Lambda And Lambdoids 31 Lambda

Lambda waves are sharp monophasic or biphasic waveforms that resemble the Greek letter lambda. They have a duration of 160 to 250 ms, an amplitude of 20 to 50 ^V, and usually appear over bi-occipital leads, although occasionally may be unilateral (Fig. 7). Lambda waves depend on rapid saccadic eye movements with eyes open. This variant pattern is usually generated when the patient scans a complex patterned design in a well-illuminated room, for instance, dotted ceiling tiles in the laboratory. They block by staring at a featureless white surface or by closing the eyes. They are much more common in children 2 to 15 yr of age than in adults.

Slow lambda of youth, also known as shut-eye waves and posterior slow wave transients associated with eye movements, are associated with eye blinks in children. They appear over occipital channels as single, broad monophasic or diphasic, mainly surface negative waves. These morphologies last 200 to 400 ms and have an amplitude of 100 to 200 ^V. They may be asymmetric. They are usually found in children younger than 10 yr of age.

Wicket Spikes
Fig. 6. Wicket spikes. Wicket spikes are often seen over temporal channels in drowsy or sleep recordings in adults. Reprinted from Goldensohn et al., 1999 with permission.

3.2. Lambdoids

Lambdoids, also called positive occipital sharp transients of sleep (POSTS) have a checkmark (biphasic) morphology with initial surface positivity and often appear in trains up to 4 to 5 Hz (Fig. 8). POSTS are usually synchronous but can be asymmetric in size. There are most commonly seen between 15 and 35 yr of age, and usually in light sleep. They may appear before the alpha rhythm completely evaporates in drowsiness. POSTS are a commonly encountered variant pattern on routine EEGs.

Slow lambdoids of youth, also known as cone-shaped waves or O-waves, are high voltage, diphasic slow transients seen over the occipital contacts and frequently with the occipital delta activity in deeper sleep states (Fig. 9). As the name implies, they are cone-shaped. They can be seen up to 5 yr of age.

Occipital Cone Waves
Fig. 7. Lambda. This is from a 42-yr-old subject. Note the bilateral triangular-shaped morphologies over the occipital derivations evident with eyes open in waking.

Fig. 8. Positive occipital sharp transients of sleep (POSTS). These derive from a drowsy 26-yr-old subject. (A) Illustrates POSTS occurring in a biposterior train at 2 to 4 Hz. In this anterior-posterior bipolar montage, POSTS have a check-mark appearance. The initial deflection is surface positive. This is better demonstrated in (B), which illustrates the same fragment reformatted in an average-reference montage. Note the frequent positive waveforms over the bi-occipital contacts.

Fig. 8. Positive occipital sharp transients of sleep (POSTS). These derive from a drowsy 26-yr-old subject. (A) Illustrates POSTS occurring in a biposterior train at 2 to 4 Hz. In this anterior-posterior bipolar montage, POSTS have a check-mark appearance. The initial deflection is surface positive. This is better demonstrated in (B), which illustrates the same fragment reformatted in an average-reference montage. Note the frequent positive waveforms over the bi-occipital contacts.

Fig. 9. Cone-shaped (O) waves. This is from a 3-mo-old child during sleep. Note the frequent high amplitude cone-shaped morphologies evident over the bi-occipital contacts in this tracing.
Posterior Slow Waves Youth
Fig. 10. Posterior slow waves of youth (youth waves). This is from the tracing of an 11-yr-old girl. Note the complex waveforms seen over the biposterior contacts. They illustrate the fused waveforms involving a delta waveform with a superimposed or "fused" alpha rhythm on top.

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