Letters to the Editor
The central thesis of Hill et al (1) in their recent publication in this journal proposing that “definite retinal migraine, as defined by International Headache Society (IHS) criteria, is an exceedingly rare cause of transient monocular visual loss” conforms to my clinical experience. What factors can account for the persistent and unjustified inclusion of the concept of “retinal migraine” in the clinician's diagnostic toolbox? I suggest two factors: 1) the legitimacy conferred by the IHS classification (2) and 2) the rare but incontrovertible identification of the apparently true existence of monocular migraine (3).
The most facile explanation for the perception that migraine is monocular is the frequent misperception that homonymous visual phenomena are coming from only one eye. This quandary was plainly stated in 1865 by G. B. Airy, the Astronomer Royal, who sketched and described his own migrainous fortification spectra and averred that “I have never been able to decide with certainty whether the disease really affects both eyes. The first impression on the mind is that only one eye is affected” (4).
A second explanation for the perception that migraine is monocular comes from the possibility that spreading depression (SD) is confined to the most anterior portion of the primary visual cortex, giving rise to teichopsia limited to the temporal crescent of the visual field in one eye (5). Until we have accurate functional neuroimaging during monocular migraine, SD limited to the approximately 10% of the calcarine cortex subserving the monocular temporal crescent remains plausible but unconfirmed.
Rigorous adherence to IHS criteria aids immeasurably in limiting the diagnosis of retinal migraine (IHS 1.4), but there is considerable ambiguity in descriptors of “positive visual phenomena” (criterion B). Do “scintillations” identify retinal phenomena and fortification spectra identify cortical phenomena? On the basis of Richards' speculation in 1971 that migrainous fortifications arise from the columnar organization of visual cortex neurons specializing in “detection of lines of a particular length and orientation” (6), the “columnless” retina is judged as an unlikely site of origin for teichopsia. The distinction between the appearance of migrainous positive visual phenomena arising from cortex versus retina is blurred by reports of “simple” phosphenes heralding the cortical dysfunction of classic migraine (7).
Must we require that all retinal migraines be accompanied by headache (IHS criterion C)? The IHS concedes that “some cases without headache have been reported” (2). We should allow for the existence of “acephalgic retinal migraine” given that we allow for acephalgic migraine or “typical aura without headache” (IHS 1.2.3) in referring to painless visual disturbances of migraine.
An accurate estimate of the frequency of retinal migraine in the differential diagnosis of transient monocular visual disturbance will require more than conformation to “strict IHS criteria” (1). A revised and more precise semiology of the visual world of migraineurs is needed. For example, distinctions between the appearance (and complexity) of scintillations, photopsias, sparks, and phosphenes remain ill-defined and impede accurate classification of positive migrainous visual symptoms. Although greater precision in the description of migrainous visual phenomena will improve the nosology of migraine, accurate localization and a thorough grasp of the neurophysiology of migraine aura remain elusive.
Although Penfield and Rasmussen were unable to reproduce “zigzag outlines of migraine images” with their bipolar electrodes (8) during cortical stimulation, functional MRI mapping of blood oxygenation level-dependent (BOLD) events during migraine aura has demonstrated retinotopic progression at a characteristic SD velocity of 3.5 =/− 1.1 mm/min (9). Surprisingly, BOLD imaging in a single migraineur revealed that the initial part of the visual aura correlated with changes in the extrastriate cortex (area V3A), not in the primary visual cortex. The BOLD signal changes in migraine aura are posited to be a surrogate for SD, lending credence to Milner's insightful theory that the scintillating scotoma corresponds to SD (10). The contemporary theory of migraine as a primarily neuronal disorder is built on the compelling consonance of functional neuroimaging, SD, and Lashley's measurement of the “march” of his own scintillating scotoma (11). This “spreading depression theory” (12) remains ascendant despite the inability to record SD in uninjured human neocortex or retina in vivo. (1)
As neuroimaging and neurophysiologic tests for migraine evolve from the investigational stage, our contemporary clinical predicament is evocative of the approach to epilepsy before electroencephalography. Lacking a comprehensive atlas of the migraine aura, the lessons of functional neuroimaging and the solicitation of more precise descriptors of patients' visual symptoms will go a long way toward refining the diagnosis of migraine and distinguishing retinal migraine from other monocular disturbances of vision.
Frederick E. Lepore, MD
Departments of Neurology and Ophthalmology UMDNJ/Robert Wood Johnson Medical School New Brunswick, New Jersey firstname.lastname@example.org
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