*7.3.2 Interictal abnormalities*

Generalized interictal features during the wakefulness and sleep are mandatory for diagnosis of LGS [11].

In wakefulness, high-amplitude, diffuse, and synchronous SSW at 1.5–2.5 Hz is typical. Slow SSW has maximal amplitude over frontal areas and ranges in duration from a few seconds to a few minutes or sub-continuous. The complexes typically consist of a spike (duration < 70 ms) or a sharp wave (70–200 ms), followed first by a positive deep and then by a negative wave (300–500 ms) [81]. Such stimuli, as eye opening, noise, calling the patient's name, and pain, tend to decline the occurrence or terminate SSW [81]; on the other hand, relaxation and drowsiness favor their occurrence. Hyperventilation (HV) and intermittent photic stimulation (IPS) usually have little influence on the SSW activity.

Characteristic features during sleeping are:


• "Paroxysmal fast activity" (PFA), which consists of sequences of rhythmic activity at 10–25 Hz and lasts for a few seconds (2–10 seconds) during NREM sleep. PFA is an essential diagnostic criterion.

These may be subclinical or accompanied by subtle change of axial muscle tone, which is detectable only by EMG electrodes as the ictal expression of a tonic seizure [81]. Interictal abnormalities and seizures decrease in REM sleep.

Focal abnormalities are usually present in patients with structural lesions; they are non-specific and depend on the underlying pathology: focal or multifocal spikes, spike-waves, polyspikes, slow waves, and focal bursts of rapid rhythms.
