While the exact mechanism remains uncertain, it is reported that there may be a greater involvement of hypoactivity of the parasympathetic nervous system that leads to BEM. Some authors have postulated that BEM is due to decreased activity of the parasympathetic (iris sphincter), while other authors have hypothesized that an increased activity of the sympathetic nervous system (iris dilator) is the cause. With BEM, there may be a change in either of these two systems resulting in pupillary size imbalance. Normal pupil size is dependent on a balance between the sympathetic and parasympathetic nervous systems controlling the iris musculature. The pathophysiology behind BEM is not well understood.
The diagnosis of BEM however does not require concomitant migraine history.Ĭase reports describing BEM predominantly describe it affecting women more frequently, but it has been described in children as well with a reported age range of 5 to 53. Some reports have described BEM to be a migraine aura or an ophthalmoplegic migraine. In most reports, there is an increased risk of BEM with a personal or family history of migraines as well as an increased frequency of episodes with migraine. The connection between BEM and migraines is ill defined. The underlying etiology of BEM is not well understood however, it has been frequently associated with an accompanying migraine in females. Unilateral BEM is called Benign Episodic Unilateral Mydriasis (BEUM), but it can also occur bilaterally (BEBM) with either or both pupils being dilated during subsequent BEM events. BEM has been reported worldwide, with most cases affecting one eye (unilateral). In contrast to physiologic anisocoria which is often asymptomatic, BEM can be noticeable to patients and physicians. Physiologic and benign pupil asymmetry is seen in up to 20% of healthy individuals. In addition if the anisocoria is isolated and episodic, then this anisocoria is benign and referred to as benign episodic mydriasis (BEM).
If there are no accompanying symptoms or signs (e.g., no headache, no ptosis, no diplopia or ophthalmoplegia) and the anisocoria is completely neurologically isolated it is much more likely to be benign. Anisocoria may be benign (e.g., pharmacologic dilation, tonic pupil) or may be an alarming sign of a life-threatening disorder (e.g., aneurysm, arterial dissection). Unequal size of pupils (anisocoria) often presents as a diagnostic challenge for clinicians because the underlying etiology may stem from a variety of factors.
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