform discharges (109). The combination of a shaky history and an overinterpreted EEG is especially pernicious. The diagnosis of epilepsy may suffer from incorrect attribution of causation. Focal seizures can cause a postictal transient hemipare-sis (Todd's paresis) (92,93), but cerebrovascular insufficiency can directly cause hemiparesis and a seizure (122). Bilateral carotid occlusive disease can cause brief loss of consciousness (123). Distinguishing primary epilepsy from epilepsy secondary to cerebrovascular disease can be difficult. A setting conducive to cerebrovas-cular disease is influential, as is rate of recovery (more rapid after seizures), a history of prior seizures, TIAs, or strokes. Similarly, seizures can induce cardiac arrhythmias (124), as well as result from them (125).

The novice diagnostician tends to overemphasize the rare. Most staring spells are simple daydreaming. Most explosive outbursts in children are temper tantrums. Most episodes of a previously well person losing consciousness and falling to the ground are syncope. The diagnostic probabilities are altered when it is known that an individual suffers from epilepsy. As an example, temporal lobe seizure (78,126) should be considered as an etiology of loss of consciousness in a per son with known complex partial epilepsy; however, it should be far down on the differential diagnosis of syncope in a person with no prior history of seizures. Primary pain is a rare symptom of epileptic seizures (127-129), and seizures should not be on the usual differential diagnosis of pain.

The most difficult diagnostic cases tend to be those with mixed disorders. A certain percentage of individuals with documented psychogenic seizures may, at other times, exhibit epileptic seizures. The incidence of mixed epileptic and nonepileptic events has been estimated, at times, to be as high as 37% (130) but is more likely 10% or less (131,132). In these cases, it may be that the epileptic seizures and their aftermath somehow became a "template" for subsequent nonepileptic spells. By documenting lack of EEG changes during a generalized seizurelike episode, video-EEG monitoring can show that the episode under observation is nonepileptic in etiology, but it can never prove the etiology of prior episodes. Inference by analogy is imprecise. Even after establishing a diagnosis of nonepileptic attacks, the experienced clinician remains vigilant for the possibility of a mixed disorder. As a practical matter in this circumstance, it often suffices to remove anticonvulsants with the understanding that epileptic seizures may emerge and require reevaluation.

The improvement of spells with anticonvulsants gives incomplete testimony as to the nature of the disorder. Placebo effects are significant in any medical disorder, and especially in those with psychogenic components. The efficacy of antiepileptic drugs is not limited to seizures. Carbamazepine and sodium val-proate have long been recognized as useful mood stabilizers (133,134). Of the newer antiepileptic medications being used for mood stabilization, evidence is available for lamotrigine (135-137). Limited supportive information is available for topiramate, oxcarbazepine, zon-isamide, and tiagabine (137), while gabapentin has mixed reviews (138,139) for mood stabilization. Phenobarbital and benzodiazepines are effective both as anti-convulsants and as tranquilizers. Phenytoin can suppress ventricular arrhythmias. When a positive response to an antiepileptic agent is encountered, the clinician should consider what else besides epilepsy might be under treatment. Conversely, some patients with presumed epilepsy worsen with increasing doses of antiepileptic drugs. This can be a clue to underlying psy-chogenic seizures (140).

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