The pathogenesis of PPM is still a mystery. Some theories suggest a transitory membrane, composed of strands from major circulus arteriosus of the iris with the mesenchyme failing to regress at the end of eight and a half months of gestation age [8]. This membrane, which replaces the vascular tunica, lies between the corneal endothelium and lens epithelium. The remnants between the iris collarette formed PPM [9]. The diagnosis is quite straight forward clinically, however, some rare congenital conditions should be considered as differential diagnosis. For example, pupillary-iris-lens membrane with goniodysgenesis and congenital idiopathic microcoria were all presented with excessive iris tissue but not extended from the iris collarette [10-11]
Treatment options include conservative treatment with or without mydriatics. Pharmacological mydriasis, occlusion therapy and close observation all played some role in selected cases [2, 6, 12]. Another non-surgical option is YAG-laser [13]. Besides the risk of hyphema and pigments dispersion, it may not be efficacious in thicker PPM. Most importantly, it is reserved for older patients and mainly for cosmetic reasons. Therefore, it is not practical in these cases of visual development. Previous studies considered a pupillary diameter of 1.5 mm necessary for visual development and refrained from amblyopic [12]. So, in our practice, the indications for an operation mostly followed this rule.
Surgical techniques and equipment varied according to the surgeon’s preferences. Incisions ranged from a single 4mm to 1.5mm wound, to a pair of small incisions [9,14-16]. We suggested “cataract-extraction- like’’ incision wounds, which is one 2.2mm main wound with another smaller side port incision [17]. The advantage is pediatric surgeons are familiar with this intraocular operation setting because it is the same as in cataract surgery. Further, much more working space can be achieved compared with a single entry. More importantly, if an underlying cataract was discovered during operation, sequential cataract operation can be done without changing wound settings. As for separating PPM from the iris, vitrectors, vitrectomy scissors, and intraocular scissors are all recommended instruments [14-16]. We prefer intraocular scissors for two main reasons. First, nonautomated equipment reduces the risk of iatrogenic complication, as supported by previous research [14]. Second, the cost is much lower compared with vitrectors and non-retina specialists may be unfamiliar with vitrectors and vitrectomy scissors.
In our study, the associated ocular abnormalities differed from previous cohorts. To our knowledge, no other study has reported congenital nystagmus, optic nerve coloboma, IOOA and keratoconus associated with dense, thick PPM. Further studies may determine whether a curtain genetic effect or association may contribute to these conditions.
Lens opacity following the operation is the main concern because removing iris strands from the anterior lens capsule might inevitably cause some minor trauma which contributes to this complication. However, our cases had no induced lens opacities in a relatively long period of follow-up time. Other studies also reported comparable results. Courtney et.al reported 10 eyes with no cataract occurrence after a mean postoperative follow-up of 5.3 (range, 2.5 to 8) years [14]. Lee et.al showed 32 eyes with only 6.3% lens opacity rates in a mean follow-up of 6.5 (range, 4.0 to 14.8) years [15]. One hypothesis came from a histopathological finding reported by Ramappa et al. The theory suggested the lens capsule was special in some cases, and the surface of the iris strands adhering to the lens can have identical histologic features from the lens epithelium itself [9]. It also explained removing a firmly attached PPM from the lens capsule with an intact lens epithelium.
Visual rehabilitation comprises all kinds of interventions, focusing on improvement of visual abilities, visual development, and coping with visual disabilities [18]. The best result comes with a multidisciplinary approach, involving physicians, optometrists, occupational therapists and most important of all, the patients. The program can be either doing at home or trained in the hospital by a teacher. In this case series, all patients were introduced to curtain training programs as soon as possible. As expected, the best outcome (Final BCVA of 0.00 logMAR) was found in patients with only refractive errors as associated ocular abnormalities (Case 1 and 2). Even in the case with very high astigmatism (>7D), the outcome was excellent. However, in highly myopic (>6D) cases (Case 4), the outcome could be less favorable with final BCVA all < 0.18 logMAR in our and other reports [6]. Strabismic amblyopia can also be avoided through timely surgical correction (Case1) [19]. The same results are also supported by a previous study [6]. In cases with other ocular abnormalities other then refractive status, the outcomes are more variable. PPM is considered less important in causing amblyopia if treated promptly and properly. Case 4 was the clearest example that amblyopia was caused by large optic disc coloboma and congenital nystagmus. Timing PPM removal and a visual rehabilitation program were fruitless in this case.
The primary limitation of this study is its retrospective nature, but those with detailed and regular followed up cases were included. Clear and detailed data were available for analysis. The second limitation is the lack of a control group. Due to the concern of deprivation amblyopia with delayed treatment, a more proactive indication was made. Reynolds et.al suggested prompt therapeutic intervention for PPM, even within the first several months [20].
The strength of this paper is its long-term follow up period and focus on how associated abnormalities influence visual outcomes. There were no significant complications in any patient using our technique. The surgical setting and equipment are much easier to handle and more familiar to pediatric surgeons.