Clinical studies using Humphrey 30-2 perimetry in patients with pituitary adenoma have been reported by several researchers, including Lee et al 4 and Boland et al. 13 The former analyzed the Humphrey results qualitatively, classifying them as normal, unreliable, bitemporal, mixed (bitemporal and additional defects), monocular, homonymous, nonspecific, or other. The latter did not analyze light sensitivity itself using Humphrey 30-2 perimetry but rather the sum of the total or temporal scores of the neurologic hemifield test (NHT), which was assessed by the original ranking scales for probability of pattern deviation obtained from Humphrey 30-2 perimetry.
Total deviation, which is the reduction in dB below the expected age matched level, may be preferable to study light sensitivity at different test points in the visual field in many subjects of different age 16 because it eliminates age and eccentricity effects. In the current study we analyzed light sensitivity values instead of the total deviation because the objective of the study is inter-eye comparison of light sensitivity at corresponding test points in the same subjects. The results demonstrate that visual field defects were asymmetric between left and right eyes in about 80% of cases in the temporal or nasal hemifield or both.
Left-right visual field defect asymmetry may be caused by asymmetric enlargement of pituitary tumors, which is sometimes evidenced in the coronal section on MRI (Fig. 6). Lee et al 4 suggested that asymmetric compression of the chiasm by the tumor on MRI could predict visual field asymmetry with a sensitivity of 44% and with a specificity of 80%. Although statistical significance was not achieved (P = 0.06), Boland et al. 13 reported a possible relationship between tumor asymmetry on MRI and asymmetric visual field defects in the two eyes.
Apart from detailing temporal hemifield defects, the present study revealed that visual field defects extend into the nasal hemifield, which also demonstrates left-right asymmetry in many cases. Advanced bitemporal visual field defects seen in pituitary adenoma patients is likely to be followed by visual field loss in the nasal hemifield. 9 This could be attributed to damaged uncrossed nerve fibers, either because the growing pituitary tumor may successively damage directly the uncrossed fibers or because the growing tumor compresses the uncrossed fibers in the lateral part of the optic chiasm against the surrounding structures.
In terms of upper-lower asymmetry, it has been reported that the mean of the total deviation of Humphrey 30-2 perimetry results tends to be greater in the superotemporal quadrant compared to the inferotemporal quadrant, although statistical analysis failed to reveal significant differences. 17 In the present study, by investigating the frequency of severe scotoma instead of total deviation, we were successfully able to demonstrate statistically significant upper-lower asymmetry with upper-dominant deterioration in the temporal hemifield and lower-dominant deterioration in the nasal hemifield. While the upper-lower asymmetry was demonstrated with the definition of severe scotoma as light sensitivity of 5 dB or lower in the present study, the statistical results were the same when the threshold of zero dB or 10 dB was adopted as the definition of severe scotoma.
The pattern of asymmetry showing upper-dominant deterioration in the temporal hemifield and lower-dominant deterioration in the nasal hemifield may conform to Hedges’ report, 9 in which visual field damage in the inferonasal quadrant precedes damage in the superonasal quadrant. The mechanism for higher frequencies of severe scotoma in the inferonasal compared to the superonasal quadrant of the visual field may implicate compression of the prechiasmal optic nerve against the falciform ligament, which has been reported recently in neurosurgical literature. 10-12, 18
The optic nerve is roofed by the falciform ligament, which forms the sharp edge of a dural fold where it enters the optic canal anterior to the chiasm. It has been reported that compression of the optic nerve against the falciform ligament caused by the ectatic internal carotid artery may result in visual field defects, which could be resolved with microvascular decompression surgery. 11, 12, 18 Meningiomas that elevate the optic nerve from below and cause compression against the falciform ligament have also been reported to cause inferior altitudinal hemianopsia. 10 When the optic chiasm is elevated by an inferiorly-placed pituitary tumor, the superotemporal portion of the optic nerve fibers might become more damaged by the compression against the falciform ligament, leading to visual field damage in the inferonasal quadrant.
The limitations of the present study include its retrospective nature and relatively small amount of patient data. The data in the study were obtained using three different types of Humphrey algorithms. However we do not think that it would affect the comparison between right and left eyes, which were tested using the same algorithm. The order of perimetric examination of right to left or left to right was not controlled. The leaning effect in Humphrey perimetry might have influenced the results of inter-eye asymmetry. We were unable to use MRI data to assess horizontal tumor deviation and thereby investigate the relationship with left-right visual field defect asymmetry. Future large-scale prospective studies undertaken with the cooperation of neuroradiologists may be expected to lead to a more precise understanding of visual field defect pathophysiology in patients with pituitary adenoma.