Glaucoma is the second leading cause of blindness worldwide, with an estimated 67 million people affected, and almost half of these being caused by primary angle closure (PAC) glaucoma (PACG).1 Primary angle closure glaucoma according to the International Society of Geographic and Epidemiological Ophthalmology is defined as the presence of iridotrabecular contact for 180 degrees or more, with structural glaucomatous changes in the optic nerve and corresponding field loss. The prevalence of PACG is especially high in East Asians, with the Chinese having the highest reported incidence rate.2–6 Ocular risk factors for PACG include a shallow anterior chamber depth (ACD), short axial length (AL), and a thick and anteriorly positioned lens.7–10 A positive family history of PACG has also been described as a predisposing risk factor.11–13
It was reported that approximately 10% of people who have anatomically narrow angles develop angle closure.14 This presents a challenge for clinicians to identify patients likely to progress to PAC or PACG from PAC suspects. The rationale for this study stems from the knowledge that a positive family history of PACG is a known risk factor, with 3.5 to 6 times higher prevalence of occludable angles in first-degree relatives.15 Inheritance is believed to be polygenic, although autosomal dominant and autosomal recessive inheritance patterns are seen in pedigrees with a high prevalence of PACG.12 In view of the high prevalence in first-degree relatives of patients with PACG, this study was undertaken to evaluate the efficacy of the darkroom prone provocative test (DPPT) in identifying family members of PACG patients at risk of developing the disease.
A screening test that is capable of identifying individuals who are likely to develop PAC is desirable. Nonpharmacologic tests have been shown to accurately reproduce physiological conditions as compared with those that involve pharmacologically stimulating the sphincter and dilator muscles.16 A darkroom test physiologically dilates the pupil and induces susceptible angles to become narrower, whereas the prone test causes anterior displacement of the lens.17,18 A combination of the darkroom and prone provocative test (= DPPT) has been shown to increase the outcome of positive results.19
In the present study, we investigated the ocular biometric parameters and DPPT response in family members of patients with PACG. In addition, we studied the correlation between the ocular biometric parameters and DPPT response to determine the risk factors associated with a positive response to DPPT.
A total of 74 family members of 25 index patients diagnosed as having PACG who attended the eye clinic of a teaching hospital from 2006 to 2008 were screened for glaucoma. Inclusion criteria for the family members of PACG patients were being in the age between 40 and 80 years with best-corrected visual acuity equal or better than 6/12. Exclusion criteria include a history of intraocular surgery including cataract surgery, ocular disease or trauma, cases of secondary angle closure, and a history of laser iridotomy. This study was conducted in accordance with the tenets of the Declaration of Helsinki. Ethical approval was obtained from the hospital institutional review board, and informed consent was taken from all participants.
A detailed history and complete examination including best-corrected visual acuity, ultrasound ocular biometry, slit lamp biomicroscopy, Goldmann applanation tonometry, fundus examination, and nonindentation gonioscopy was performed with a Goldmann 2-mirror goniolens. Lastly, the DPPT was carried out. The analyzed parameters using the A-mode applanation ultrasound were ACD, lens thickness (LT), and AL. Intraocular pressure (IOP) was measured using the Goldmann applanation tonometer.
In the DPPT, baseline IOP was recorded as the mean of three IOP readings taken using Goldmann applanation tonometry during the slit lamp examination. Subjects were instructed to sit with their head prone on a table, without any pressure exerted on their eyes, and the room was darkened for 1 hour. The subjects were repeatedly spoken to, ensuring that they did not sleep and to keep their eyes open throughout the test. Intraocular pressure recording was repeated after the test in the same darkroom, with the only source of light being that of the tonometer. Again, the mean of three readings was taken. Ultrasound biometry and gonioscopy were then repeated.
Darkroom prone provocative test is considered positive if the IOP rise above baseline values is more than or equal to 8 mm Hg, with more than 180 degrees of iridocorneal touch on nonindentation gonioscopy; borderline if the IOP rise above baseline values is 6 to 7 mm Hg, with more than 180 degrees of iridocorneal touch; and negative if the IOP rise above baseline values is less than or equal to 5 mm Hg, with less than 180 degrees of iridocorneal touch.18
The parameters assessed were ACD, LT, AL, lens position (LP), and relative LP (RLP). Lens position is defined as ACD + 1/2 LT. Relative lens position is defined as LP/AL. Darkroom prone provocative test results were taken as positive, borderline, and negative.
