Clinical assessment of the optic disc at risk of nonarteritic anterior ischaemic optic neuropathy

Abstract

Editor, Nonarteritic anterior ischaemic optic neuropathy (NAION) is an acute optic neuropathy occurring predominantly in small optic nerve heads in elderly patients (Beck et al. 1984; Hayreh et al. 1994; Hayreh 1996; Jonas & Xu 1993). Recently, phosphodiesterase-5 inhibitor drugs such as sildenafil have been mentioned as a potential risk factor for NAION (Pomeranz & Bhavsar 2005; Hayreh 2008). If indeed the intake of phosphodiesterase-5 inhibitor drugs is a risk factor for NAION, the detection of subjects at risk of the development of NAION prior to the intake of the drugs may be helpful to give a warning not to use the medication. Subjects at risk of NAION may be detected by an ophthalmoscopic exa-mination searching for subjects with a small optic nerve head. Because the assessment of the optic nerve head and the detection of discs at risk of NAION are markedly dependent on the examiner, we performed this study to assess the inter-observer variability in the ophthalmoscopic examination of the optic nerve head in classifying risk of NAION. The clinical observational study included monoscopic optic disc photographs of 40 healthy eyes of Caucasian subjects without any health problems. The horizontal cup/disc diameter ratio as measured on the photographs was 0.48 ± 0.11 (mean ± standard deviation) (median: 0.50; range: 0.18–0.76). The optic disc photographs were presented to three groups of examiners. The first group consisted of 10 experts in the field of neuro-ophthalmology. The second group included 16 comprehensive ophthalmologists who had been working in private office for at least 10 years. The third group included 35 residents in ophthalmic training with an experience in ophthalmology ranging between 1 and 5 years. All examiners were asked to assess whether the optic disc represented a risk of the development of NAION. Before the test, the association between a small optic disc with a slightly unsharp optic disc border and NAION was discussed. The 10 experts showed a relatively high degree of agreement of about 80% in classifying the discs with a cup/disc diameter ratio of 0.40 or less for NAION (Table 1). In a corollary way, they showed a relatively high degree of agreement of about 80% in classifying discs with a cup/disc diameter of 0.56 or higher as not at risk of NAION. For the intermediate discs with a cup/disc diameter ratio ranging between 0.40 and 0.55, the agreement rate was roughly 60%. For the 16 practitioners when compared with the experts, the agreement was generally lower and did not vary significantly (p > 0.05) from the agreement rate of the residents (Table 1) (Fig. 1). Scattergram showing the distribution of the agreement (percentage of yes-answers) among 16 ophthalmologists in private practice in classifying optic nerve heads as discs at risk of nonarteritic anterior ischaemic optic neuropathy (NAION), in relation to the horizontal cup/disc diameter ratio. The scattergram shows a wide variation in the decision, whether the individual eye had an optic disc at risk of NAION. The results suggest that ophthalmologists in private practice and ophthalmic residents show a relatively high inter-observer variability in classifying optic nerve heads as discs at risk of the development of NAION. Potential limitations of our study should be mentioned. The artificial examination conditions for the study participants who knew that they were tested may have led to an increased concentration of the study participants and to falsely good results. Another limitation of the study may be the term ‘disc at risk’ itself. According to Hayreh, it is not the small disc that poses the risk to develop NAION, but instead it may be a drop in arterial blood pressure, occurring mostly at night (Hayreh et al. 1994; Hayreh 1996). This drop in blood pressure leads to an initially circumscribed ischaemic event in the optic nerve head. Because of the lack of space in small discs, this normally unremarkable event may trigger a vicious circle with an ischaemic oedema-induced compression of the tissue in the vicinity of the circumscribed malperfusion, progressively increasing the regional ischaemia. In conclusion, the diagnostic precision of ophthalmologists in detecting an optic disc at risk of NAION may not be sufficient to reliably counsel a subject about the risk of NAION. Supported by the Paul and Evanina Bell Mackall Foundation, a Senior Scientist Award (AML) and an unrestricted grant from Research to Prevent Blindness Inc. and a grant-in-aid from Pfizer Inc.