Diagnosing glaucoma – the process
In a recent scientific report published by nature.com, which considered the “impact of ‘natural blind spot location’ on perimetry“,
The natural blind spot location (NBSL) represents an absolute scotoma in the visual field (VF) induced by the area of the optic disc on the retina, which lacks the light detecting cells. NBSL is routinely measured in standard VF testing so as to interpret the VF test results – especially for the diagnosis of glaucoma.
Glaucoma, the second leading cause of irreversible blindness, is a group of ocular diseases characterised by optic disc excavation for which elevated intraocular pressure is a major risk factor.
Functionally, patients with glaucoma develop a loss of VF sensitivity over space, known as “visual field loss”. In typical clinical glaucoma testing, standard automated perimetry measures the VF the patient’s response to white light stimuli with different luminance intensities at fixed locations in the field of vision. The pattern of functional VF loss caused by glaucoma is weakly associated with the structural damage of retinal nerve fiber layer (RNFL) bundles. In glaucoma diagnostics, the evaluation of both functional VF loss and structural damage of RNFL is usually combined to assess the severity of the disease.
Generally, tracing RNFL bundles is difficult due to poor visibility in retinal images. It has been shown that the optic disc location relative to fovea is related to RNFL geometry and should be considered for RNFL profile norms. The diagnosis of glaucoma might be potentially improved by including the variability of the optic disc position relative to fovea. The functional equivalent of the optic disc location, the NBSL is routinely measured in standard with automated perimetry devices as part of the VF measurement.
In general, the distribution of blind spots is asymmetric. In particular, it is a bimodal distribution of NBSL in the vertical plane. Previously, a possible bimodality would not have been detectable due to small samples sizes.
Discrepancies between various results may suggest the blind spots distribution variability between populations, their health conditions and other intervening factors that aid clinicians in precisely interpreting VF measurements.
Whilst the position of the optic disc in relation to the fovea has an impact on the RNFL trajectories, the NBSL in relation to fixation also has a systematic effect on the spatial distribution of VF sensitivity.
Suffice to say, a specialist will base his diagnosis on many more factors and analyses than one might initially think. It is therefore important to consult an ophthalmologist as soon as you notice any discrepancy in your VF, in addition to the regular consultations, which might not produce any spectacular results.