Evaluation of changes in anterior segment morphology using Sirius Scheimpflug-Placido disk topographer following laser peripheral iridotomy in primary angle-closure disease

Purpose: To assess the morphological changes in the anterior segment following laser peripheral iridotomy (LPI) in primary angle-closure disease (PACD) using Sirius Scheimpflug-Placido disk corneal topographer. Methods: This was a prospective observational study. A total of 52 eyes of 27 patients with PACD who underwent LPI were analyzed for iridocorneal angle (ICA), anterior chamber depth (ACD), anterior chamber volume (ACV), horizontal visible iris diameter (HVID), corneal volume (CV), central corneal thickness (CCT), and horizontal anterior chamber diameter (HACD) 1 week following LPI, using Sirius Scheimpflug-Placido disk corneal topographer. Data analysis was done using Statistical Package for the Social Sciences (SPSS) software version 19.0, and paired t-test was used to assess the statistical significance. Results: Laser peripheral iridotomy was performed in 43 eyes with primary angle-closure suspect (PACS), six eyes with primary angle closure (PAC), and three eyes with primary angle-closure glaucoma (PACG). The analysis of the data showed statistically significant changes in anterior segment parameters of ICA, ACD, and ACV. Post-laser increase in ICA from 34.13° ± 2.64° to 34.75° ± 2.84° (P < 0.041), mean ACD increase from 2.21 ± 0.25 to 2.35 ± 0.27 mm (P = 0.01), and mean ACV increase from 98.19 ± 12.13 to 104.15 ± 11.16 mm³ (P = 0.001) were noted. Conclusion: Significantly quantifiable short-term changes in the anterior chamber parameters of ICA, ACD, and AC volume were seen after LPI in patients with PACD on Sirius Scheimpflug-Placido disc corneal topographer.

Keywords: Anterior chamber depth; anterior chamber volume; iridocorneal angle; laser peripheral iridotomy; primary angle-closure disease; Scheimpflug topographer

Glaucoma is a chronic progressive multifactorial optic neuropathy causing gradual irreversible diminution of vision. It was predicted that primary angle-closure disease (PACD) would affect 21 million people by the year 2020; among them, 87.6% would be Asians.[[1]] One population-based survey in India revealed that 0.7% of the people over 30 years had PACG and 1.4% of people over 30 years of age had occludable angles.[[2]] The angle-closure disease can be defined as obstruction of the trabecular meshwork by the peripheral iris (iridotrabecular contact [ITC]), causing blockage of aqueous outflow. According to the International Society for Geographical and Epidemiological Ophthalmology (ISGEO) classification, which is followed widely, the PACD has been categorized as primary angle-closure suspect (PACS), primary angle-closure (PAC), and primary angle-closure glaucoma (PACG).[[3]]

Laser peripheral iridotomy (LPI) is regarded as the first-line intervention for PACD.[[4]],[[5]] It reverses appositional angle closure by relieving the pupillary block, and thus helps to control the intraocular pressure (IOP). Early diagnosis and LPI prevent the progression of PAC into PACG.[[6]]

The morphological changes in the anterior segment following LPI have been described with anterior segment optical coherence tomography (AS-OCT), ultrasound biomicroscopy (UBM), and Scheimpflug-based imaging devices (Pentacam and Sirius topographer) by different investigators.[[7]],[[8]] Among these, the AS-OCT and the UBM acquire only cross-sectional images of the anterior segment in particular sectors.[[8]] Among the Scheimpflug-based devices, anterior segment evaluation using Pentacam is commonly studied compared to Sirius Scheimpflug corneal topographer, and it was found that Sirius was comparable and probably more accurate than Pentacam in assessing the anterior segment details.[[9]] The Sirius Scheimpflug-Placido disk topographer is a high-precision, noncontact device with a 3D rotating camera attached with Placido rings. This machine gives a precise measurement of the morphological changes occurring after LPI in all sectors and a rapid 3D reconstruction of the anterior segment, including angle recess. The rotating Scheimpflug camera takes up to 50 cross-sectional images of angles from 0 to 180° in a single scan, acquiring 25,000 data points in approximately 2 s. The purpose of the current study was to evaluate using Sirius Scheimpflug-Placido disk topographer the anterior segment changes following LPI in patients with PACD attending the glaucoma clinic of our hospital.

Methods

We conducted a prospective, observational study in a tertiary care center in South India. It was carried out after obtaining approval from the Institution Review Board and Institute Ethics Committee, and it adhered to the tenets of the Declaration of Helsinki. Written informed consent was obtained from all the participants. Patients with PACD above 18 years of age attending the ophthalmology department outpatient services were included.

