Diagnosis and differential diagnosis of epilepsy

Diagnosis of epilepsy in patients with mental retardation follows basically similar rules to those used with other epileptic patients. Thus, the diagnosis of epilepsy is clinical and includes two or more unprovoked seizures during a relatively short period of time. However, some details concerning particularly the differential diagnosis of epilepsy differ in patients with mental retardation compared with the epileptic population with normal intelligence.

Clinical investigation of epileptic patients routinely includes the collection of historical data, physical and mental examination, EEG, and laboratory tests such as determinations of blood glucose and electrolytes. In babies with epilepsy, attention should be paid to changes of skin colour or cardiac rhythm, sucking and smacking, which all may be epileptic phenomena. In people with mental retardation it may be difficult or impossible to obtain an accurate clinical history from the patient. The clinician often has to depend on relatives or other professionals involved in the care of the patient. In addition to a description of the seizures, a history of the age at onset of epilepsy and the complete clinical picture of the epileptic syndrome are of value.

General factors that provoke seizures include fever, infection, hypoglycaemia, stress, alcohol withdrawal, hyperventilation, some medications, sudden discontinuation of sedative drugs, and specific activity. Fevers associated with infections such as those of the ears, sinuses, upper respiratory tract, or urinary tract are quite common. If seizures are exacerbated, ensure that any treatable infection is identified. The situation is further confused by the fact that seizures can produce the fever, which, however, resolves within an hour. Withdrawal seizures may be precipitated by a sudden discontinuation of drugs such as benzodiazepines which have an antiepileptic effect, although they may have been prescribed for another reason. Some seizures may be associated with a specific activity, especially if this activity induces excitement or anxiety. Exercise-induced seizures occur regularly in some patients.

Many people with mental retardation have abnormal behaviour that resembles epileptic seizures but is not epileptic in origin. The diagnostic and other problems caused by non-epileptic seizures or pseudoseizures are well known.(3) Different dyskinesias, psychogenic attacks, and other non-epileptic episodes may be manifested as pseudoseizures at different ages (T§ble...1). Sudden aggression and other epilepsy-like conditions (Tabled) are in practice the most important reasons for the overdiagnosis of epilepsy, and consequently also for overmedication and subsequent intoxication in patients with mental retardation. On the other hand, non-convulsive epileptic phenomena and even partial seizures (Table.3) may be difficult to diagnose in people with mental retardation. The situation is more complicated when patients with intractable epilepsy have both real epileptic seizures and pseudoseizures, for example psychogenic seizures. In such cases the recognition of psychogenic seizures(4) (Table.,.4) helps to identify appropriate treatment.(5)

Table 1 Salient non-epileptic episodes at different ages

Sudden aggncsiion

Self-ibwe

Biuvc behaviour

Abnormal motgr Ictflfy

Surihj

E^blinking

NysUfcmuJ

Exigenced mnle

InterttTfitMrVt lethar^

Table 2 Conditions often misdiagnosed as epilepsy in mentally retarded subjects

Abut« itiiurM

N&KtfivulsJvt nuitt upiif

Seizures wuh periodic headudie

Seiiures with iciu 'ai with paraeichesu

Sem. -e; with vicor.ii and vcjccicivt disturbance?

Low tf emotioful wnnil

POHitMl iffiCCi

& Tipli- partial Hiuurtt

Complex partial iurci

Table 3 Underdiagnosis of epilepsy in mentally retarded subjects

Table 4 Differential diagnosis of epileptic and psychogenic seizures

Using high-resolution magnetic resonance imaging ( MRI), it is possible to identify structural brain abnormalities, including neoplasms, dysplasia, heterotopia, or diseases in the brainstem and/or posterior fossa. If MRI is not available, CT is recommended.

Prolonged video-EEG monitoring of the patients is of use in selecting candidates for epilepsy surgery or in distinguishing between epileptic and non-epileptic seizures. Basically, this enables any behaviour to be analysed in relation to the EEG changes. If this investigation is not available, portable cassette recording of the EEG may also be of considerable value. The diagnosis of subclinical seizures, including minimal behavioural or cognitive changes in the absence of any obvious clinical seizures, can be demonstrated as lengthened reaction times during EEG discharges in the Romny test.

Positron emission tomography, functional MRI, magnetic resonance spectroscopy, and magnetoencephalography are the most informative of the other methods of measuring brain function. The role of the last three new techniques in defining epileptic seizures and syndromes has yet to be evaluated fully. Interictal and ictal single-photon emission CT may also be of use.

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