The accurate diagnosis of events occurring during sleep requires knowledge of normal and abnormal sleep, skills in recording specialized forms of polysomnography, and experience in interpreting multiple physiological signals coupled to video and audio recordings. A careful history from the patient with a collateral history from a bed partner, parent, or caregiver is essential to determine whether a sleep study is necessary and to provide perspective into its subsequent significance. In some cases, the diagnosis may be obvious on history alone, whereas in others, a range of studies, including wake and sleep electroencephalography (EEG), brain magnetic resonance imaging (MRI), and neuropsychometric tests, may be needed. Solving the problems posed by complex parasomnias or sleep-related movement disorders may take considerable skills spanning the fields of sleep medicine, psychiatry, epileptology, and neurodegenerative disorders.
The recording technique most commonly used for elucidating nocturnal events is often referred to as video-EEG polysomnography, 1 a term that reflects the multiple signals recorded during such studies. A standard polysomnogram (PSG) includes two electro-oculographic (EOG) derivations, three EEG derivations, channels recording chin and anterior tibial electromyography (EMG), surrogate measurements of airflow by nasal pressure and oronasal thermal sensors, measurement of oxyhemoglobin saturation by pulse or ear oximetry, assessment of thoracoabdominal movement by inductance plethysmography, and a recording of the electrocardiogram (EKG). For the recording of nocturnal spells, 16 EEG derivations are usually added to provide the ability to detect epileptiform abnormality. At least one additional EMG channel is included to record arm movements, often a derivation recording signals from the extensor digitorum communis muscles. To allow for adequate spatial resolution, only 16 channels at a time are usually displayed, but readers can move from the more conventional PSG display with an additional EMG channel to a display of whole-head EEG with relative ease using modern digital machines. Although EEG and EOG derivations are recorded with similar parameters to those used in a video-EEG monitoring (VEM) unit, the EMG pass-band is set at 10 to 100 Hz to reduce movement artifact without significantly affecting the amplitude of an interference pattern. Ideally, separate EMG derivations should be used to record right and left limb EMG, placing the electrodes 2 to 3 cm apart over the middle of the muscles; however, limitation on the number of channels often results in the use of one anterior tibial and one arm muscle derivation (often extensor digitorum), linking the two limbs. The main distinction from a conventional PSG, however, is the recording of video images and sound.
Although it is usual to monitor a patient undergoing a conventional sleep study by means of a camera and a microphone, the recordings are rarely stored or replayed during review of the study. When studying patients with nocturnal spells, however, the high-quality recording of behaviors is essential. Most digital PSG machines allow for real-time correlation of images and sounds with polygraphic tracings, permitting the ...