Subjects and Methods

The Kumejima Study, conducted between May 2005 and August 2006, was a population-based epidemiologic survey and followed the tenets of the Declaration of Helsinki and regional regulations and was approved by the ethics board of the regional council. All participants provided written informed consent before the examinations. All eligible residents (4632 subjects) aged 40 years or older were encouraged to participate. Kumejima had 5249 residents aged 40 years or older in 2005 according to the official household registration database. After excluding 617 residents who died, moved, or could not be located during the study period, 4632 residents were eligible for the study. For the UBM study, 10% of the residents (461 subjects) were randomly sampled using the random number method because of limited examination time.

In the screening examination, refraction was measured using an autorefractometer (ARK-730, Topcon, Tokyo, Japan); IOP was measured 3 times using a Goldmann applanation tonometer and the median value was recorded. The peripheral anterior chamber depth (ACD) was scored according to the van Herick method and the gonioscopic findings were scored according to Shaffer’s grading system using a Goldmann 2-mirror lens by experienced ophthalmologists. Based on gonioscopic findings, PAC and PAC suspects were identified. If the posterior trabecular meshwork was not visible during static gonioscopy in at least three quarters of the angle circumference in primary gaze without manipulation or indentation, the angle was classified as occludable. An eye was classified with PAC when an eye with an occludable angle had 1 or more of the following: an IOP exceeding 21 mm Hg; peripheral anterior synechiae (PAS) reaching the scleral spur or beyond; a history of an acute IOP elevation, including the presence of iris atrophy, glaukomflecken, dilated nonreactive pupils, or a previous diagnosis of PAC according to the combination of the subject’s reliable statement and the objective signs in their eyes. In some of the cases, we could confirm the history in their medical records. If an eye that was not classified as having an occludable angle had PAS in an area of a narrow chamber angle of Shaffer grade 2 or less, the eye also was classified with PAC and secondarily added to occludable angle. When an eye with an occludable angle was not classified with PAC, the eye was classified as a PAC suspect.

UBM measurements were performed using UD-1000 (Tomey, Aichi, Japan) with a 40-MHz transducer probe at the 12-o’clock (superior), 3-o’clock (nasal), 6-o’clock (inferior), and 9-o’clock (temporal) positions of the right eye of each subject in the supine position by 1 of 2 experienced ophthalmologists (I.H., C.U.). After topical anesthesia was applied, an eye cup containing hydroxyethyl cellulose and physiologic saline was mounted on the globe, and the transducer was applied gently to the limbal area with care to avoid compression of the globe. UBM assessments were carried out in room light (about 1400 lux, measured with an illuminometer [IM-2D, Topcon]) and repeated in the same manner 3 minutes after the room was darkened to about 3 lux. Variations in accommodation were minimized by fixation to targets located on the ceiling with a standard distance. The lateral and axial resolutions were both 50 μm. A 9 × 6-mm field was imaged and transferred to a personal computer for analysis.

The UBM parameters evaluated in the current study had been defined by Pavlin and associates. The images transferred to a personal computer were analyzed using originally developed software for image analysis. UBM images were calibrated as 414 × 276 pixels and 9 × 6 mm in size. On a UBM image magnified on the monitor of a personal computer, the scleral spur was first identified by an examiner and the circular lines with the diameter of 250 and 500 μm centered on the scleral spur were automatically drawn. According to the circles, the following points were identified by the examiner and the distance and area were automatically determined by the software. Among the parameters described by Pavlin and associates, the angle-opening distance of 250 or 500 (AOD250, AOD500, respectively) was defined as the distance between the posterior corneal surface and the anterior iris surface measured on a line perpendicular to the trabecular meshwork 250 or 500 μm, respectively, from the scleral spur; the trabecular-iris angle (TIA) was defined as the angle measured with the apex in the iris recess and the arms of the angle passing through a point on the trabecular meshwork 500 μm from the scleral spur and the point on the iris perpendicularly opposite; and the trabecular-ciliary process distance (TCPD) was defined as the length of the line extending from the corneal endothelium 500 μm from the scleral spur perpendicularly through the iris to the ciliary process. The thickness of the iris was measured on the same line ( Figure 1 ). Reproducibility of the UBM parameters determined with this software was preliminarily evaluated: the coefficient variation of AOD250, AOD500, TIA, TCPD, and the iris thickness in the 5 UBM images serially obtained in 10 subjects averaged 12.5%, 8.9%, 9.5%, 8.6%, and 10.3%, respectively (unpublished data), which were almost comparable to that in a previous report in which the parameters were manually determined.

