Year : 2006 | Volume
: 54 | Issue : 3 | Page : 165--168
Smith-method assessment of anterior chamber depth for screening for narrow anterior chamber angles
Turki M Al-Mubrad, Kelechi C Ogbuehi
Department of Optometry and Vision Sciences, College of Applied Medical Sciences, P.O. Box 10219, Riyadh 11433, Saudi Arabia
Turki M Al-Mubrad
Department of Optometry and Vision Sciences, College of Applied Medical Sciences, P.O. Box 10219, Riyadh 11433
Purpose: To compare the axial anterior chamber depth (ACD) using the Smith method, in patients under treatment for primary open-angle glaucoma (POAG) and primary angle-closure glaucoma (PACG), with an age-matched control group.
Materials and Methods: Triplicate just-touching-slit-length (JTSL) measurements of the axial anterior chamber depth were determined in 198 eyes of 99 patients (39 control; 36 POAG; and 24 PACG) recruited from King Saud University clinics, Riyadh, Saudi Arabia. Goldmann tonometry and gonioscopy were carried out as a part of the patient«SQ»s routine examination. Subjects with a history of intraocular surgery for glaucoma or any other anterior segment disease were excluded form the study. The average ACD estimate by the JTSL method were compared among the various groups.
Results: The average JTSL estimates were: Control group 2.33±0.68 mm (axial ACD estimate = JTSL estimate ´ 1.4); POAG group 1.98±0.97 mm; PACG group 0.65±0.41 mm. There was no significant reduction ( P = 0.068) of the JTSL estimate in the POAG group, compared to the control group. There was a statistically significant ( P < 0.001) reduction of the JTSL estimate in the PACG group, compared to both the control and POAG groups.
Conclusion: The Smith-method JTSL technique may be used for non-invasive rapid screening, to help identify patients at risk of developing angle-closure, during routine examination of patients in the ophthalmology clinic.
|How to cite this article:|
Al-Mubrad TM, Ogbuehi KC. Smith-method assessment of anterior chamber depth for screening for narrow anterior chamber angles.Indian J Ophthalmol 2006;54:165-168
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Al-Mubrad TM, Ogbuehi KC. Smith-method assessment of anterior chamber depth for screening for narrow anterior chamber angles. Indian J Ophthalmol [serial online] 2006 [cited 2023 Mar 22 ];54:165-168
Available from: https://journals.lww.com/ijo/pages/default.aspx/text.asp?2006/54/3/165/27066
Over the past two decades, axial anterior chamber depth (ACD) estimates have assumed importance in screening for narrow anterior chamber angles, a major predisposing factor for the development of angle-closure glaucoma., This is especially relevant in the 30 year (and above) age group, where there is a significant decline in axial ACD, presumably due to the continually increasing lens thickness, or in individuals who are otherwise at risk of developing angle-closure.
The relationship between shallow anterior chambers and an increased risk of developing acute glaucoma, is well established.,, Several studies have also found significantly reduced axial ACDs in individuals with primary open-angle glaucoma (POAG)., Generally, eyes with axial ACDs of 2.5 mm or less are more likely to develop primary angle-closure glaucoma (PACG),, especially after mydriasis.
Several factors are known to influence the mean anterior chamber depth, including race,, refractive error,, and gender.,, Generally, men and myopes have deeper axial ACDs than women and hyperopes, respectively. Furthermore, Caucasians have deeper axial ACDs than Asians.
Gonioscopy, ultrasound biomicroscopy and optical coherence tomography are techniques available for the definitive diagnosis of angle status. However, all three techniques are demanding and time-consuming, hence there is a need for techniques based on simplicity, ease of application and interpretation, general availability and speed of use. The penlight shadow technique as well as the van Herick method lack a numeric measurement and are subjective. Accurate estimates of axial ACD can be achieved by pachymetry and ultrasonographic techniques. The ultrasonographic apparatus is too expensive to be widely available in ophthalmology clinics. The pachymeter, while more suited to clinical practice, is a costly, often incompatible accessory of the slit-lamp.
The method of ACD assessment suggested by Jacobs and modified by Smith, is simple and inexpensive. The Smith method and its variations have been validated by a number of authors., It allows the clinician to obtain reliable estimates of axial ACD, without any attachments to the slit-lamp biomicroscope. The axial ACD estimates that are accurate to within ± 0.25 mm, ± 0.2 mm, and ± 0.33 mm, as compared to pachymetry and to within ± 0.42 mm, as compared to ultrasonography. There is no effect of the central corneal thickness on the ACD estimate made using this method. Smith just touching slit length (JTSL) axial ACD estimates are free from inter and intra observer variation.
The purpose of this study was to assess the axial ACD by Smith JTSL estimate in POAG and PACG patients and to compare these readings with an age-matched control sample.
Materials and Methods
The data for this study was collected over a five-month period, from June to October 2004. Ninety-nine subjects recruited from King Saud University Clinics-Saudi Arabia, were included in this study.