Family members were divided into:
Family members in this group were defined as the following:
(a) PAC: gonioscopically occludable angles with more than 180 degrees of pigmented trabecular meshwork not visible gonioscopically without manipulation or indentation, along with a steep peripheral iris configuration, with raised IOP and/or primary peripheral anterior synechiae (PAS).
(b) Acute PAC: the presence of an acute rise in IOP, along with the presence of associated symptoms such as severe unilateral headache, blurred vision, and nausea/vomiting in the presence of closed angle and corneal edema.
(c) PACG: presence of signs seen in PAC, along with glaucomatous optic neuropathy and corresponding visual field damage with or without elevated IOP.
Family members in this group had occludable angles, more than 180 degrees of pigmented trabecular meshwork not being visible gonioscopically without manipulation or indentation, along with a steep peripheral iris configuration, but no evidence of PAS, pigment clumping. Intraocular pressure was within normal limits (between 10 and 21 mm Hg), and there is no evidence of glaucomatous optic neuropathy.
Family members in this group have gonioscopically open angles (angle recess, >25 degrees), with IOP within normal limits and no evidence of glaucomatous optic neuropathy.
Statistical analysis was performed using SPSS 18.0 (SPSS Inc., Chicago, IL), and statistical significance was defined as p < 0.05. Frequency histograms and the one-sample Kolmogorov-Smirnov test were used to assess the distribution of numeric data for parametric characteristics. Comparison of ocular biometric measurements was performed using one-way analysis of variance and Tukey post hoc analysis or Kruskal-Wallis analysis of variance by rank and Mann-Whitney U test where appropriate when nonparametric analyses were required. Spearman correlation test was used to correlate biometric parameters and IOP difference before and after DPPT. The Spearman rank correlation coefficient is used to find the strength of a link between two series of data.
The 25 index PACG patients consisted of 20 female and 5 male subjects. The mean age was 67.3 ± 12.4 years. The ethnic composition was 18 Chinese, 3 Malays, and 4 Indians. Among the 74 family members who took part, the ratio of male to female was 1:1.6, with a mean age of 57.3 ± 12.1 years. Most family members were Chinese (93.2%), the remaining being Malay (2%) and Indian (3%). The demographic characteristics of the family members are summarized in Table 1.
Table 2 details the breakdown in terms of numbers of patients in each of these three groups of DPPT responders (positive, borderline, and negative). There is a significant difference in terms of IOP rise among the unaffected family members compared with the affected and PAC suspects (p < 0.05). None of the affected family members had a negative DPPT test, and none of the unaffected family members had a positive DPPT test.
The mean age of family members with positive DPPT was higher than that of family members with borderline or negative DPPT, although the difference was not significant. Family members with a positive DPPT had the shallowest ACD, greatest LT, shortest AL, and most anterior LP as evidenced by RLP compared with those with borderline and negative DPPT (Table 3). These biometric parameters in the positive, borderline, and negative groups showed statistical significance. However, RLP was not significantly different between the three groups of DPPT responders.
Correlation coefficient analysis was performed between ocular biometric parameters and DPPT (Table 4). Among the parameters, ACD showed the most significant correlation with DPPT. The correlation between DPPT and LT, LP, and RLP was significant (p < 0.05) but with a slightly weaker correlation coefficient. In contrast, AL was not statistically significantly correlated with DPPT.
An early diagnosis of PACG is essential to prevent ocular morbidity. In this study, DPPT has been an effective test to detect PAC/PACG at an early stage among first-degree relatives for treatment to be instituted. Family members were divided into three main groups: affected (those with PAC), PAC suspects, and unaffected (those with wide open angles).
