Hypersynchronous Slowing
▲ Hyperventilation Hypersynchrony
Description
Hyperventilation may produce bilateral, rhythmic, in-phase, low-frequency activity, which is termed hypersynchronous slowing. In general, hypersynchrony denotes a highly regular, monomorphic rhythm that is superimposed upon or eclipses the background activity. The timing and duration of hyperventilation hypersynchrony (HVHS) is highly variable. It does not always occur, and, when it occurs, it may quickly follow the beginning of hyperventilation, it may occur for only a few seconds at the end of hyperventilation, and it may also continue beyond the end of hyperventilation. Furthermore, it may appear and disappear during hyperventilation or be a subtle, regular, slow rhythm that is superimposed on the background activity. The amount of HVHS partly depends on the duration of hyperventilation and the patient’s effort. General recommendations are for hyperventilation to be performed for at least 3 to 5 minutes with deep respirations at 15 to 20 per minute (American Clinical Neurophysiology Society, 1994), (Mendez and Brenner, 2006). When HVHS continues beyond the hyperventilation, it usually resolves within 1 minute. Persistence for longer than 90 seconds beyond hyperventilation is abnormal. Regardless of the duration beyond the end of hyperventilation, HVHS does not normally return after its resolution. This abnormality is termed the “re-buildup” phenomenon.
HVHS’ onset and frequency are its most characteristic features (Niedermeyer, 1999). Often, the onset is not a run of rhythmic activity; instead it typically occurs as individual, bilateral, sometimes sharply contoured, slow waves that recur and gradually meld together into a regular rhythm with continued hyperventilation. HVHS’ frequency at onset may be in either the theta or delta frequency ranges, and it progresses into a slower rhythm with continued hyperventilation. The slowest frequency is usually between 1.5 and 4 Hz, and the lower frequencies in this range occur more commonly in children. Children and young adults are much more likely to manifest HVHS, and the age range with maximum prevalence is 8 to 12 years (Takahashi, 1999). HVHS in children and adults differ with a posteriorly maximum field in children and an anteriorly maximum field in adults (Fisch, 1999). Amplitudes in children may reach 500 μV, which is much higher than in adults (Kiloh et al., 1981). HVHS may demonstrate a shifting asymmetry with an overall distribution that does not show lateralization (Blume et al., 2002).
HVHS results from hypocapnia with respiratory alkalosis and cerebral vasoconstriction producing decreased cerebral blood flow (Yamatani et al., 1994; Zysno and Buttner, 1971). It is independent of oxygenation and is facilitated by cerebral ischemia (Fisch, 1999). Overall, HVHS is more likely to occur in the context of hypoglycemia (Markand, 1984). Serum glucose concentrations less than 80 mg/dl facilitate HVHS, whereas concentrations greater than 120 mg/dl hamper it (Takahashi, 1999).
Distinguishing Features
• Compared to Intermittent Rhythmic Delta Activity
There is no morphologic or distribution difference between HVHS and intermittent rhythmic delta activity that occurs independently of hyperventilation. The association with hyperventilation is the key distinguishing feature.
• Compared to Generalized Interictal Epileptiform Discharges
HVHS may be mistaken for generalized interictal epileptiform discharges (IEDs) (Benbadis and Tatum, 2003). Generalized IEDs that are provoked by hyperventilation are similar to HVHS in frequency, maximal field, and the paroxysmal occurrence during hyperventilation. Furthermore, the occasional occurrence of
impaired verbal recall and motor responsiveness during HVHS in children without epilepsy complicates the differentiation issue by including behavioral changes that can appear as seizures (Epstein et al., 1994). When a clearly visible spike or polyspike precedes each slow wave, differentiation is straightforward. However, identifying an IED is still possible when the spike is not distinct from the slow wave by using the IED spikes’ time-locked location on the slow waves’ slopes. A low-amplitude spike that is not distinct may appear as a notch along the slow wave’s up slope, and this can be differentiated from the superimposition of faster frequencies on the slowing that can occur with HVHS. The difference is that HVHS notches occur with variable timing with respect to the subsequent slow wave’s apex and may have more than one notch along the slow-wave slope. The time interval between IED spikes’ notches and the slow waves’ apices is fixed because the two waves comprise a complex and are not independent and superimposed activities.
impaired verbal recall and motor responsiveness during HVHS in children without epilepsy complicates the differentiation issue by including behavioral changes that can appear as seizures (Epstein et al., 1994). When a clearly visible spike or polyspike precedes each slow wave, differentiation is straightforward. However, identifying an IED is still possible when the spike is not distinct from the slow wave by using the IED spikes’ time-locked location on the slow waves’ slopes. A low-amplitude spike that is not distinct may appear as a notch along the slow wave’s up slope, and this can be differentiated from the superimposition of faster frequencies on the slowing that can occur with HVHS. The difference is that HVHS notches occur with variable timing with respect to the subsequent slow wave’s apex and may have more than one notch along the slow-wave slope. The time interval between IED spikes’ notches and the slow waves’ apices is fixed because the two waves comprise a complex and are not independent and superimposed activities.
Co-occurring Patterns
Glossokinetic muscle artifact from the swallowing of accumulated saliva sometimes occurs at the end of hyperventilation. More often, hyperventilation leads to evidence of mild drowsiness, which is most often evident in this context by the absence of a posterior dominant rhythm and decreased muscle or movement artifact.
Clinical Significance
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