Sleep Waveforms and PSG Scoring Vocabulary

Fundamentals of EEG Waveform Measurement

• Primary Waveform Measures: Technologists use four core parameters to define and differentiate EEG waveforms:

  • Amplitude: The measured voltage, representing the height of the wave.

  • Frequency: The speed of the voltage fluctuations, measured in the number of cycles per second.

  • Waveform Shape: The characteristic morphology of the wave (e.g., sinusoidal, sharp, or irregular).

  • Distribution/Location: Where the activity is recorded on the scalp (e.g., frontal, central, or occipital).

• AASM Measurement Standards:

  • The American Academy of Sleep Medicine (AASM) manual provides specific amplitude and frequency requirements, though few waveforms have rigorous, firm criteria.

  • Waveform shape is considered an imprecise, subjective measure because waveforms are recognized by the human eye, leading to potential discrepancies between scorers.

  • Standard scoring requires measuring from the frontal, central, and occipital leads.

• Amplitude Calculation: The amplitude of a wave is determined by subtracting the peak value from the lowest value (peak-to-peak measurement).

• Frequency Calculation Methods:

  • Counting Method: Used when multiple waves occur per second. For example, if a spindle waveform contains $13$ waves between two markers set $1.0\,s$ apart, the frequency is $13\,Hz$.

  • Duration Method: Used when there are only one or two waves per second. Frequency is calculated as $1 / \text{duration in seconds}$.

  • Zero Crossing: A technique to measure duration where the count begins when the waveform reaches $0\,\mu V$, goes through a negative half-wave, then a positive half-wave, and returns to the zero line.

  • Example: A K complex lasting $0.5\,s$ would have a frequency of $1 / 0.5 = 2\,Hz$.

  • Spindle Example: A wave lasting $0.075\,s$ results in a frequency of $1 / 0.075 \approx 13\,Hz$.

Alpha Rhythm Characteristics and Clinical Significance

• Definition: Alpha rhythm is the resting background activity of the brain during wakefulness. It is most prominent when a patient is relaxed and has their eyes closed.

• Key Identification Criteria:

  • Frequency: Must be between $8\,Hz$ and $13\,Hz$. (e.g., a sample with $11$ waves in $1.0\,s$ is $11\,Hz$).

  • Shape: Described as sinusoidal, meaning it is smooth, regular, and has rounded peaks.

  • Distribution: Occipital prominence. The amplitude is typically highest in the O2 electrode because it is closest to the source.

  • Amplitude: While no formal amplitude criterion is given, it must be large enough to be visible (typically ranges from $20\,\mu V$ to $40\,\mu V$).

• Age and Individual Variations:

  • Children: Those under $3$ years old generally have a background rhythm slower than $8\,Hz$, known as the dominant posterior rhythm. Frequency increases with age, reaching an average of $9\,Hz$ by age $9$.

  • Elderly: Frequency may slow with age but usually remains within the $8-13\,Hz$ alpha range.

  • Low Voltage Alpha: Approximately $15\%$ of the population has very low amplitude alpha rhythm or none at all.

• Distribution Patterns:

  • Localized: High amplitude only in the occipital channel.

  • Diffuse: Extends forward into the temporal lobes. If the source is close to the M1 reference electrode, alpha may appear in the frontal and central channels.

  • Paradoxical Distribution: If alpha amplitude is high at both the occipital and reference (M1) electrodes, the differential amplifier may subtract the inputs, making the occipital signal appear lower than other channels.

• Reactivity: Alpha rhythm is reactive to eye opening. When eyes open, the rhythm decreases in amplitude or disappears, a process called attenuation. If no EEG change occurs between eye opening and eye closing, the patient is considered to have no alpha rhythm.

Transition to Sleep: LAMF and Stage N1 Markers

• Low Amplitude Mixed Frequency (LAMF): Signals the transition from wakefulness to light sleep (Stage N1).

  • Frequency: Predominantly $4-7\,Hz$, though faster and slower frequencies may occur.

  • Amplitude: Generally below $20\,\mu V$, with occasional higher amplitude waves.

  • Appearance: Diffuse, irregular, and seen in all EEG channels. It also serves as the background for REM sleep.

• Vertex Sharp Waves:

  • Duration: Less than $0.5\,s$ (frequency of $2\,Hz$ or higher).

  • Morphology: Sharp appearance ("something that would hurt if you sat on it").

  • Distribution: Arise from the vertex (CZ electrode placement) and are highest in the central channel.

  • Occurrence: Large enough to stand out from the LAMF background; they first appear during Stage N1.