PACD was defined as PACS–ITC in three or more quadrants with a normal optic disk, IOP, and visual field; PAC–three or more quadrants of ITC with raised IOP and/or peripheral anterior synechiae and normal optic disk and field; and PACG–ITC in three or more quadrants, with glaucomatous optic neuropathy.[[3]]

Patients with a history of previous laser or surgical Surgical Peripheral iridectomy / Laser Peripheral iridotomy (PI) and iridoplasty, intraocular surgery like cataract surgery, and trabeculectomy were excluded. Patients with secondary angle-closure glaucoma, like uveitic glaucoma, neovascular glaucoma, and postsurgical glaucoma, were also excluded.

Procedure

All patients underwent a complete ophthalmological evaluation, including medical and ophthalmic history, uncorrected visual acuity (UCVA), and best-corrected visual acuity (BCVA), slit-lamp examination to examine the anterior segment, Goldmann applanation tonometry (GAT) to measure the IOP, gonioscopy (Posner four-mirror indirect handheld gonioscope; Volk, OHIO, USA), optic disk evaluation with +90 D (Volk lens) in undilated state, and anterior segment evaluation with the Sirius Scheimpflug-Placido disk topographer (version 2.6.3; CSO, Kleinostheim, Germany) system. Gonioscopy was performed by an experienced glaucoma consultant (KS) with a gonioscopy lens (Posner four-mirror indirect handheld gonioscope, Volk) in a dim-lit room. Anterior segment parameters including iridocorneal angle (ICA), anterior chamber depth (ACD), anterior chamber volume (ACV), central corneal thickness (CCT), horizontal visible iris diameter (HVID), horizontal anterior chamber diameter (HACD), and corneal volume (CV) in both the eyes were recorded using the glaucoma analysis program of Sirius Scheimpflug-Placido disk corneal topography.

LPI was performed by a glaucoma consultant (KS) using neodymium: yttrium–aluminum–garnet laser (Zeiss VisulasYag III; Carl Zeiss Meditec, Jena, Germany) with prior constriction of pupils using pilocarpine (2%) eye drops, four instillations for every 15 min. The settings used for LPI were 12–15 mJ, one to three shots, pulse III mode using an ocular Abraham iridotomy contact lens (Western Ophthalmics, Lynnwood, WA, USA) in the superior region between the 10 o' clock and 2 o' clock position [Figure 1] The patency of the PI was validated by the flow of aqueous and pigments through the iridotomy site. IOP was measured 1 h post-laser. All patients were prescribed prednisolone 1% eye drops four times a day and timolol 0.5% twice a day for 1 week. Repeat gonioscopic evaluation and Sirius topography to assess the anterior segment were performed 1 week later [Figure 2].{Figure 1}{Figure 2}

Sample size calculation

The sample size was estimated with a minimum expected mean difference in ACD of 0.04 with a standard deviation of 0.12 observed in a previous study at a 5% level of significance and 80% power.[[10]] The sample size was calculated for the different parameters ACD, ICA, and ACV. Since the difference in ACD yielded the highest sample size, it was selected. The sample size was calculated using nMaster 2.0 software and was found to be 52.

Statistical analysis

The outcome variables ICA, ACD, ACV, HACD, HVID, CV, and CCT were analyzed using Statistical Package for the Social Sciences (SPSS) version 20.0 (SPSS Inc, Chicago, IL, USA). Paired t-test was used to assess the statistical difference in the anterior segment changes after LPI, and values were expressed in terms of mean and standard deviation as per distribution.

Results

A total of 52 eyes of 27 patients were analyzed in our study. Both eyes of patients who satisfied the inclusion criteria were taken for the study (both eyes for 25 participants and one eye for two participants) [Table 1]. The mean age of the patients was 53.11 ± 7.06 years (range 39–68 years). Male: female distribution in our study was 13 (48.14%):14 (51.86%). Among 52 lasered eyes, 43 eyes (82.7%) were PACS, six eyes (11.5%) were PAC, and three eyes were PACG (5.7%). In the study participants, 40 eyes (76.9%) had vision better than 6/9, seven eyes (13.5%) had a vision from 6/9 to 6/60, and five eyes (9.6%) had a vision worse than 6/60. The mean IOP before LPI was 20 ± 2.81 mmHg, which was reduced to 15 ± 2.3 mmHg 1 week post-laser (participants were given timolol eye drops following LPI for 1 week). Differences were observed in ICA, which increased from 34.13° ± 2.64° to 34.75° ± 2.84° (P < 0.041). The ACD increased from 2.21 ± 0.25 to 2.35 ± 0.27 mm (P < 0.01) and the ACV increased from 98.19 ± 12.13 to 104.15 ± 11.26 mm3 (P < 0.001). No statistically significant changes were noted in HVID, HACD, CCT, and CV [Table 2].{Table 1}{Table 2}

Discussion

LPI is the current first-line treatment for PACD. Evaluation of the anterior chamber angle following LPI is very important as 30% of patients' post-LPI may continue to progress.[[11]] Gonioscopy is recommended for it 1 week following LPI.[[10]] Since gonioscopy is a subjective evaluation, anterior segment imaging by AS-OCT or a Scheimpflug-based device may help to record the changes accurately and objectively with the added advantage of it being a noncontact device. In our study, morphological changes in the anterior segment after LPI were evaluated in 52 eyes of 27 patients with PACD using Sirius Scheimpflug-Placido disk corneal topography. It was found that ICA, ACD, and ACV showed statistically significant changes following LPI among PACD patients.