Patients were included from the subjects attending the clinic, either for an assessment of refraction or an evaluation of their glaucomatous status. Informed consent was obtained from each subject before the measurements were carried out and the study was conducted in conformance with the ethical considerations laid out in the 1964 declaration of Helsinki.
Slit-lamp examination of the anterior segment was performed on each eye to exclude any other anterior segment disease and developmental or acquired anomaly that could alter the axial ACD. The slit-lamp examination was always performed before tonometry, to avoid examiner bias and the interference of the residual fluorescein with JTSL assessments.
Goldmann applanation tonometry was performed to assess each patient's entry intraocular pressure (IOP).
The glaucoma patients in this study had been previously diagnosed with the disease and were under drug therapy to control their IOP. None of the patients had undergone surgical procedures to control their IOP.
Each patient had their anterior chamber angle graded independently by gonioscopy, according to the scheme described by Shaffer, in which angle width is graded in five categories from 0 (closed) to 4 (wide open) and according to the van Herick technique, as described.
The Smith method for estimating anterior chamber depth was carried out as described elsewhere. The procedural exceptions in this study were as follows:
i) The JTSL measurements were not performed in the masked fashion described in Osuobeni et al ., which was specifically designed to prevent the examiner from estimating the end point of each JTSL measurement on the basis of tactile clues.
ii) Three instead of five JTSL measurements were made for each eye and the mean JTSL measurement was used for further analyses.
iii) The 'JTSL estimate' term used in this study is not the axial ACD estimate from the Smith method (which is the JTSL estimate ´ 1.4). It only represents the length of the slit-beam, at which the focused horizontal corneal-imaged slit and the out-of-focus Crystalline lens-imaged slit appear to just touch.
iv) The examination was performed in a blind fashion. The examiner never knew the status of a patient before performing his/her axial ACD assessment.
Analyses carried out on our data include: Student's t-test; single-factor analysis of variance; Tukey-Kramer multiple comparison test; Spearman's and Pearson's correlation coefficients.
There were no statistically significant ( P = 0.633, single-factor ANOVA) differences between the ages of the three groups (control, POAG, PACG). The age characteristics of the subjects are given in [Table 1]. Average values for the JTSL estimate and IOP for the three groups (Control, POAG, PACG) are listed in [Table 2]. No difference was found ( P = 0.068, Tukey-Kramer multiple comparison test) between the JTSL estimate of the control and POAG groups. However, the control and POAG groups had significantly ( P P P P values for the multiple group comparisons using the van Herick technique were identical to the values for the gonioscopy estimates. The average values for gonioscopy and van Herick anterior chamber angle grading are listed in [Table 3].
[Figure 1] is a scatter graph of the JTSL ACD estimate and van Herick anterior chamber angle estimate, a plot against the gonioscopy anterior chamber angle estimate.
The significance of axial (and peripheral) ACD assessment is to isolate the population at risk of developing angle closure glaucoma. This assumes greater relevance in subjects with ocular hypertension and a narrow angle. Wishart and Batterbury suggested that subjects with an axial ACD of 2.2 mm or less and a van Herick Grade 2 or less, should be considered for Nd:YAG laser iridotomy. Eyes with PACG have shallower anterior chamber depths (1.8 mm) than normal eyes (2.8 mm).
The actual mechanism of angle-closure glaucoma (defined as "primary", only if it is caused by pupillary-block glaucoma), is by contact-occlusion by the peripheral iris of the posterior trabeculum. The myriad of mechanisms that could result in angle closure glaucoma (including lens block-, ciliary block-, plateau iris configuration- and multi-mechanism- glaucoma), blunt the effectivity of axial ACD measurements in identifying populations at risk of developing angle-closure glaucoma.
However, for those patients with ocular hypertension, a shallow axial ACD may be an invaluable correlate-with a narrow van Herick estimate-for referral for diagnostic testing and possible surgical management.
The 95% confidence interval for the JTSL estimate of the PACG group was -0.2 to +1.5 mm, which would convert to an ACD estimate range of -0.3 to +2.1 mm (JTSL ´ 1.4). For the POAG group, the JTSL estimate range was +0.20 to +3.84 mm (axial ACD of +0.28 to +5.38). The 95% confidence interval for the JTSL estimate of the control group was +1.0 to +3.7 mm, which would convert to a confidence interval for the axial ACD estimate of +1.4 to +5.2 mm.
These results suggest that eyes with axial ACDs as low as 1.5 mm may be normal, but that at axial ACDs of 2 mm and below, the risk of progression to angle-closure is significant.
We propose that the Smith-method JTSL technique be used as a non-invasive, quantifiable, rapid screening technique to identify patients at risk of developing angle-closure, during routine examination of patients in the ophthalmology clinic, especially in primary eye-care clinics, where sophisticated equipment may be unavailable to properly assess the risks of glaucoma.
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