The Mechanism of Macular Sparing

Abstract

Patients with homonymous hemianopia sometimes show preservation of the central visual fields, ranging up to 10°. This phenomenon, known as macular sparing, has sparked perpetual controversy. Two main theories have been offered to explain it. The first theory proposes a dual representation of the macula in each hemisphere. After loss of one occipital lobe, the back-up representation in the remaining occipital lobe is postulated to sustain ipsilateral central vision in the blind hemifield. This theory is supported by studies showing that some midline retinal ganglion cells project to the wrong hemisphere, presumably driving neurons in striate cortex that have ipsilateral receptive fields. However, more recent electrophysiological recordings and neuroimaging studies have cast doubt on this theory by showing only a minuscule ipsilateral field representation in early visual cortical areas. The second theory holds that macular sparing arises because the occipital pole, where the macula is represented, remains perfused after occlusion of the posterior cerebral artery because it receives collateral flow from the middle cerebral artery. An objection to this theory is that it cannot account for reports of macular sparing in patients after loss of an entire occipital lobe. On close scrutiny, such reports turn out to be erroneous, arising from inadequate control of fixation during visual field testing. Patients seem able to detect test stimuli on their blind side within the macula or along the vertical meridian because they make surveillance saccades. A purported treatment for hemianopia, called vision restoration therapy, is based on this error. The dual perfusion theory is supported by anatomical studies showing that the middle cerebral artery perfuses the occipital pole in many individuals. In patients with hemianopia from stroke, neuroimaging shows preservation of the occipital pole when macular sparing is present. The frontier dividing the infarcted territory of the posterior cerebral artery and the preserved territory of the middle cerebral artery is variable, but always falls within the representation of the macula, because the macula is so highly magnified. For physicians, macular sparing was an important neurological sign in acute hemianopia because it signified a posterior cerebral artery occlusion. Modern neuroimaging has supplanted the importance of that clinical sign but at the same time confirmed its validity. For patients, macular sparing remains important because it mitigates the impact of hemianopia and preserves the ability to read fluently. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Measurement of Cerebral Glucose Metabolism in the Visual Cortex Predicts the Prognosis of Hemianopia

“…Macular sparing may occur when glucose metabolism in the posterior striate cortex is maintained at a certain level because injury to the posterior striate cortex due to blood flow is maintained by collateral flow. 25 Macular sparing, which maintains the central visual field, may occur immediately after the onset. The major cause of macular sparing is perfusion of the calcarine cortex is by multiple arteries.…”

“…The occipital pole, which generally corresponds to the macula, receives blood from posterior cerebral artery. 25 However, the calcarine cortex often receives collateral blood flow from the middle cerebral artery and pial branch and remains perfused even after occlusion of the posterior cerebral artery, resulting in macular sparing. 25,30-32 In addition, the range of the corresponding striate cortex is large.…”

“…25 However, the calcarine cortex often receives collateral blood flow from the middle cerebral artery and pial branch and remains perfused even after occlusion of the posterior cerebral artery, resulting in macular sparing. 25,30-32 In addition, the range of the corresponding striate cortex is large. The central 10° region occupied half of the surface area of the posterior striate cortex.…”

“…8 Moreover, macular sparing, which is often observed in patients with homonymous hemianopia, may occur within the central 10°. 25 We calculated the residual visual field area at the center of the 10° region and the periphery using image analysis software (ImgWorks, Tamanomori) in each individual (Figure 1). At the start of observation and 1 year after onset, the total area of the residual visual field on the injured side of the right and left eyes was compared in the central and peripheral areas.…”

Vertical and horizontal reading training in patients with hemianopia and its effect on reading eye movements

“…1 c). The existence of a macular sparing had been discussed controversially over decades, for an overview see 10 . It was assumed to be a perimetric artefact due to fixational eye movements during perimetry, which cause a shift of the field defect towards the hemianopic side 11 .…”