Macular Sparing in Parietal and Temporal Lobe Lesions
Macular sparing in parietal and temporal lobe lesions occurs because the fibers carrying macular vision are deeply located in the optic radiation, forming a thin but wide layer in the vertical plane that is less vulnerable to partial lesions affecting these regions.
Anatomical Basis for Macular Sparing
The optic radiation is structured in three layers: the superficial layer corresponds to homolateral peripheral vision, the intermediate layer to contralateral peripheral vision, and the deepest layer to central vision from both hemimaculae 1.
These fibers fan out sequentially around the posterior part of the lateral ventricles, with the thick fibers transmitting peripheral vision displacing upwards and downwards, leaving the thin fibers for central vision as the sole constituents of the central and terminal part of the optic radiation 1.
Due to the width and depth of this macular layer, even in extensive lesions of the optic radiation, some of its fibers can escape injury, explaining macular sparing without requiring bilateral macular representation 1.
Parietal Lobe Lesions and Macular Sparing
In parietal lobe lesions, visual field defects tend to affect the lower quadrants, which may be congruous or incongruous 1.
Deep parietal lesions typically affect the deeper strata of the optic radiation (macular vision and/or contralateral quadrants of peripheral vision), but the wide distribution of macular fibers allows some to remain intact 1.
Studies of patients with PCA strokes support the concept that the central 15° of the visual field are represented in the posterior 25% of visual cortex, explaining why partial lesions in parietal regions may spare macular vision 2.
Temporal Lobe Lesions and Macular Sparing
Temporal lobe lesions affecting the optic radiation may produce various visual field defects ranging from no defect to upper homonymous quadrantanopia, lower homonymous quadrantanopia, or homonymous hemianopia 1.
These field defects can be either congruent or incongruent depending on the extent and location of the lesion 1.
As temporal lobe lesions typically encroach on the optic radiation from its outer side, visual field defects tend to be larger in the homolateral field, often sparing the macular representation 1.
Clinical Significance of Macular Sparing
Macular sparing, if present, is typically less than 0.4 degrees wide, making it a subtle clinical finding that requires precise testing methods like fundus perimetry 3.
In some cases, apparent macular sparing may be related to small eccentric fixation shifts during visual field testing 3.
Among patients with posterior cerebral artery infarctions showing macular sparing, approximately half had lesion-sparing confined to the posterior 25% of visual cortex, supporting the anatomical basis for this phenomenon 2.
Diagnostic Considerations
When evaluating patients with suspected parietal or temporal lobe lesions, visual field testing should include careful assessment of central vision to detect subtle macular involvement or sparing 2, 4.
Goldmann perimetry, Humphrey perimetry, and auto-plot tangent screen examinations are useful for defining central visual field disturbances within the central 10° of vision 4.
Advanced neuroimaging with high-definition MRI is essential for correlating visual field defects with the precise location of lesions in the visual pathway 2.
Clinical Patterns in Different Lesion Locations
Lesions in the posterior portion of the medial occipital area tend to cause incomplete central homonymous hemianopia 4.
In contrast, lesions in the occipital tip typically produce complete central homonymous hemianopia or quadrantanopia 4.
The fibers related to the central visual field appear to be sparse in the posterior portion of the medial area compared to the occipital tip, where they have a high concentration 4.