Conjunctival structure of glaucomatous eyes treated with anti-glaucoma eye drops: a cross-sectional study using anterior segment optical coherence tomography

Large clinical trials have revealed that reduction of intraocular pressure (IOP) in patients with glaucoma prevents the progression of glaucomatous optic neuropathy and visual field defects [1‐3]. In the past decade, various anti-glaucoma eye drops have been developed, such as prostaglandin analogs, β-blockers, α-adrenergic-receptor agonists, carbonic anhydrase inhibitors (CAIs), Rho kinase inhibitors, and fixed combinations including β-blockers/CAIs and ß-blockers/prostaglandin analogs. Most patients with glaucoma are treated with various anti-glaucoma eye drops to maintain IOP reduction.

In most cases, long-term administration of anti-glaucoma eye drops is necessary for efficient IOP control. Although most anti-glaucoma eye drops do not cause severe systemic effects, some studies have demonstrated that long-term administration of anti-glaucoma eye drops induces histopathological and inflammatory changes in the conjunctival tissues. These changes are deleterious because altered histology of conjunctival tissue was shown to be a risk factor for surgical failure in patients who underwent trabeculectomy [4‐12].

Anterior segment optical coherence tomography (AS-OCT) has facilitated the non-invasive imaging of conjunctival structures [13]. We previously demonstrated that AS-OCT images can detect conjunctival inflammatory responses and subconjunctival scarring after phacoemulsification and pars plana vitrectomy [14, 15]. However, no studies using AS-OCT have reported on the conjunctival structure after long-term administration of anti-glaucoma eye drops. Therefore, our study aimed to determine whether the structural features of conjunctival tissues after long-term administration of anti-glaucoma eye drops can be quantified using the AS-OCT images.

In vitro and ex vivo studies have demonstrated that long-term administration of anti-glaucoma eye drops induces significant histopathological and inflammatory changes in conjunctival tissues [4‐12]. However, these findings have used human or animal histological specimens that did not provide in vivo measurements of the conjunctival structures. Our study is unique because the structural features of conjunctiva caused by long-term administration of anti-glaucoma eye drops were non-invasively quantified by AS-OCT.

This study identified the use of prostaglandin analogs and the fixed combinations of β-blocker/prostaglandin analog eye drops as prognostic factors for decreasing the preservation rate of the borderlines between the conjunctival stroma/Tenon’s capsule and Tenon’s capsule/sclera, suggesting that prostaglandin analogs are the main prognostic factor for lower preservation rates. Prostaglandin analogs are currently considered the first line of administration for glaucoma patients because of their efficacy, systemic tolerability, and high patient adherence to the once-daily treatment. However, prostaglandin analogs provoke a conjunctival inflammatory reaction because human-leukocyte-associated antigen (HLA)-DR is expressed in the conjunctiva even after a short period of administration, independent of the types of prostaglandin analogs used [16]. Preservative-free latanoprost also promotes the activation of P38-NF-κB signaling and upregulates the expressions of cytokines, followed by CD4+ T cell infiltration in the mouse conjunctiva [17]. Matrix metalloproteinases (MMPs) are upregulated, but tissue inhibitors of metalloproteinase (TIMPs) are downregulated in rat conjunctival tissue when prostaglandin analogs are administered, suggesting that prostaglandin analogs may enhance extracellular matrix degradation [18]. An experiment on the effects of prostaglandin analogs on human conjunctiva showed that the altered expressions of MMP-3 and TIMP-2 from the fibroblasts resulted in the remodeling of the extracellular matrix [19]. Moreover, human conjunctiva treated with prostaglandin analogs contains amorphous material [20]. These data indicate that the lower preservation rate of the borderlines between the conjunctival stroma/Tenon’s capsule and Tenon’s capsule/sclera in eyes treated with prostaglandin analogs or fixed combinations of β-blockers/prostaglandin analogs may reflect the remodeling of extracellular matrices in the conjunctiva due to prostaglandin analog-induced inflammation.

Interestingly, the α2-receptor agonist was significantly associated with higher preservation rates of the borderlines than other types of eye drops. In rat conjunctiva treated with brimonidine, the concentrations of inflammatory cytokines, such as IL-1β, IL-2, and IL-6, were significantly lower than rates in the control conjunctiva [21]. Additionally, there were significantly fewer inflammatory cells in the conjunctiva treated with brimonidine than in conjunctiva treated with timolol or latanoprost [22]. The reduced inflammation in the conjunctiva may contribute to the higher preservation rates of the borderlines of the conjunctival stroma/Tenon’s capsule and Tenon’s capsule/sclera in the α2-receptor agonist.

Preservatives present in eye drops suppress microbial activity and preserve the sterility of ophthalmic formulations for multidose drug bottles. However, long-term administration of anti-glaucoma eye drops with preservatives induces significant histopathological and inflammatory changes in the conjunctival tissues, such as increased numbers of fibroblasts and inflammatory cells. These conditions induce postoperative fibrotic responses in the conjunctival tissues, resulting in bleb failure and a reduced success rate of filtration surgery. The inflammation induced by preservatives may reduce the preservation rates of the borderlines between the conjunctival stroma/Tenon’s capsule and Tenon’s capsule/sclera. Interestingly, Purite®, the preservative in brimonidine, is known to cause less lymphocytic infiltration in the conjunctival stroma than other preservatives [22]. The reduced lymphocytic infiltration in the conjunctiva due to Purite® may contribute to the higher preservation rates of the borderlines of conjunctival stroma/Tenon’s capsule and Tenon’s capsule/sclera.

The present study has some limitations. First, although the AS-OCT images quantified the disruption of the bulbar conjunctival borderlines, the associations between the observations detected by the AS-OCT images and the surgical outcomes of trabeculectomy remain unknown. Before trabeculectomy, AS-OCT images of the conjunctiva should be taken in eyes that were treated with various anti-glaucoma eye drops to identify the relationship. Second, the present study was unable to determine whether the disruption of the bulbar conjunctival borderlines is reversible because the study design was cross-sectional. If the disruption is reversible, the cessation of anti-glaucoma eye drops would be beneficial for eyes that will be treated with a trabeculectomy. Third, there was no histological confirmation of what the disruption of conjunctival layer borderlines caused by long-term administration of anti-glaucoma eye drops represents. A longitudinal study is required for further information.

In conclusion, AS-OCT images display the disruption of the bulbar conjunctival borderlines in eyes that have experienced long-term administration of anti-glaucoma drops. Assessments of the conjunctiva based on AS-OCT images may contribute to better therapeutic management for glaucoma patients treated with anti-glaucoma eye drops.