Anterior chamber depth measurement by optical pachymetry: systematic difference using the Haag-Streit attachments

Haag-Streit recommends that for the most accurate anterior chamber depth (ACD) results, the corneal thickness, as determined with attachment No I, should be deducted from a measurement from corneal epithelium to the anterior lens surface, made with attachment No II (method A). Often ophthalmologists use the measurement made from the corneal endothelium to the anterior lens surface, using only attachment No II (method B), which is a simpler and faster method. This study examined agreement between methods A and B.

Two studies were conducted independently by each author. PHA measured 127 (27 men, 100 women; mean age, 66.9 years) consecutive Danish patients referred for evaluation because of possible angle closure. RRB measured 109 subjects (30 men, 79 women; mean age, 61.3 years) consecutively from a population based glaucoma survey in Rom Klao, Thailand.

Using method A, mean ACD was 1.97 mm (SD 0.29) in the Danish study and 2.59 mm (SD 0.27) in the Thai study. ACD measured with method B was significantly (p<0.001) deeper than method A in both studies (Danes: difference = 0.118 (95% CI: 0.109 to 0.127); Thais: difference = 0.166 mm (95% CI: 0.158 to 0.174)). With an increase in ACD of 1 mm, the methodological difference increased by 0.052 mm (regression formula: difference (B - A) = 0.0667 x mean ACD - 0.0148; R(2) = 0.31). This positive correlation did not differ significantly between the two studies.

The relevance and importance of estimating ACD as a risk factor in primary angle closure glaucoma suspects and patients has been repeatedly emphasised. This is the first empirical study to quantify the difference in ACD using these two methods in two samples, one clinic based (angle closure suspects) and the other population based. The size of the methodological difference has a level that corresponds to the age reduction of ACD per decade, or to about 6% of ACD in a given eye. These findings highlight the importance of clinicians recognising that these two methods will give different results and recommend that one should standardise ACD measurements using the Haag-Streit optical pachymeter.