The determination of the parameters on the ultrasound biomicroscopy image. AOD250 and AOD500 = angle-opening distance 250 and 500, respectively; TIA = trabecular-iris angle; TCPD = trabecular-ciliary process distance, ID = thickness of the iris.

Results

Reliable UBM images were obtained in 301 right eyes (80.4%) of the 374 eligible subjects after excluding 87 with a history of intraocular surgeries (cataract surgery in 59 eyes, laser iridotomy in 1 eye, and trabeculectomy in 1 eye), ocular trauma (4 eyes), or iritis (1 eye); a physical or mental disability making the UBM measurement difficult (15 eyes); or refusal to undergo UBM measurements (4 before and 2 during the UBM measurements) from the randomly sampled 461 subjects ( Figure 2 ). Of the 301 eyes, 8 (2.7%), 37 (12.3%), and 256 (85.0%) eyes were diagnosed with PAC, as PAC suspects, and with nonoccludable angles, respectively. The diagnosis of 8 eyes with PAC was made based on the presence of PAS (7 eyes) and the combination of PAS and IOP exceeding 21 mm Hg (1 eye). No eye had a history of an acute episode of PAC or laser iridotomy, because eyes with history of surgery or laser procedures had been excluded from the current study.

Selection of the 301 eligible subjects from all 4632 residents aged 40 years or older in Kumejima. The dotted lines show the exclusions from the studied subjects.

Mean values of the UBM parameters in different age groups are shown in Table 1 . All parameters were significantly negatively correlated with age ( P < .001, Pearson’s correlation coefficient), suggesting that older subjects had narrow peripheral angle, anteriorly located ciliary body, and thin iris. In Table 2 , the comparisons of the UBM parameters between the PAC/PAC suspects and nonoccludable eyes are summarized. Subjects with PAC/PAC suspect were older ( P < .001, unpaired t test) and had more hyperopic refractive error ( P < .001) than nonoccludable angle eyes, and women tended to be more prevalent ( P = .052), while IOP did not differ significantly ( P = .560). Between 45 eyes with PAC/PAC suspect and 256 with nonoccludable angles, the AOD250, AOD500, TIA, and TCPD under light and dark conditions were significantly smaller in PAC/PAC suspect than in nonoccludable angles ( P < .001 for all comparisons by the unpaired t test), while the iris thickness showed no significant intergroup difference ( Tables 3 and 4 ). Between 8 eyes with PAC and 37 eyes of patients who were PAC suspects, no UBM parameters differed significantly ( P > .1 for all comparisons), with the exception that the AOD250 under light conditions was significantly greater in PAC suspects than in those with PAC ( P = .001).

TABLE 4

Comparison of Ultrasound Biomicroscopy Parameters Among 8 Eyes with Primary Angle Closure, 37 With Primary Angle Closure Suspects, and 256 With Nonoccludable Angles Under Dark Conditions

	Occludable Angle				Nonoccludable	P Value b
			P Value a	PAC+PACS
AOD250 (mm)	PAC	0.056 ± 0.035	.435	0.065 ± 0.036	0.138 ± 0.076	<.001
	PACS	0.067 ± 0.036
AOD500 (mm)	PAC	0.094 ± 0.029	.38	0.085 ± 0.041	0.223 ± 0.113	<.001
	PACS	0.083 ± 0.043
TIA (degrees)	PAC	8.4 ± 2.4	.343	7.6 ± 3.5	18.7 ± 8.5	<.001
	PACS	7.4 ± 3.7
TCPD (mm)	PAC	0.645 ± 0.115	.828	0.636 ± 0.117	0.768 ± 0.149	<.001
	PACS	0.635 ± 0.119
ID (mm)	PAC	0.426 ± 0.046	.407	0.440 ± 0.059	0.460 ± 0.062	.040
	PACS	0.443 ± 0.061

AOD = angle-opening distance; ID = iris thickness; PAC = primary angle closure; PACS = primary angle closure suspect; TCPD = trabecular-ciliary process distance; TIA = trabecular-iris angle.

Values, except for P values, are expressed as the mean ± standard deviation.

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