Stage N2 Markers: K Complexes and Sleep Spindles

• K Complexes:

  • Duration: Equal to or greater than $0.5\,s$ (frequency of $2\,Hz$ or less).

  • Structure: Consists of an initial sharp negative (upward) wave followed by a slower positive (downward) wave. Faster waveforms may sometimes ride on top of the K complex.

  • Distribution: Prominent in the frontal cortex, though often seen in the occipital channel.

  • Arousal Association: A K complex is associated with an arousal if the arousal begins within $1.0\,s$ after the K complex ends. This is often confirmed by brief increases in chin or limb EMG tone.

• Sleep Spindles:

  • Frequency: $11-16\,Hz$ (most commonly $12-14\,Hz$).

  • Duration: Train of waves lasting at least $0.5\,s$ and less than $3.0\,s$. Most bursts last $1.0$ to $2.0\,s$.

  • Clinical Warning: Spindles longer than $3.0\,s$ are scored as arousals. Spindles faster than $18\,Hz$ are considered excessive and may be artifacts or misidentified.

  • Morphology: Sinusoidal with regular waves and rounded peaks.

  • Distribution: Primarily central and frontal.

• Distinguishing Spindles vs. Alpha:

  • Spindles are typically central/frontal bursts ($1-2\,s$).

  • Alpha is typically occipital and sustained (seconds to minutes).

  • Ambiguity can occur in patients with severe sleep apnea who exhibit brief arousals with short alpha bursts.

Deep Sleep and REM Markers

• Slow Wave Activity (Stage N3):

  • Frequency: $0.5\,Hz$ to $2.0\,Hz$ (duration of $0.5\,s$ to $2.0\,s$).

  • Amplitude: Minimum of $75\,\mu V$, measured peak-to-peak in the frontal channel.

  • Scoring Tip: Some scoring systems use horizontal guidelines at $\pm 37\,\mu V$ (total height $74\,\mu V$) to assist technologists. If a wave exceeds these lines and meets frequency criteria, it is a slow wave. K complexes meeting these criteria should also be counted as slow waves.

• Sawtooth Waves:

  • Frequency: $2\,Hz$ to $6\,Hz$.

  • Location: Central channel.

  • Shape: Resemble the teeth of a handsaw.

  • Significance: Strongly supportive of Stage R (REM) sleep but cannot be used as the sole criteria. Stage R requires LAMF EEG, rapid eye movements, and muscle atonia. Sawtooth waves without these other markers may indicate that Stage R is approaching.

Eye Movement Recording and Physiology

• Ocular Dipole: The eye acts as a battery; the front (cornea) is the positive pole, and the back (retina) is the negative pole.

• Recording Placements: Electrodes are placed near the outer canthus.

  • Left electrode: $1.0\,cm$ below the outer canthus.

  • Right electrode: $1.0\,cm$ above the outer canthus.

• Bio-Calibrations (Bio-Cals):

  • Performed before and after the study to verify channel accuracy.

  • Patients are asked to look left, right, up, down, blink, and clench teeth.

  • Looking Left: Front of eye (positive) moves toward the left electrode, causing a downward deflection. Back of eye (negative) moves toward the right electrode, causing an upward deflection.

• Types of Eye Movements:

  • Blinks: Cause a reflex upward rotation of the eye known as Bell's Phenomenon. This creates a positive downward deflection in eye channels and can also be seen in the frontal EEG leads.

  • Reading Eye Movements: Pattern of slow scanning followed by a sharp return to the next line.

  • Slow Eye Movements (SEM): Initial component lasts more than $0.5\,s$. Common in Stage N1 and sleep onset.

  • Rapid Eye Movements (REM): Sharp initial component, usually occurring in bursts. Characteristic of Stage R and active wakefulness (e.g., watching TV).

Electromyography (EMG) and Atonia

• Chin EMG Tone:

  • Highest during wakefulness.

  • Decreases progressively through Stage N1 and N2.

  • Lowest during Stage R (muscle atonia).

• Muscle Twitches:

  • Defined as lasting less than $0.25\,s$.

  • Commonly seen during sleep onset and Stage R.

  • In Stage R, twitches often coincide with bursts of rapid eye movements and increase the certainty of the stage score, although they do not affect the EEG epoch scoring directly.

Questions & Discussion

• Incidental Observations and Dialogue:

  • A student or observer noted "All I saw was white blonde hair."

  • A clarification was made: "All I remember you saying is I'm sorry you feel that way."

  • Situational context mentioned: "Morning. The sun is just starting to… Neighbors, the rich neighborhoods."

  • Technical Issue: A system message "Access Denied" with Request ID "5B6ZPVDWKVS1W9W4" appeared during the instructional session.