Primary angle closure glaucoma has been reported to be more prevalent in South East Asian countries, whereby the highest incidence rate can be found among the Chinese people.2 This was representative in our study population, whereby the majority of our PACG patients (18 of 25, 72%) are Chinese. The mean age in the affected family members was significantly higher than those of the PAC suspects and the unaffected family members. Unaffected family members were the youngest, suggesting that, with time and thickening of lens, some of these eyes could also show evidence of PAC.
The elevation of IOP during the darkroom test was caused by the closure of the narrow angles in the dark and thus is physiologically relevant to the pathophysiology of PAC.20 In addition, the prone test allows forward movement of the lens and iris, thus provoking angle closure in susceptible individuals.
In our study, ocular biometric parameters in family members of PACG patients showed that 19% (14 of 74) of them had narrow angles on gonioscopy. This prevalence is lower than that reported by Sihota et al.18 who found a prevalence of 66.4% (99 of 149) in predominantly Indian ethnic subjects. A possible reason for this is our smaller sample size compared with that of Sihota et al. In a study on Chinese ethnic subjects, Amerasinghe and coworkers15 found that 32% of first-degree relatives had narrow angles. This figure is in-between our study and Sihota et al.18 They also found that siblings of Chinese patients with PAC or PACG have an almost 50% probability of having narrow angles and are more than seven times more likely to have narrow angles compared with the general population.15
An attack of angle closure glaucoma depends on various factors, with the most important factor being a shallow ACD.21 This, in part, may explain the presence of a significant correlation between positive DPPT and ACD. Apart from studies by Alsbirk22 and Lowe12 who concluded that biometric characteristics such as ACD are heritable, Tu et al.23 went on to show that a shallow ACD has a 93% heritability rate among Chinese families with PACG. It was interesting to note that, in this study, we found 28% (17 of 60) unaffected family members had a borderline DPPT response. A subgroup analysis was performed, and we found that those unaffected family members who had borderline DPPT response were slightly older (mean age, 57.9 vs. 54.9 years) and had shallower mean ACD (2.88 ± 0.43 mm vs. 3.03 ± 0.34 mm) compared with the unaffected family members who had a negative response for their DPPT test. Lens thickness and AL were not significantly different within this subgroup. This further illustrates the importance of ACD in the etiology of IOP rise in DPTT despite open angles on gonioscopy and lens size.
As with individuals with PACG, affected and PAC suspects had the shallowest ACD, thickest lens, shortest AL, and most anteriorly positioned lens. In this study, ACD and LT showed a significant correlation with positive DPPT. A shallow anterior chamber could be a result of increased lens thickness, phacomorphic pupil block, or a forward displacement of the whole lens or a combination of all factors.24 It was postulated that thicker lenses are more likely to move forward during the prone position test.25 Lowe24 estimated that an increase in lens thickness causes 0.35 mm of anterior chamber shallowing, whereas forward LP causes 0.65 mm of shallowing, thus accounting for a total of 1-mm difference in ACD as compared with the normal eye.
In this study, family members suspected to be at risk of angle closure showed 87.5% positive or borderline DPPT response. All of the affected family members had a positive or borderline response to DPPT whereas none had a negative response. These results contradicted the findings by Sihota et al.18 who found negative responses in 6 (10.9%) of 55 affected patients. Foulds26 stated that a negative darkroom provocative test does not rule out the existence of PAC, but a positive test has more value.
Even though initial screening showed that a total of 60 family members were not affected with PAC, DPPT revealed that approximately 28% of them were capable of developing PAC. We propose a longitudinal study to determine if these individuals would go on to develop PAC later in life to substantiate the reliability of the DPPT in identifying individuals at risk of PAC. Although the DPPT test may be impractical in an everyday clinic setting because of its requirement of time and staff resources, we feel that it may play an important role especially in an academic setting. Because this was a small pilot study, more observations are needed to corroborate our preliminary results.
Department of Ophthalmology
Faculty of Medicine
University of Malaya
50609 Kuala Lumpur
The authors thank Dr. Mohamed Reza Peyman for his assistance in the technical aspects of the manuscript preparation.
This study was supported by University Malaya Research Grant RG485/12HTM.
Received August 3, 2013; accepted February 6, 2014.
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