Our results showed that IOP and CCT were similar in healthy controls and patients with PACS. As expected, ACD, ACV and iridocorneal angle values were lower in PACS patients compared to healthy controls. In this study, we also investigated early changes in IOP, corneal topography and retinal OCT findings after LPI in PACS patients. While ACD showed no change after LPI, ACV and iridocorneal angle increased. The mean foveal retinal thickness increased significantly one month after LPI. Further, the mean RNFL thicknesses in superior and temporal quadrants showed significant increase at first month visit after LPI. There was no significant change in the mean GC-IPL thickness values of all quadrants and the average and minimum measurements.
Previous studies have shown different results regarding ACD after LPI in patients with narrow angle. Acet et al. reported deepening of ACD after LPI [6]. Unlike our study, they also included patients with primary angle closure (PAC), PAC glaucoma, acute angle closure glaucoma and iris plateau configuration. However, other two studies involving a similar patient group as our study found no significant change in ACD after LPI [7, 8]. We thought that the discrepancy in results was related to the different methodology of these studies. In line with the literature [6–8], ACV and iridocorneal angle were increased after LPI in our study.
The mechanism of optic neuropathy in PACG has not been fully elucidated. However, mechanical compression in general is considered to be the main mechanism of PACG, especially in acute form. Excessively high IOP is claimed to cause apoptosis in ganglion cells and associated axons [9]. The branches of the central retinal artery provides nutrients and oxygen to the inner retinal layers. The superficial retinal capillary plexus by these branches supply the RNFL, ganglion cell layer and inner plexiform layer [10]. While RNFL consists of the axons of ganglion cells, GC-IPL consists of the dendrites of ganglion cells. Thus, the retinal ganglion cells receive most of their oxygen supply from the superficial retinal capillary plexus [11]. A previous OCT angiography (OCTA) study showed that there was a positive correlation between parapapillary perfusion and the thicknesses of RNFL and GC-IPL in healthy population [12]. However, Rao et al. [13] reported similar vessel densities in eyes with PAC compared to those of control eyes, but thinner RNFL thickness in superotemporal region in eyes with PAC. On top of that, they hypothesized that high IOP might affect RNFL measurements earlier than retinal vessel densities. Zha et al. [14] reported that RNFL was thicker in eyes with PACS than in healthy eyes. They stated that previous studies had different results in RNFL and RNFL could remain same or increased first then decrease late during an acute IOP spike episode. Due to the variable results in previous studies we assessed the change in RNFL thickness after LPI and found an improvement after LPI in patients with PACS. These results suggest that RNFL can be damaged before PAC develops, but this can be prevented by LPI.
In a longitudinal study (72 months) in eyes with PACS, the authors reported a 47% reduction in the risk of primary angle-closure or acute attack after LPI, and no long-term side effects of LPI [15]. Thus, they stated that prophylactic LPI provides a modest benefit given the very low event rate observed and the reduced harm for most of the endpoints achieved over the period of performing an iridotomy. In three studies (153 eyes, 11–46 months follow-up) investigating the change in IOP after Li in eyes with PACS, no increase in IOP was reported in any of the eyes [16–18]. However, in one study including 239 eyes with a 56-months follow-up it was reported that 18% of eyes had increased IOP and 7% required further treatment [19]. Studies evaluating the angle with gonioscopy have reported persistent angle closure ranging from 11–25% in eyes with PACS after LPI [20]. However, side effects like increased IOP and permanent angle closure were not observed in our group, at least in the short-term follow-up period.
Limitations of the present study include its relatively small sample size, not measuring lens thickness and lens vault, and short-term follow-up of the patients. In summary, ACV and iridocorneal angle increased in one week after LPI and remained stable at first month. One month after LPI foveal retinal thickness showed significant reduction, RNFL thicknesses of superior and temporal quadrants were increased. Our results suggest that LPI in patients with PACS provides improved retinal thickness and RNFL thickness, as well as anterior chamber parameters.