The Hidden Gamma Zone
Dr. Rashmi Nagar, Dr. Sarang Lambat, Dr. Prabhat Nangia, Dr. Vinay Nangia
Suraj Eye Institute, 559 New Colony, Nagpur
A male 58 years of age, a known case of primary open angle glaucoma presented to us with complaints of blurring of vision in both eyes. He was not on any anti-glaucoma medications. He had undergone Phacoemulsification and trabeculectomy with mitomycin C in both eyes in 2017. On slit lamp examination there was a good filtering bleb in both eyes. Intraocular pressure on Goldman applanation tonometry was 12 mmHg in the right eye (RE) and 9 mmHg in the left eye (LE).
On fundus evaluation, a vertical cup-disc ratio (VCDR) of 0.8:1 with superior rim loss in RE and 0.8:1 with inferior rim loss in LE was noted. Peripheral retina was found to be normal.
The loss of RNFL inferiorly is known to be much more frequent than superiorly, and also tends to occur earlier. However, loss of RNFL superiorly is also known to occur first but much less frequently than inferior RNFL loss. This case is interesting because the RE shows more significant loss superiorly and the left eye shows more significant loss inferiorly. Clinically one may keep a watch for such an occurrence. This finding also tells us that the causes of RNFL loss occurring inferiorly may not play out in a similar manner even in the two eyes of the same patient. One may consider that this is an example of optic discs that show significant glaucomatous damage, however the greater extent of the RNFL damage does indicate the possibility that the superior RNFL in RE and inferior RNFL in the LE may also have been the initial areas where the damage began and progressed.
The GCL circles shown above are important because they are close to the fovea. The horizontal diameter of the outer circle of the GCL as shown in figures (5,6) above is (1.857 mm – 6.63 degrees approximately) and the vertical diameter of the outer circle is (1.59 mm – 5.68 degrees approximately). The clinical importance of significant loss of GCL (less than one percentile value) so close to the fovea may be understood in the appropriate clinical context, because it influences our management of the patient and may also influence our indications for surgery based on a constellation of clinical findings. There is an area of the inferior optic disc macular RNFL segment that is often more vulnerable to glaucomatous damage, which is connected with the ganglion cells placed just beneath the foveal area as seen in the LE of this subject. The associations of the RNFL axons with the GCL location may be important in understanding the structural and functional loss that may occur. This would play an important role in the management of our glaucoma patients.
- Hou HW, Lin C, Leung CK. Integrating macular ganglion cell inner plexiform layer and parapapillary retinal nerve fiber layer measurements to detect glaucoma progression. Ophthalmology. 2018 Jun 1;125(6):822-31. DOI: 10.1016/j.ophtha.2017.12.027
- Nouri-Mahdavi K, Nowroozizadeh S, Nassiri N, Cirineo N, Knipping S, Giaconi J, Caprioli J. Macular ganglion cell/inner plexiform layer measurements by spectral domain optical coherence tomography for detection of early glaucoma and comparison to retinal nerve fiber layer measurements. American journal of ophthalmology. 2013 Dec 1;156(6):1297-307. https://doi.org/10.1016/j.ajo.2013.08.001
Dr Vinay Nangia
MS, FRCS, FRCOphth
Suraj Eye Institute