The longer symptom duration of macular hole indicates a larger hole diameter theoretically. It would appear that we can identify the symptom duration according to the hole diameter, but this is not the case. There was no correlation between the symptom duration and hole diameter (ref. 19) according to Ullrich et al. This finding can be explained by the pathogenic mechanism of macular hole. Hole expansion will be further caused by the traction in the tangential direction on the surface of the posterior vitreous cortex, and the contraction of proliferative glial cells on the internal limiting membrane (ref. 20, 21) after macular hole (MH) formation. Therefore, hole diameter may mainly depend on traction instead of the duration of the macular hole. Patients have subjective estimates on symptom duration, so macular hole may have persisted for a long time before the diagnosis is confirmed, which showed that the hole diameter on OCT image is not accurate in the diagnosis of chronic macular hole.
Diameter is one of the most important indicators to assess the morphological characteristics, and surgical outcomes of macular holes. The minimum diameter and basal diameter are among predictors of macular hole closure and visual acuity after surgery (ref. 19, 22–25). A study found that the macular hole with a larger minimum diameter and basal diameter was more likely to experience type 2 closure (retinal edges were flat, and there was a defect of the neurosensory retina on the fovea) (ref. 26). Liang X et al. found that the minimum diameter was a valuable prognostic factor for postoperative metamorphopsia (ref. 27). Minimum diameter has been reported in literature as the only parameter predicting surgical success, and is associated with postoperative visual acuity (ref. 28). The chronic group had a larger MLD and BD than the control group in this study, and the differences were statistically significant. Statistically significant differences were noted only in the MLD between the two groups after adjustments basald on age, sex, and hole stage. This is consistent with the result of previous studies (ref. 29). MLD was identified as a factor closely associated with the diagnosis of chronic IMH after logistic regression analysis. This shows that the MLD is more meaningful than the BD to predict chronic macular holes, and a larger MLD instead of a larger BD suggests a larger likelihood of chronic macular holes.
Hole height can be used as an indicator to evaluate the morphological characteristics of macular holes in the perpendicular direction. Previous studies have reported that HH is not associated with the hole closure or postoperative visual acuity. Our study found that chronic IMH had a smaller height than acute IMH, and statistical differences were still noted between the two groups after adjustments basald on age, sex, and hole stage. The possible reason for chronic IMH to have a smaller height is as follows: According to the pathogenic mechanism of macular hole, the vitreous body will be completely separated from the fovea as the hole progresses. Traction in the perpendicular direction of the hole is relieved, but in the horizontal direction is still there. The hole height no long increases, but the hole diameter continues to increase. Retinal atrophy occurs at the edge of chronic MH, the intraretinal fluid decreases, and the edge of chronic MH intimately adheres to the retinal pigment epithelium (ref. 29), resulting in a smaller height of chronic MH, compared with the height of acute MH.
Researchers have proposed more indicators to present the morphological characteristics of MH basald on measurements of the hole diameter and height. MHI is defined as the ratio of hole height to maximum basal diameter, which reflects the combined effect of the tractions in both the perpendicular and tangential directions on morphological characteristics of MH (ref. 30). MHI is an important predictor of visual outcome after MH surgery, and has been reported to be positively correlated with the postoperative visual acuity. Compared with MHI < 0.5, the MHI ≥ 0.5 indicates a better visual acuity (ref. 30, 31). THI is expressed as the ratio of hole height to minimum diameter, which also reflects the ratio of traction in the perpendicular direction to that in the tangent direction. HFF can reflect the relationship between the sum of linear lengths of the photoreceptors detached from both sides of MH, and the basal diameter. The larger HFF indicates a larger likelihood for tissues on both sides of the MH to completely cover the bare retinal pigment epithelium after surgery, as well as the larger likelihood of well-healed macular holes, and improved postoperative visual acuity. Studies have shown that THI and HFF have positive correlations with the best corrected visual acuity (ref. 12, 19, 32). Patients with chronic macular holes had a smaller MHI, THI, and HFF than patients with acute macular holes, and the differences were still statistically significant between the two groups after adjustments basald on age, sex, and hole stage in this study. Parameter changes on OCT in the chronic group were consistent with the low closure rate, and poor visual prognosis after surgery for chronic macular holes. DHI is the ratio of minimum hole diameter to maximum basal diameter, which reflects the traction in the tangent direction. DHI is in slightly positive correlation with the healing process of the macular hole. The larger DHI indicates the larger likelihood of no macular holes. No correlation has been found between DHI and postoperative visual acuity (ref. 28, 33). In this study, DHI was found to increase in the chronic group, but no statistical difference was found between the control group and chronic group after adjustments basald on age, sex, and hole stage.
Many OCT parameters can presently be used to evaluate the morphological characteristics of macular holes. The hole diameter and height are the most readily available parameters on OCT images. MLD and HH was identified as a factor closely related to the diagnosis of chronic macular holes, and area under ROC curve of the diagnostic model reached 0.880, suggesting the excellent diagnostic efficacy after logistic regression analysis in our study. Characteristic changes of quantification parameters on OCT images of chronic IMH have been reported in our study. Chronic macular holes have a larger minimum diameter, smaller hole height, MHI, THI, and HFF. MLD and HH is closely correlated with the diagnosis of chronic macular holes, which is easy to apply in clinical practice, and does not require complex measurements or calculations.
This study still has some limitations. The enrolled chronic IMHs were confirmed by OCT to have persisted for ≥ 1 year. The long duration of macular holes was identified. Surgery within 6 months after the onset of macular hole symptoms will have obvious benefits (ref. 34), so the symptom duration of less than six months was used as the time node to define the acute IMH (control group). The possibility cannot be ruled out that the macular hole may have persisted for a long time before the diagnosis is confirmed, due to the inaccurate duration of subjective symptoms provided by patients although we have limited the symptom duration as an inclusion criterion for the control group. At the same time, the sample size of this study is small. A prospective study with a larger sample size is therefore needed to confirm the conclusions of this study.