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Data collection for this project was carried out between May and August 1995. Ethical approval for this project was obtained from the Mongolian Ministry of Health. The work was carried out in accordance with The World Medical Association's Declaration of Helsinki.

2.2.1 Sampling

At the time this work was carried out, the country was divided into 22 provinces (Aimags), which in turn were sub-divided into districts (Sums). The smallest geographical division was a “bag”, an area roamed by a group of semi-nomadic families. Three to eight of these bags surround the town at the centre of the district, with each bag having a population of 1-2,000 people. Hovsgol (42,150 sq.

miles) is the most northerly province in Mongolia. The population in May 1995 was 117,587 (over 40's 20,896). The regional capital is Moron, population 27,230 (over 40's 4,860).

One thousand subjects aged 40 years and over were chosen for examination (4.78% of the province's population in this age group) (Figure 13). Forty percent were drawn from the aimag capital. Moron (8.2% of its population aged 40 and over). Within the town of Moron subjects were selected in simple random fashion within 3 of the 6 Horo (city districts). There are three predominant ethnic groups in the 24 districts of Hovsgol. Districts (somons) outside Moron were stratified according to their ethnic majority and one district chosen at random from each of the three groups. Two hundred people aged 40 and over were selected in each somon. An "urban" group from the district’s town and a “rural” group from the surrounding bags were chosen in proportions which reflected the distribution of the population in that district. In the town centres, simple random selection was used. However in the rural communities, where random sampling would have presented insurmountable logistical problems, systematic sampling was used; starting in the bag closest to town all residents aged 40 and above were identified from census data and the entire group called for examination. If necessary, the process was repeated in the next nearest bags until the required number of subjects had been examined. Selection was always strictly determined by position on the census list. Subjects who did not attend for examination were not replaced by others from further down the this list. Table 7 gives a summary of the eligible population in

Moron and the selected somons.

A further 300 subjects were recruited in the three selected somons (Tunel, Galt and Renchinlhumbe, see Figure 14) for measurement of anterior chamber depth and corneal thickness. One hundred people aged 10 to 39 years were chosen by random selection from the population register of each somon centre. These people therefore represent an “urban” sample.

Figure 13. Moron, the provincial capital of Hovsgol Aimag

Figure 14. Tunel Somon centre, Hovsgol Aimag

Table 7. Details of sample size and selection in Hovsgol Aimag, Mongolia

Location Population aged 40

and over

Sample size Bags or Horos* sampled / Total number

Sample method

Moron (Provincial capital) 4860 400 3 / 6 Horo Simple Random

Tunel (Center) 191 66 1 /1 Horo Simple Random

Tunel (Rural) 394 134 2 / 6 Bags Systematic

Galt (Center) 157 36 1 /1 Horo Simple Random

Galt (Rural) 731 164 2 / 6 Bags Systematic

Renchinlhumbe (Center) 182 58 1 /1 Horo Simple Random

Renchinlhumbe (Rural) 441 142 4 / 6 Bags Systematic

“Horo” is the term given to an urban district. “Bag” is the term for a rural area occupied by a group of nomadic families.

2.2.2 Questionnaire and visuai acuity

An eight-point questionnaire was administered in a standardized manner by two Mongolian ophthalmologists to glean a possible history of symptoms of intermittent angle-closure and family history of blindness. Visual acuity was measured at 6 metres using a Snellen chart, with distance correction if worn. A pinhole was used if the visual acuity was less than 6/18. The World Health Organization definition of blindness was used, wherein subjects with a visual acuity of less than 20/400 (3/60) or a visual field of <10° were classified as either blind or unilaterally blind. Cause of blindness was classified as the pathological process felt to be

responsible for the majority of visual loss. Cataracts of grade 3 or 4 (Minnasian’s classification degenerative/uncorrected high myopia with an axial length of >26mm, posterior pole staphylomata and a refraction of greater than -6D, grade 3 & 4 climatic droplet keratopathy (Johnson’s classification^^), and macular

degeneration characterised by a large disciform response in a subject aged over 80 years were all accepted as causes of blindness.

2.2.3 Visuai fieid testing

The central 25° visual field was assessed in all subjects using a Henson CFA 3000 static, semi-automated field analyser (Tinsley Medical Instruments, Croydon, United Kingdom). A 26 point static threshold-related suprathreshold test was routinely used. If one or more points were missed, this was extended to a 66 point test. Subjects with a field test graded “suspect” or “defect” by the instrument’s internal classification algorithm were re-tested using a 52 point threshold analysis. No refractive correction was used during testing. The presence of a glaucomatous field defect was determined by threshold test results. Definite glaucomatous field defects were defined as having a nerve fibre bundle pattern, at least 5 dB below threshold and at least 10° wide. If smaller or less dense but still of typical nerve fibre bundle pattern the defect was classified as probably glaucomatous. Equivocal field defects, or those typical of other pathology were classified as non-

2.2.4 Slitlamp examination

All subjects were examined using a Haag-Streit 900 slitlamp. Anterior segment sequelae of primary angle-closure (such as glaukomflecken and iris whorling) and stigmata of secondary glaucoma were noted. Benoxinate 0.3% (Moorfields Eye Hospital Pharmacy, London, UK) was used as a corneal anaesthetic. Fluorets (Chauvin Pharmaceuticals, Romford, UK) paper strips were used to instil a small amount of fluorescein into the pre-corneal tear film. Intraocular pressure (lOP) was measured three times in each eye using a Goldmann applanation tonometer. No adjustment for corneal astigmatism was attempted when performing tonometry The tonometer was calibrated daily. Pressure in each eye was defined as the median of the three readings. A conscious effort was made to avoid "even number bias". If the tonometer head was seen to pulsate, the end point was taken at the middle of this pulsation amplitude. Measurements were made throughout the day between the hours of 8am and 7pm.