The ICA is formed between the posterior surface of the cornea and the periphery of the iris, which is wide in normal eyes and narrow in PACD. Investigators have previously reported that LPI causes a significant increase in ICA in patients with PACD.[[7]],[[12]] Our study showed a statistically significant change in ICA post-LPI. Most investigators have used AS-OCT for documenting changes in ICA after LPI with good reproducibility.[[5]],[[6]] Antoniazzi et al.[[8]] measured the anterior segment parameters following LPI in PACD using Pentacam (Oculus Pentacam, Wetzlar, Germany) and compared it to measurements using AS-OCT (Carl Zeiss Meditec) and demonstrated a significant change in ICA, which was comparable to both the imaging modalities. Mansoori and Balakrishna[[13]] demonstrated a similar change using a Scheimpflug-based device with good reproducibility.

Angle-closure glaucoma is characterized by a shallow anterior chamber due to pupillary block, which is recorded as reduced ACD. LPI usually leads to an increase in ACD in patients with PACD.[[14]],[[15]] We found a statistically significant increase in ACD following LPI in almost all our study participants. The same has been reported earlier using AS-OCT and Scheimpflug-based devices with good reproducibility.[[16]],[[17]]

ACV is the amount of aqueous humor in the anterior chamber. LPI provides a new opening between the anterior and posterior chambers and relieves pupillary block, equalizing pressure in both chambers.[[18]] In our study, ACV showed a significant increase after LPI in PACD patients, which is similar to that reported previously using AS-OCT and Scheimpflug-based devices following LPI.[[8]],[[15]],[[19]]

The glaucoma analysis program of Sirius Scheimpflug corneal topographer measures HVID, HACD, CV, and CCT along with ICA, ACD, and ACV. CV and CCT will not be altered with LPI, except in very few cases of corneal endothelial damage before/following LPI. The HVID and HACD grossly represent the white-to-white diameter, and LPI will not impact the values. Our study did not show a significant difference in HVID, HACD, CCT, and CV after LPI in PACD patients, which is expected and similar to previous reports.[[13]],[[19]]

Though gonioscopy is the diagnostic technique to evaluate the angle, it does not allow for the quantification of angle recess or anterior chamber measurements. Quantitative techniques such as UBM, AS-OCT, and Scheimpflug-based devices have the ability to detect exact dimensions and changes occurring either due to physiological states or interventions such as LPI.[[17]],[[18]] Objective analysis of these can help us understand the pathophysiology of angle closure and factors affecting its progression. Additionally, they may be valuable in the follow-up and demonstrating progression, though this needs further evaluation in long-term studies. Quantification offers the prospect of developing exact algorithms for predicting and detecting progression in angle-closure diseases.

The AS-OCT and Scheimpflug-based devices offer an advantage over UBM, being noncontact techniques. Scheimpflug-based devices have the additional advantage of quick and complete scanning of the entire anterior chamber. On the other hand, AS-OCT has the disadvantage of being technician dependent, requiring the marking of the scleral spur.[[20]],[[21]],[[22]],[[23]]

A study conducted by Nasser et al.[[9]] using the two Scheimpflug-based devices, Sirius topographer and Pentacam, in the normal population concluded that both imaging modalities could provide reproducible measurements of the anterior chamber and Sirius is superior with the presence of Placido rings and better three-dimensional reconstruction, even though the parameters obtained were similar with both the devices.

The strengths of our study were the sample size which was higher than other studies and the use of a rapid, high-precision Sirius Placido disk topographer for anterior segment evaluation. The limitation of our study was the inclusion of both eyes from subjects, which may have led to stronger effects in the results due to the effect of clustering. But since ours is a descriptive study confirming the magnitude of change after LPI of previous investigators, we feel that the effects observed may still be significant. Additionally, the study was not adequately powered to perform subgroup analysis to compare the changes following LPI between the specific groups of PAC, PACS, and PACG.

Conclusion

LPI causes significant anterior segment changes in certain parameters like ICA, ACD, and ACV, which can be reliably demonstrated using the Sirius Scheimpflug topographer. Our study dealt with short-term changes in anterior segment morphology on Scheimpflug-based imaging at 1 week post-LPI. We recommend the assessment of long-term changes in PACD to understand better the pathophysiology and progression of the angle-closure disease.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.