A sleep study, known medically as a Polysomnography (PSG), is primarily designed to diagnose conditions like sleep apnea or restless legs syndrome. While a standard PSG is not optimized for epilepsy detection, it can sometimes capture epileptic events, acting as an initial flag for a neurologist to investigate further. The distinction lies in the differing goals and technical setups of a sleep study versus dedicated seizure monitoring.
Components of a Standard Sleep Study
A typical Polysomnography monitors physiological functions during sleep to understand its architecture and identify disruptions. The study tracks breathing effort, airflow, heart rate, oxygen saturation, and muscle activity via Electromyography (EMG) electrodes on the legs and chin.
The PSG also uses Electroencephalography (EEG) electrodes to record brain wave activity, which is necessary for identifying sleep stages like REM and non-REM sleep. Standard PSG typically uses a limited number of EEG leads, often between four and eight, placed at specific locations on the scalp. This limited setup is sufficient for classifying sleep stages but is not designed for detailed neurological mapping, meaning subtle or localized electrical abnormalities may be missed.
Incidental Seizure Detection
A standard sleep study can record a seizure, but this detection is incidental to its main purpose. If a seizure is generalized, meaning it affects a large part of the brain, the resulting electrical activity is usually prominent enough to be visible on the limited EEG channels. Seizures causing significant physical convulsions are also likely to be noted because the video and muscle activity recordings capture the movement.
However, the limited number of EEG channels reduces the sensitivity for detecting focal seizures, which originate in a small, localized area. When a technician or physician reviewing the PSG suspects a seizure, the protocol involves noting the event and recommending a specialized neurological follow-up for a definitive diagnosis.
Dedicated Monitoring for Nocturnal Seizures
When nocturnal seizures are suspected, Video-EEG Monitoring (V-EEG) is the preferred diagnostic tool. V-EEG is superior because it uses a much more extensive array of electrodes, often 18 or more, covering the entire scalp according to a standardized placement system. This broad coverage allows neurologists to precisely pinpoint the origin and spread of abnormal electrical activity in the brain.
V-EEG also includes continuous, synchronized video recording to correlate the patient’s physical movements with the simultaneously recorded brain activity. This synchronization allows clinicians to differentiate epileptic events from non-epileptic ones, which can look very similar externally. When combined with PSG components, V-EEG provides the most comprehensive evaluation to diagnose both sleep disorders and epilepsy.
Conditions That Mimic Seizures During Sleep
Many sleep disorders produce behaviors that look remarkably like seizures, which is why V-EEG is often necessary for accurate diagnosis. These events, termed parasomnias, involve abnormal movements or experiences during sleep.
Examples include confusional arousals or sleep terrors, which can involve screaming or thrashing during deep non-REM sleep. REM Sleep Behavior Disorder (RBD) causes a person to physically act out vivid dreams due to the failure of normal muscle paralysis. Additionally, Periodic Limb Movement Disorder (PLMD) causes repetitive, involuntary leg twitches that can be mistaken for seizure activity.
The data collected during a sleep study, especially the EMG and video, is important for ruling out these mimics as well as confirming the presence of a seizure.