2.2.5 Opticai pachymetry of the anterior chamber and corneai thickness

Examination of all subjects included measurement of the axial anterior chamber depth (ACD) and central corneal thickness (CCI) using optical pachymeters (Devices I & II, Haag-Streit, Bern, Switzerland) mounted on a slitlamp (Model 900, Haag-Streit, Bern, Switzerland). The “touch” method of measuring CCT was used throughout, except in the first 100 subjects aged 40 years and over, when the “overlap” method was used In further analyses, CCT data on these first subjects was disregarded, and a mean age/sex/side (i.e. right or left eye) specific value for CCT obtained from subsequent individuals (when the “touch” method) was used.

The subject was instructed to maintain steady a gaze in the primary position. The brightest, narrowest illumination beam possible was used. The measurements of axial ACD and CCT were made using the pupil margin as a point of reference to ensure accurate centration. Corneal thickness (anterior epithelial surface to posterior endothelial surface) was measured using Device I at x1.6 objective magnification with +2.5 D eyepiece addition, read to the nearest 0.01 mm. ACD (anterior corneal epithelial surface to anterior lens capsule) was measured to the

nearest 0.05 mm, using Device II, x1 objective magnification and +6 D eyepiece addition. Each parameter was measured only once in each eye, as a previous study has shown minimal intra-observer variability between repeated readings. “True” anterior chamber depth from the endothelial surface of the cornea to the anterior lens capsule was calculated by subtracting the corneal thickness from the anterior chamber depth. No correction was made for corneal radius or diameter. No cycloplegic agents were used prior to taking measurements.

2.2.6 Gonioscopy

Gonioscopy was carried out in all patients using a Goldmann-style 2 mirror gonioscope. A 2% hypromellose solution in saline (Moorfields Eye Hospital

Pharmacy, London, England) was used as a coupling medium for the contact lens. Slight tilting to gain a view over the convexity of the iris was permitted, but further manipulation of the lens or redirection of gaze was avoided because of the

possibility of exerting pressure on the cornea and artificially widening the angle. The drainage angle was graded as occludable or open (as defined in section 2.1.1). This is equivalent to a modified Scheie grading^^ with a dichotomous outcome.

A four point classification was used to record the overall iris profile as steep, angulated (plateau), regular or concave with the grade chosen to best reflect the entire 360° architecture. This scheme differed from Spaeth's only in that

subjects with a Spaeth grade “s” (steep) peripheral iris profile were divided into two groups. One group with a typical steep, convex iris profile with little divergence between the iris and corneo-sclera and a centrally shallow anterior chamber; in the second group, the iris rises steeply from its insertion but then makes an abrupt angulation away from the corneo-scleral wall, resulting in a relatively deep axial anterior chamber and a centrally flat iris plane. This allowed the identification of subjects with a “plateau” iris configuration, in whom prophylaxis and treatment of angle-closure may be different from that in cases predominantly attributable to pupil block. Manipulative or indentation gonioscopy using the Goldmann or Zeiss lens respectively was used (unless the angle was wide open) to detect peripheral anterior synechiae (PAS), graded as present or absent in each quadrant. If

excess pigment was seen on the superior trabecular meshwork, this was recorded. In subjects with an occludable angle, but no stigmata of PAC (i.e. normal lOP, disc and field with no iris/lens ischaemic sequelae, symptoms or PAS), a dark-prone provocation test was performed. The subject laid face down with eyes shut and bandaged for one hour, taking care to avoid pressure on the globe. The intraocular pressure was measured at the end of this time, and a rise of > 8 mm Hg was considered a positive test result.

All slitlamp examinations (including optic disc evaluation), tonometry and

gonioscopy were carried out by the same investigator (PJF), therefore eliminating any potential for inter-observer error. Glaucoma or glaucoma suspect cases among the first 350 subjects were also examined by a glaucoma specialist experienced in the assessment of angle-closure glaucoma cases (Dr Poul-Helge Alsbirk). Inter-observer agreement between these two investigators for tonometry and gonioscopy was assessed.

2.2.7 Optic disc examination

The optic disc and posterior pole were examined using a Volk +90D lens through the un-dilated pupil. If a satisfactory view was not obtained, or if an inexplicable field defect was detected, the pupil was dilated with 0.5% tropicamide (AIcon Laboratories, UK) after gonioscopy, provided the angle was not judged to be occludable. The vertical cup-to-disc ratio (CDR) was judged by observing disc contour (when a stereoscopic view was obtained) and angulation of blood vessels which crossed the disc rim. Standard photographs and measurement graticules were not used. Disc haemorrhages or notching (localised tissue loss at superior or inferior pole) were recorded.

2.2.8 Ultrasound ocular biometry

Anterior chamber depth, lens thickness and axial length of the globe were measured in all subjects aged 40 years and older using an A-mode ultrasound device with a hard-tipped corneal contact probe (Model 820, Zeiss Humphrey, San Leandro, CA, USA). This was mounted on a tonometer set to the intraocular pressure in that eye. At least five readings were taken from each eye and the longest, high-quality trace was chosen for measurement.

2.2.9 Management of Glaucoma

Any subject at risk of glaucoma or having the disease was given advice,

medication and referral for further management, and in the case of manifest or latent PACG, treated in the field with a Carl Zeiss Visulas II portable YAG laser to produce a peripheral iridotomy. When necessary, arrangements were made for patients to undergo filtering surgery.