Blackouts are diagnosed clinically, but some investigations can support the diagnosis, identify the underlying cause, and clarify the epilepsy syndrome diagnosis. However, investigations rarely substitute for an accurate history and eye-witnessed account.
ECG is the most important initial investigation. Although the interictal ECG is normal in patients with epilepsy, and no particular ECG features predict propensity to SUDEP, its value is to identify or exclude epilepsy mimics, e.g., long QT syndrome, hypertrophic cardiomyopathy. These conditions, though rare, are potentially preventable causes of sudden death. Furthermore, specialized cardiology tests may then be indicated, e.g., head-up tilt-table testing, echocardiography, and prolonged heart rate monitoring.
Electroencephalography, initially developed in the early 20th century, analyzes small electrical changes produced by the superficial cerebral cortex, measured by using scalp electrodes. Patients and doctors often wrongly regard EEG as a diagnostic test for epilepsy and a routine investigation for blackouts. However, unless an actual seizure is captured on the recording, an EEG can only support a clinical diagnosis of epilepsy.
"Epileptiform" activity on an EEG does not necessarily mean that the diagnosis is epilepsy; 0.5-3% of the healthy population have an abnormal EEG.12 Epileptiform activity without epilepsy may accompany brain tumors, severe intellectual disability, congenital brain injury, and may follow brain surgery. Conversely, the EEG is often normal in people with epilepsy. Even during seizures, surface EEG is may not capture abnormal electrical discharges (rarely) if the epileptic focus is buried deeply within the cerebral cortex.
Therefore an EEG must only be interpreted in the correct clinical context; a full description of the attacks is essential to the EEG reporter. A routine baseline, interictal EEG lasts 20-30 minutes and involves recording during wakefulness (including a period of hyperventilation and of photic stimulation). Video recording of the procedure is helpful, because it provides a visual record of any events. Recording soon after the last seizure can increase the EEG yield further, as can repeating the awake recording, recording during sleep following prior sleep deprivation, or making more prolonged recordings over days.
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