Using similar methodology as in Case 1, we found that the lesion causing the visual field defect was located in the upper rostral portion of the primary visual cortex (Slice 12, Fig. 4C), sparing a small anterior part of the visual cortex as well as the occipital pole. According to the cytoarchitectonic maps, the probability was very low that the optic radiation was also affected (Fig. 4B).
HVFDs typically involve the central 10° of vision. To the best of our knowledge, excluding reports of damage to the monocular temporal crescent, there are only 3 documented cases of homonymous scotomas beyond 25° from fixation (4–6). Various reasons may account for the paucity of such reports. First, peripheral HVFDs may remain unnoticed by the patient because of low spatial resolution of the peripheral visual field. Second, the rarity of pure peripheral HVFDs may be due to anatomical variations leading to collateral circulation in cases of calcarine artery occlusion (19–22). Third, when a retrochiasmal lesion is suspected, automated static perimetry is usually performed within the central 30° of fixation. In both of our cases, the presence of single-defect points on the outer border of the 30° visual field in both eyes aroused suspicion of a homonymous pattern and led to examination of the peripheral visual field. Such subtle defects may sometimes escape detection or be erroneously interpreted as perimetric artifacts. With semiautomated kinetic perimetry in Case 1 and supraliminal automated static perimetry within 90° of fixation in Case 2, the peripheral homonymous defects were accurately detected.
As to the most appropriate perimetric technique for detecting occipital pole lesions, Wong and Sharpe (23) found that manual kinetic perimetry, either with tangent screen or Goldmann technique, coupled with automated static perimetry (Humphrey Field Analyzer, central 30-2 threshold program) are satisfactory screening tests. Our results confirm this observation.
Three retinotopic maps based on individuals with occipital lobe lesions have been proposed regarding visual field representation in the striate cortex. According to Holmes map, approximately 25% of the surface area of the striate cortex is devoted to the central 15° of vision (1,24,25). This has been corroborated with CT (26–28) and positron emission tomography (29). Horton and Hoyt (2) advocated a revised map based on MRI data of 3 patients with occipital lobe lesions, in which the central 15° of vision are represented by approximately 70% of the total surface area of the human striate cortex. Wong and Sharpe (3) challenged this finding by reviewing MRI data from 14 patients with occipital lobe lesions and concluded that the central 15° of vision occupy 37% of the total surface of the human striate cortex.
To determine the location of homonymous scotomas in our patients, we correlated the MRI findings with the proposed retinotopic maps. The map of Horton and Hoyt predicted that the scotomas would begin around 10° from fixation, the refined map of Wong and Sharpe predicted that the scotomas would be located at 15°, and Holmes map predicted that the scotomas would be located at 25°. Our data are consistent with the Holmes map, and support the hypothesis of Mejico et al (4), suggesting that the Horton–Hoyt and the Wong–Sharpe maps may overestimate the area of the striate cortex devoted to the central visual field. In order to exactly localize the brain lesion onto the visual pathway (optic radiation and primary visual cortex) and to investigate its relation to the functional (perimetric) outcome, we used a lesion analysis that combined established reconstruction techniques (17) with the stereotaxic probabilistic cytoarchitectonic atlas developed by the Jülich group (13–16). This methodical approach has been previously used in studies investigating the anatomy of the pupillary light reflex pathway (30), the functional topography of early periventricular lesions in regard to cerebral palsy and reorganization of language (31), the topography of unilateral tactile agnosia (32), and the involvement of damaged white matter fiber tracts in acute spatial neglect (33). Cytoarchitectonic maps are now available for a variety of brain areas, including primary motor, somatosensory, and visual cortices (34–36). In contrast to plotting the lesion onto the reference brain of the Talairach and Tournoux atlas (37), or the Montreal Neurological Institute (MNI) single-subject or group templates (38), these probabilistic cytoarchitectonic maps are based on the analysis of the cytoarchitecture of a sample of 10 human postmortem brains (http://www.fz-juelich.de/ime/ime_brain_mapping) and provide stereotaxic information on the location and variability of cortical areas in the MNI reference space. The technique used in our study overcomes the considerable intersubject variability of anatomical landmarks that occurs in the striate cortex (39).
The prognosis and recovery of function with visual field defects depends on the localization of occipital lobe infarctions. Celebisoy et al (40) found that striate cortex involvement was associated with poor prognosis. Similarly, Messing and Ganshirt (41) suggested that the best recovery was recorded after lesions of the occipital pole, while those in the striate area had the poorest prognosis. However, caution is needed when correlating the anatomical imaging data with the functional perimetric findings because the areas of infarct may be surrounded by edema especially in the acute phase. This might confound the delineation of the actual lesion area, where irreversible neuronal death has occurred (3,7). The cytoarchitectonic maps support the localization of brain lesions (optic radiation and striate cortex) but do not provide any detail of retinotopic mapping. They provide a useful adjunct to retinotopic maps and conventional MRI.
The authors are indebted to Mrs Elke Krapp and Regina Hofer for their help in preparation of this manuscript.
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