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A male patient CB (date of birth 31/12/1962) attended for a routine eye examination in July. The refraction was isometric and unremarkable (R -1.50/-0.75 x 10 (6/5), L -2.00/-0.25 x 5 (6/5)), pressures within normal limits (R 11 mmHg, L 12mmHg with non-contact tonometry) and a Henson suprathreshold screening revealed no field defect (threshold established at 32 dB).
Ophthalmoscopy did, however, show a significant asymmetry of the optic discs (Figure 1). The right disc has a vertical cup/disc ratio of 0.5, variable margin definition and early peripapillary atrophy. The overall disc appears paler than the left, though the neuroretinal rim appears adequately perfused. The left disc has a much smaller cup, a ratio of around 0.2 and a more oval and regular margin. The disc appears a little darker, possibly mostly to do with the smaller cup. The patient was referred routinely to rule out the possibility of any neuropathic disease of the right eye.
After fundoscopy and a full threshold field assessment, the ophthalmologist concluded that there was no sign of any disease, that the discs were indeed asymmetric and that future regular eye examinations were in order, ideally to 'include a GDx assessment'.
Nerve fibre layer assessment
Optician arranged with Zeiss Instruments UK for a screening of the patient at its headquarters. As well as the fundus photos shown, a full threshold field assessment (SITA Fast 24-2 central plot) was undertaken and, despite three fixation losses in the right eye, all values were within normal limits and the plots appeared normal and equal (Figures 2a and b). A frequency doubling technology (FDT) perimetry was undertaken. It is thought that the loss of ability to detect a doubling in spatial frequency of a succession of out-of-phase grating targets is an early glaucomatous sign. The FDT is a useful central vision screener with a good sensitivity related to primary open-angle glaucoma. Results for the patient were normal (Figure 3).
As recommended by the opthalmologist, both discs were scanned using the GDx. This interprets reflections from layered structures, such as the retinal nerve fibre layer, and so can then offer dimensional detail. By taking a reading from around the macula, followed by one from around the disc, the machine is able to compensate for the birefringence resulting from the layers of the cornea (a significant source of error in earlier models of GDx).
Figure 4 shows the results from the GDx scan. The table at the top shows reliably (p > 5 per cent) that the thickness values around the discs were within normal limits and that the asymmetry was minimal.
The nerve fibre thickness map shows superior and inferior thicker areas of nerve fibre layer as would be expected in the normal eye, and a slight difference in the distribution of these fibres related to the different shape of the discs (hotter colours are thicker, colder thinner). The deviation maps show some minimal thinning compared with the normal eye, notably superiorly on the right.
All these findings were well within the normal expectation and not suspicious. The summary graphs at the bottom of the printout show that, though the discs have a different profile (note the thinner nasal aspect for the left eye for example), the two retinal nerve fibre profiles fall well within the normal limits.
Discussion
This case is interesting because the retinal nerve fibre layer data reassures the practitioner that, despite appearances, both discs appear to have a healthy nerve fibre distribution.
Disc asymmetry, in the absence of any explanation such as large anisometropia, is always to be considered as significant by eye care practitioners. Loss of the nerve fibre layer contributes to extension of the cup (vertically in degenerative neuropathies such as glaucoma) and is therefore referable even in the absence of tonometry or field results.
The results here show that, despite an apparent difference in disc structure, the equality of the nerve fibre layers makes glaucoma most unlikely. Routine monitoring over the coming years will be sufficient to detect any change in nerve fibre layer thickness and warrant re-referral. As it stands, however, the GDx in this instance would have reassured the practitioner and reduced the need for a referral at this stage. It is also interesting to note that the ophthalmologist has recommended regular review by GDx.
