The phantom spike and wave (PhSW) takes its name from the low amplitude, often not clearly discernible, spike within a repeating spike and slow wave complex that can hamper the identification of the pattern as a complex. Without the spike, the pattern appears to be a paroxysmal 6 Hz rhythm. Phantom conveys the same characteristic here as miniature and larval. Clarifying the meaning of the name, the alternative “wave and phantom spike” also has been used. This name reverses the order of waves within the complex, but it avoids the misunderstanding that the slow wave also is easy to overlook. When the spike element is visible, the PhSW is easily identified as a complex.

Morphologically, the PhSW is similar to the 3 per second spike and wave complex that accompanies genetic generalized epilepsies, and, most characteristically, childhood absence epilepsy. To this end, PhSW has been called a miniature version of the 3 per second spike and wave pattern. The crucial differences are only the faster frequency, which usually is 5 to 7 per second, and the low amplitude of the spike and the wave. However, these differences are not absolute and some overlap exists with generalized interictal epileptiform discharges (IEDs). One potential overlap is the frequency. About a quarter of PhSW pattern occurrences have repetition rates of 4 per second, which is a common rate for generalized IEDs. Another overlap is the spike amplitude. Generalized IEDs may have low-amplitude spikes that may appear as notches within the slow wave. Nevertheless, the spike of PhSW is essentially characterized by being consistently small. Its amplitude usually is less than 40 μV and its duration usually is shorter than 30 milliseconds, and about a quarter of PhSW’s spikes are less than 25 μV, which is why the spike sometimes is not evident. The slow wave of PhSW typically is less than 50 μV, but it usually has higher amplitude than the spike. Although the slow wave of PhSW has lower amplitude than the slow wave of generalized epilepsies and differs more from generalized IEDs than does the spike, the slow wave’s amplitude is still high enough that it is evident within the EEG record.

The typical manifestation of PhSW is as a bilaterally synchronous burst, that lasts up to 2 seconds and usually occurs as one of the two types that are distinguished according to four features: Amplitude, location of the complex, gender of the patient, and wakefulness of the patient. The two forms are named according to descrip­tive acronyms. WHAM is Waking, High amplitude, Anterior, usually Male (WHAM), and FOLD is usually Female, Occipital, Low amplitude, and Drowsy. High-amplitude spikes are defined as greater than 45 μV. Although PhSW usually is either frontally or occipitally predominant, about 20% of occurrences are generalized.

PhSW is an uncommon pattern that occurs during 0.5% to 1% of EEGs, and is slightly more likely to occur in females (Santoshkumar et al., 2009). The age range during which it is most likely to occur is adolescence and young adulthood, with an occurrence rate in this group of about 2.5% (Radhakrishnan et al., 1999). The pattern may occur during wakefulness and sleep, but it is most likely to occur during drowsiness. When occurring during sleep, it occurs far more commonly in NREM sleep than REM sleep.