Unanswered Questions in Headache: So What Is Photophobia, Anyway?

Photophobia is a subjective experience that alters sensory per-ception of light. Patients often describe photophobia in differentways. For example, some will say “light hurts my eyes” or “lightmakes my headache worse,” while others will not report associ-ated pain or pain aggravation, but unpleasantness, and state “Icannot stand even low levels of light.” However, regardless of thesubjective experience reported, they all manifest a very consistentbehavior; they retreat to a dark place or protect their eyes fromlight.This variability in the subjective experience of photophobicpatients is not surprising, considering that many different disor-ders can result in photophobia.We would like to put out the idea that photophobia involvesnot only pain pathways, but also limbic system pathways thatsuperimpose an emotional processing of discomfort leading tolight avoidance. The interaction of these pathways could providethe drive to seek out darkness. This is especially important in thecontext of migraine, where photophobia may reflect an allodynicresponse when even very low levels of light become painful.However, this avoidance can also be protective. Everyone expe-riences the need to avoid exposure of the eyes to high lightintensity. For example, looking directly at the sun is unpleasant,and most people will squint or look away to avoid retinaldamage. Likewise, the glare of skiing on a sunny day withoutsunglasses is unpleasant no matter how good the snow is. Wesuggest that these protective pathways are likely to be the sameones activated by low light in migraine and other pathologiesassociated with photophobia.So, how might the brain integrate light with pain and emo-tional circuits to yield the phenomena of photophobia? In thepast few years, we have begun to get glimpses of potential path-ways and the direction of needed future research. The cascadebegins with light activation of non-image-forming melanopsinretinal ganglion cells.