Discussion

The study gave to somewhat unexpected results. Namely, the evidence of the significant difference observed between headache and non headache subjects in terms of mean AHI (p< 0.01), SpO2 (p < 0.01) and T <90% (p < 0.01) with better respiratory parameters among headache sufferers, particularly amongst the migraineurs. In fact, when we had planned the study, we were looking for possible endogenous elements able to induce and/or transform headache and we hypothesized that a sleep breathing disturbance might be one of this factors. On the contrary, it emerged that headache patients have a better respiratory condition during sleep, also in allodynic cases.

The analysis of sleep behavior in different groups showed that migraineurs took more time to fall asleep and awake more frequently during night with a reduced global sleep satisfaction. In conclusion, if compared to controls, migraineurs seem to sleep worse but to breathe better. However, the hypothesis that there is an allostatic function of migraine and allodynia could also be made: the presence of these conditions might inhibit deep sleep, and thus avoid prolonged apneas. The observation that allodynic patients complain of a poor subjective satisfaction by sleep with frequent awakenings and difficulties in starting sleep may be the time when an allostatic load (episodic migraine) becomes an allostatic overload (allodynic migraine), or it may correspond to a further allostatic adjustment to maintain an equilibrium: migraine is "sufficient" until the metabolic unbalance is such, that allodynia is needed. Allodynia is more frequently observed among subjects with chronic/ transformed migraine, but it is also present in a large portion of episodic migraineurs. Probably transformed migraine is the true manifestation of the overwhelmed allostatic capacity of migraine (allostatic overload) while the presence of allodynia is still a marker of a functional modification. Overall, using an allostatic perspective, migraine may be considered a functional strategy to maintain equilibrium and to reverse situations potentially dangerous for the hyperexcitable and hypoenergetic migraineurs brain [08]. Transformed migraine may than represent the failure of this strategy (allostatic over-load), without the capacity to counteract the energetic unbalance.

Allodynia (the perception of pain by non-painful stimuli) is largely considered as a marker of migraine transformation, but the observation that it is frequently present also among episodic migraineurs, offers another possible way to interpret this symptom. Allodynia may be an additional manifestation of migraine in an extreme effort to correct a metabolic or energetic or homeostatic disequilibrium, nocturnal sleep related blood oxygenation included.

To explore this unexpected hypothesis, we grouped both headache and non-headache subjects by sleep subjective satisfaction, sleep latency, and presence of awakenings. Comparing groups (headache subjects with short vs long sleep latency, with frequent vs sporadic awakenings and satisfied vs non-satisfied) no significant difference emerged, at least after Bonferroni correction. Probably the relatively small cohort dimension may have influenced the analysis.

The fact that patients with chronic headaches have a high prevalence of sleep complaints is well documented [09] and a high frequency of headache among patients with pathological breathing during sleep is well defined [02], but in the transitional phase toward sleep breathing disturbances, allodynia may be a useful para-physiological modification instead of a symptom of migraine transformation/chronification.

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