Dr Adrian Bruce
BScOptom PhD FAAO FACO
Australian College of Optometry and The University of Melbourne
Australian College of Optometry
Optic nerve head cupping and more specifically advancing cupping are widely accepted as one of the key signs of glaucoma.
The condition is often known as glaucomatous optic neuropathy (GON), on the principle that the optic nerve is the fundamental site of the disease. With this approach in mind, clinicians may not consider glaucoma if there is a smaller degree of cupping, for example, a cup-disc ratio below 0.5.
Because structural loss usually precedes visual field loss,1,2 in the absence of obvious cupping the possibility of glaucoma may be discounted. There are many factors to consider when assessing for glaucoma, but to disregard the need for further investigations based on a ‘small’ cup-to-disc ratio could result in misdiagnosis.
In a retrospective analysis, Sherman and colleagues presented several cases of glaucoma in the absence of significant cupping.3 They demonstrated how new technologies at the time such as the Heidelberg Retinal Tomograph and GDx were being used in the diagnosis of these cases of glaucoma without cupping.
In this report, we present a case of glaucoma without cupping, this time using optical coherence tomography (OCT) to assist with diagnosis.
A 63-year-old Caucasian male attended the general clinic of the Australian College of Optometry for an annual glaucoma review. He had a family ocular history of glaucoma; his father and aunt had been diagnosed and treated for the disease.
The patient enjoyed excellent unaided vision, with R 6/6, L 6/6. Gonioscopy showed angles to be open to scleral spur in both eyes. There was no pseudoexfoliation syndrome or pigment dispersion syndrome. Cup-to-disc ratios were R 0.25, L 0.3 and the disc diameter was small, estimated at about 1.4 mm, although the neuro-retinal rims were seen as full.
The intraocular pressure (IOP) was R 20 mmHg L 20 mmHg and central corneal thickness (CCT) measured R 534 µm and L 539 µm. In previous years, his IOP had been in the range of 19-22 mmHg. The borderline IOP and the family history had led to the ongoing review (Figure 1).
An OCT examination was performed to assess both the retinal nerve fibre layer (RNFL) at the optic discs and the inner retinal layers (ganglion cell and so on) at the maculae. The Nidek OCT Macula Map showed normal findings in the right eye and a distinct inferior arcuate defect on the normative database map in the left eye (Figure 2). The left eye analysis of the macular inner retinal layers (GChart) highlighted the superior-inferior hemifield asymmetry.
Figure 2. Nidek OCT Macula Map: left eye showing inferior hemifield defect with an arcuate defect of the macula ganglion cell layer
The Nidek OCT Disc Map of the RNFL was normal in the right eye, but for the left eye flagged thinning of the inferior-temporal quadrant (Figure 3).
Figure 3. Nidek OCT disc map flagging the left inferior and temporal RNFL quadrants
Visual field assessment using the Medmont Central Fast Threshold Test showed a left, superior nasal defect in the perimacular area (Figure 4). This defect was repeatable, with reliable indices. The field defect corresponded with the Nidek OCT macula scan findings of an inferior perimacular defect in the ganglion cell layer as well as the disc map defect in the inferior-temporal RNFL quadrants.
Figure 4. Medmont Central Test left visual field, with corresponding superior nasal perimacular defect
As a result of these findings, the patient was referred to an ophthalmologist who confirmed the diagnosis and commenced treatment for primary open angle glaucoma.
Optic disc size
NHMRC guidelines give the average vertical disc diameter as 1.6 to 2.0 mm.4 The disc size can be measured clinically using a fundus lens, with low powered lenses (like a Volk 60 D) being the most accurate.5 Alternatively, most OCT RNFL analyses give an estimate of the disc area. The OCT scan in this case gave the disc areas as R 1.30 mm2 and L 1.21 mm2.
Optic disc area may be compared to the disc diameter most accurately via an ellipse formula, although a circle formula is a simple approximation (area = pr2, where r = radius). When calculated using the NHMRC’s guidelines for disc diameter,4 the average optic disc is predicted to be 2.0 mm2 to 3.1 mm2. This patient’s optic disc areas were significantly less.
This case demonstrates the importance of considering the optic disc size in conjunction with the cup-disc ratio. Clinically, it is tempting to link smaller cupping with a low risk for glaucoma. Just as the corneal thickness provides context for interpretation of the IOP measurement, the optic disc size does the same for the cup-disc ratio. The consequence of not allowing for disc size could be a delay in diagnosis, until more significant optic nerve damage or functional loss has occurred.
1. Quigley HA, Katz J, Derick RJ and et al. An evaluation of optic disc and nerve fibre layer examinations in monitoring progression of early glaucoma damage. Ophthalmology 1992; 99: 1: 19-28.
2. Samarawickrama C, Hong T, Jonas JB, Mitchell P. Measurement of normal optic nerve head parameters. Survey of Ophthalmology 2012; 57: 4: 317-336.
3. Sherman J, Bass S, Slotnick S. Glaucoma without cupping. Optometry 2004; 75: 677-708.
4. National Health and Medical Research Council. NHMRC guidelines for the screening, prognosis, diagnosis, management and prevention of glaucoma, 2010.
5. Lim CS, O’Brien C, Bolton NM. A simple clinical method to measure the optic disc size in glaucoma. J Glaucoma 1996; 5: 4: 241-245.
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