In our population-based study on adult Chinese in Beijing, the prevalence of AH was 0.9% per eye or 1.6% per subject. AH was associated with elder age (P = 0.014, OR 1.057), thicker lens (P = 0.032, OR 3.887), and higher spherical equivalent (P = 0.017, OR 1.396). AH was not associated with the systemic parameters of blood pressure, glucose, diabetes, cholesterol, and triglycerides.
Previous studies of the prevalence of AH were in autopsy series and clinical populations. However, the best source for prevalence estimates in a general population is population-based studies. To our knowledge, this is the first large population-based study reporting on the prevalence of AH in Asian populations. Our prevalence (1.6%, 95% CI: 1.1%, 2.0%) results are consistent with previous studies of AH. In the Beaver Dam Eye Study of 4,952 patients, AH was present in 1.1% (95% CI: 0.9%, 1.5%) of the total population. The Australian Blue Mountains Eye Study of 3,654 patients confirmed a similar 1.0% (95% CI:0.7%, 1.3%) prevalence of AH. In the autopsy series of 10,801 eyes studied at UCLA, AH had a prevalence of 1.96%.
Our data show that AH was associated with elder age (P = 0.014). Prevalence increased significantly with age from 0.7% in the 50–59 year age group to 2.7% in the 80–93 year age group. Several clinical-based studies[10, 12, 13] concluded that the occurrence of AH was significantly associated with elder age. In the Beaver Dam Eye Study, the prevalence of AH increased from 0% of persons aged less than 55 years to 2.1% of persons aged 75 years or older. In the Australian Blue Mountains Eye Study, the prevalence of AH also increased with age from 0.2% in subjects 43–54 years, to 2.9% in subjects 75–86 years. These findings confirm our results. Komatsu and coworkers used samples of AH obtained in vitreous surgery and observed with light and electron microscopes and processed by the focused ion-beam method. They thus concluded that AH was produced not only by changes in ionic tension in the vitreous fluid but also by changes in the vitreous matrix in the aging process and diseases.
Our data also show that AH was associated with a higher spherical equivalent (P = 0.017). Bergren and coworkers performed a cross-sectional study of 12,205 patients and reported patients with AH were more hyperopic than control subjects. One explanation is that the complete posterior vitreoretinal interface may be important for the formation of asteroid zona pellucida. Therefore, the older the age, the more likely the individuals who maintain posterior vitreous attachment are to develop AH. This may explain why AH is associated with a higher spherical equivalent because PVD is more common in myopia and occurs at an earlier age. However, we did not find an association between AH and PVD (P = 0.417) in our study. Another possibility is that the presence of AH may arrest the process of vitreous collapse or contraction and has a protective effect on vitreous liquefaction, which prevents PVD.
In our population-based study, AH also correlated to the thicker lens (P = 0.032). As there is no relevant report so far, the associations between AH and lens thickness have remained unclear. One of the reasons may be that the lens thickness was associated with higher age and hyperopic spherical equivalent. Previous studies have proved that the age-related increase in lens thickness was due to the continuous production of new lens fibers in the equatorial region of the lens. Besides, the higher refractive power necessary in hyperopic eyes lead to greater lens thickness . From a geometric point of view, the thicker lens partially protrudes forward into the anterior chamber and partially bulged backward into the vitreous cavity, which may affect the dynamics of aqueous humor circulation and thus cause changes in the composition of the vitreous extracellular matrix.
In our study, AH was not associated with diabetes. The prevalence of diabetes was higher in AH group (1.8% vs. 1.6%), the glucose level was higher in AH group (6.14 ± 2.10 vs. 5.61 ± 1.59, mmol/l), and the glycosylated hemoglobin level was higher in AH group (4.55 ± 1.21 vs. 4.35 ± 1.03, %), but there was no significant difference (p = 0.829, 0.129, 0.245, respectively). In fact, except for a few retrospective small sample studies[15, 21] that reported the correlation between AH and diabetes, most of the large sample studies (The Blue Mountains Eye Study, n = 3654; The Yonsei Eye Study, n = 13016; The Beaver Dam Eye Study, n = 4926) reported that there was no statistical correlation between AH and diabetes. Fawzi et al. reviewed 10801 patients in the University of California at Los Angeles (UCLA) autopsy eye database and reported that there was no correlation between diabetes and AH. They found only specific age subgroups (51–60 years, P = 0.006; 41–60 years, P = 0.004) that showed a statistically significant association between AH and diabetes. However, there was no correlation among the age groups younger than 40 years, 81–90 years, and 91 years and over (P = 0.50, 0.53, and 0.73, respectively.). Considering the irreversibility of the course of diabetes, we have reason to doubt the accuracy of this result. Elbaz et al. reported that the association between diabetes and AH was substantially attenuated from a univariate OR of 3.88 to an OR of 1.99 after adjustment for sex and age. The relationship between AH and diabetes may be explained by the increase of basal membrane permeability in patients with diabetes, which are likely to be the source of phospholipids and calcium required for asteroid formation. But the current data can not be conclusive, further physiology research is needed.
In our study, AH was not associated with other systemic parameters of body height, weight, the circumference of the waist, the circumference of the hip, diastolic blood pressure, systolic blood pressure, serum concentrations of low-density lipoproteins, high-density lipoproteins, cholesterol, creatine, triglycerides, glucose, glycosylated hemoglobin, diabetes, hypertension, hyperlipidemia, self-reported diagnosis of cerebral infarction or haemorrhage, and coronary heart disease, frequency of reported snoring, smoking. AH has purportedly been associated with several systemic diseases, including diabetes, hypercholesterolemia, hypertension, hypercalcemia, and gout. Many of these studies have been case series, case-control studies, or performed in clinic populations and may reveal a selection bias for diseases that are more prevalent in patients presenting for ophthalmic and vitreoretinal evaluations. Thus, these findings cannot readily be extrapolated to the general population. Besides, the prevalence of bilateral AH was 18.9% in our study. The fact that AH mainly occurs unilaterally[3, 10–12] does not support an association with systemic parameters.
Attention should be paid to AH, since it may prevent PVD and have a protective effect on vitreomacular traction. Besides, AH generally only has a minor impact on vision and thus may be a useful model for better understanding the interaction between incident light and intravitreal structures. Further studies on AH may help in understanding the pathogenesis of ABs by showing the correlations between AH and other ocular and general parameters.
This is the first population-based investigation searching for the prevalence and associations between ocular and systemic parameters and AH in the Chinese population. Despite the advantages of this population-based study, potential limitations of our study should be mentioned. First, a major concern regarding any prevalence study is non-participation. The Beijing Eye Study 2011 had a reasonable response rate of 78.8%, although differences between participants and non-participants can lead to a selection bias. Second, the presence of AH was determined either from the presence of typical ABs seen at the slit-lamp or on retinal examination or from the stereo fundus images of a limited number of fields. ABs may present only in peripheral areas and not be detected. Thus, we are probably underestimating the prevalence of AH. Another limitation is the cross-sectional design of the study. This prevents us from knowing the antecedent-consequent relationship between the risk factors and the endpoint.
In conclusion, in adult Chinese in Beijing, the prevalence of AH was 0.9% for eyes or 1.6% for subjects. AH was associated with elder age, thicker lens, and higher spherical equivalent.