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Editors Selection IGR 9-3

Risk Factors: Long axial length

Comment by Claude Burgoyne on:

23937 Long axial length as risk factor for normal tension glaucoma, Oku Y; Oku H; Park M et al., Graefe's Archive for Clinical and Experimental Ophthalmology, 2009; 247: 781-787

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An association between myopia and the susceptibility of the optic nerve head (ONH) to developing a glaucomatous optic neuropathy at all levels of intraocular pressure (IOP) is emerging, but is not present in all studies. If the myopic eye is more susceptible to normal and elevated levels of IOP, what is the actual causal factor? Oku et al. (740) retrospectively studied the association between one component of myopia (axial length), normal tension glaucoma (NTG) and primary open angle glaucoma (POAG) in a group of Japanese patients undergoing cataract or combined cataract and glaucoma surgeries (1,202 eyes of 1,202 patients: 87 NTG, 137 POAG, and 978 non-glaucomatous control cases). Axial length, IOP, curvature of the anterior corneal surface, age, and gender were determined at the time of surgery. They found that axial length was significantly associated with NTG (odds = 1.24, P = 0.002) and POAG (odds = 1.28, P = 0.001) and that the incidence of either POAG or NTG was significantly higher in patients with axial lengths ≥ 25.0 mm (odds = 2.29, P < 0.001, Fisher's exact test). Age was not different between groups and was weakly but significantly correlated negatively with the axial length (r = -0.24, P < 0.001, Pearson's correlation coefficient test). They concluded that axial lengths ≥ 25 mm should be considered a risk factor for NTG and POAG, and that all such patients should be carefully examined for the presence of an optic neuropathy regardless of IOP.

Axial length separately influences ONH susceptibility in glaucoma

The study is of interest because in this patient population it establishes axial length (not refractive error) as a risk factor, which only one previous study has done. The study is limited by the fact that the normal eyes were not aggressively screened by glaucoma specialists for the presence of glaucoma, nor was visual field testing performed. If rigorous case finding turned up many more cases of visual field documented glaucoma among the normal eyes, the axial lengths of these patients may have strengthened or weakened the reported associations. It is of interest that, while the axial length in NTG and POAG patients was greater than in normal eyes, there was no difference in axial length between these two groups. If axial length contributes to ONH susceptibility, one might expect it to be greatest in the NTG group or that a greater number of NTG patients would have long axial lengths. While not directly addressed, Table 3 in fact suggests that the prevalence was higher in the POAG group (40:137 or 29% compared to 20:87 or 23%) in the NTG patients. Aggressive case finding among the normal controls may importantly influence these results. This study supports the concept that axial length separately influences ONH susceptibility in glaucoma. The causal factor(s) underlying that influence ‐ i.e., large scleral shell, thin sclera, thin lamina, steeper translaminar pressure gradient, large scleral canal opening, abnormal connective tissue material properties, or abnormal scleral and lamina cribrosacyte molecular biology remains to be determined.

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