Retinal vessel diameter in young patients with open‐angle glaucoma: comparison between high‐tension and normal‐tension glaucoma

Editor, It remains controversial whether normal-tension glaucoma (NTG) and high-tension glaucoma (HTG) differ in their pathogenesis. Some investigators demonstrated structural or functional difference, whereas others reported no difference between them. Regarding the diameter of retinal vessels (RVD), two previous studies reported no difference in RVD between HTG and NTG (Mitchell et al. 2005; Amerasinghe et al. 2008). However, these studies included subjects older than 40 years. As the RVD may be influenced by age and co-existing systemic diseases, its measurement in younger patients with open-angle glaucoma (OAG) is expected to provide better comparison. Thus, we investigated RVD in young OAG patients and compared it between HTG and NTG. We reviewed the medical records of 161 consecutive patients who had been newly diagnosed as primary OAG at age ≤40 years at Korea University Medical Center from 2007 to 2010. The inclusion criteria were as follows: (i) mean deviation (MD) on initial visual field test <12 dB; (ii) refractive error <−8 dioptres; (iii) no history of hypertension, diabetes and other vascular disease and (iv) no previous ocular surgery. Those with untreated intraocular pressure (IOP) consistently <22 mmHg on three separate occasions were defined as NTG and the others as HTG. Each HTG patient was matched to one with NTG based on age (within ±5 years), sex, refractive error (within ±3 dioptre) and MD (within ±3 dB). Twenty-seven pairs of eyes were successfully matched. Diameter of retinal vessels was measured on the digital 30-degree retinal photographs taken before glaucoma treatment as described elsewhere (Knudtson et al. 2003). In brief, all arterioles and venules passing completely through a circumferential zone 0.5 to 1 disc diameter from the disc margin were measured using the software (Visupac/System, Software version 4.2; Carl Zeiss Meditec, Jena, Germany). A single grader, blinded to subject characteristics, performed all vessel measurements. Central retinal arteriolar equivalent (CRAE) and central retinal venular equivalent (CRVE) were calculated using the revised Parr–Hubbard formulas (Knudtson et al. 2003). Another observer also performed RVD measurements; the intraclass correlation coefficient (95% confidence interval) was 0.865 (0.780–0.920) for CRAE and 0.867 (0.781–0.920) for CRVE. Optic disc size, cup size, neuroretinal rim area and peripapillary atrophy area were also measured with ocular magnification corrected using the built-in program. Among the demographic data, only untreated IOP differed between NTG and HTG group (Table 1). Mean CRAE was smaller in the NTG than in the HTG group (p = 0.007). However, no significant difference was found in mean CRVE. Optic disc parameters were not different between groups (Table 2). Our finding of smaller CRAE in NTG versus HTG eyes is inconsistent with the previous studies (Mitchell et al. 2005; Amerasinghe et al. 2008). Such discrepancy may be attributed to different study designs (sample sizes, diagnostic criteria, prospective versus retrospective and population based versus hospital based). Retrospective nature of this study also precluded assessment of ocular perfusion. Nevertheless, the present study has several strengths: (i) younger age, (ii) MD matching controlled glaucomatous damage between groups and (iii) assessment of optic disc characteristics found no difference between groups. Given the fact that OAG is far less prevalent in this younger age group, the present findings are still likely to be meaningful. Unlike arterioles, retinal venular diameters did not differ between NTG and HTG in this study. Some studies suggested that the diameters of retinal arterioles may be more related to the glaucomatous optic neuropathy rather than those of retinal venules (Wang et al. 2007; Chang et al. 2011). These results, taken together, may provide further support to the view that NTG and HTG have, at least partially, different pathogenesis. However, further researches are warranted to elucidate the relevance of these findings.