Introduction

Glaucoma is a progressive optic neuropathy that causes irreversible blindness if not treated appropriately. Treatment approaches for this disorder are aimed at controlling intraocular pressure (IOP), a major modifiable factor related to disease progression. Current treatment modalities include topical medications, laser therapies, and surgical procedures [1, 2].

Trabeculectomy is now the most widely performed surgical procedure for treatment of medically uncontrolled glaucoma in adults. While wound healing is required for other types of surgery, trabeculectomy is unique because its success is linked to interruption of this wound-healing process in order to maintain patency of the new filtration pathway. This goal makes subconjunctival scarring one of the most challenging issues facing clinicians after trabeculectomy [3, 4].

Statins, competitive inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, are generally used to decrease plasma low-density lipoprotein cholesterol levels and to prevent cardiovascular disease-related morbidity and mortality. In addition to their lipid-lowering properties, statins have been shown to have a myriad of other effects on the body, including antioxidative, antiproliferative, angiogenic, and anti-inflammatory effects. The use of statins has increased sharply in recent years, and they are now one of the most commonly prescribed classes of medications worldwide [5, 6].

Researches have shown that statin use is associated with a lower risk of primary open-angle glaucoma (POAG), a lower progression rate of visual field defects [7, 8], and a significant increase in aqueous outflow through relaxation of the trabecular meshwork and the ciliary body muscle [9]. Reports have also suggested that statins play a role in fibrosis, which theoretically could affect the wound-healing process following trabeculectomy [10, 11]. This study aimed to evaluate the effect of oral statin use on patient outcomes after trabeculectomy.

Methods

The protocol for this retrospective, comparative, interventional study was approved by the Institutional Review Board of the Faculty of Medicine at Chulalongkorn University. The requirement for written informed consent was waived due to the retrospective nature of the study. Medical records of patients who underwent primary mitomycin-C (MMC)-augmented trabeculectomy and completed 2 years of follow-up were reviewed.

Results

A total of 158 eyes of 158 patients were eligible for this study (Table 1), with 47 statin users and 111 nonusers. Eighty-three (53%) patients were male, and the mean (SD) age of patients was 59.6 (13.9) years. Most eligible eyes were phakic (115/158, 73%). Nearly half of patients in both groups (69/158, 44%) were diagnosed with secondary glaucoma. There was no statistically significant difference in the type of secondary glaucoma between statin users and nonusers (P = .33) (S1 Table). The mean (SD) preoperative IOPs were 26.3 (12.4) mm Hg in statin users and 27.4 (12.2) mm Hg in nonusers. There were no statistically significant differences in sex, laterality, lens status, type of glaucoma, visual acuity, IOP, number of medications, cup-to-disc ratio, or visual field mean deviation between the groups before the primary trabeculectomy surgery. Moreover, there was no significant difference in the median of duration of MMC in both groups [median (IQR): statin users 3 (3–3.5) minutes, statin nonusers 3 (3–3.5) minutes, P = .79].

Table 1

Baseline patient characteristics.

Characteristics	Statin nonusers (N = 111)	Statin users (N = 47)	P-value
Sex*			.65
Male	57 (51%)	26 (55%)
Female	54 (49%)	21 (45%)
Age† (year)	57.2 (14.3)	65.4 (11.2)	< .001
Laterality*			.36
Right	72 (65%)	34 (72%)
Left	39 (35%)	13 (28%)
Lens status‡			.02
Phakic	87 (78%)	28 (60%)
Pseudophakic	24 (22%)	19 (40%)
Diabetes mellitus*	15 (14%)	26 (55%)	< .001
Diagnosis*			.75
Primary open-angle glaucoma	33 (30%)	15 (32%)
Primary closed-angle spectrum	30 (27%)	11 (23%)
Secondary glaucoma	48 (43%)	21 (45%)
Intraocular pressure† (mm Hg)	27.4 (12.2)	26.3 (12.4)	.61
Number of medications†	3.8 (1.0)	3.9 (0.8)	.35
Mean deviation§ (dB)	-21.45	-19.65	.43
	(-29.83 to -9.44)	(-21.86 to -10.02)
	N = 24	N = 13
Visual acuity§ (decimal)	0.29 (0.01 to 0.67)	0.29 (0.01 to 0.50)	.67
Cup-to-disc ratio§	0.8 (0.7 to 0.9)	0.7 (0.5 to 0.85)	.08

* Data presented as n (%); P-value was obtained from chi-squared test.

† Data presented as mean (standard deviation); P-value was obtained from Student’s t-test.

‡ Data presented as n (%); P-value was obtained from Fisher’s exact test.

§ Data presented as median (IQR); P-value was obtained from Mann-Whitney U test.

Kaplan-Meier survival analysis with the log-rank test (Fig 1) showed that the statin user group had a significantly higher cumulative failure probability than the nonuser group (P = .013), with a 24-month cumulative failure probability of 78.7% in statin users and 60.4% in nonusers. Identified causes for cumulative failure in both patient groups are shown in Table 2. The Cox proportional-hazards model revealed that the unadjusted hazard ratio for statin users was 1.51 (95% confidence interval (CI): 1.02–2.24, P = .042). The multivariable model showed that the adjusted hazard ratio for statin users was 1.61 (95% CI: 1.06–2.45, P = .026).

Fig 1

Kaplan-Meier survival analysis with log-rank test.

The analysis shows that the statin user group had a significantly higher cumulative probability failure than the nonuser group (P = .013).

Table 2

Causes of 24-month cumulative failure after trabeculectomy in statin users and nonusers.

	Statin nonusers (N = 111)	Statin users (N = 47)
Inadequate lowering of intraocular pressure	43 (38.7%)	23 (48.9%)
Reoperation for glaucoma	21 (18.9%)	14 (29.8%)
Hypotony	3 (2.7%)	1 (2.1%)
No light perception	3 (2.7%)	4 (8.9%)

Failure rate at 24 months was 46.8% in statin users and 39.6% in nonusers. There were no statistically significant differences in visual acuity, IOP, or number of medications between the groups at 24 months (P = .62, P = .15, and P = .99, respectively) (Table 3). Logistic mixed-model regression analysis showed an increased risk of failure at 24 months for statin users using both univariate (odd ratio [OR] = 3.72, 95% CI: 1.10–12.62, P = .035) and multivariate models (adjusted OR = 5.89, 95% CI: 1.44–23.94, P = .013).

Table 3

Clinical outcomes in statin users and nonusers at 24-month follow-up after trabeculectomy.

	Statin nonusers (N = 111)	Statin users (N = 47)	P-value
Visual acuity* (decimal)	0.29 (0.01 to 0.67)	0.4 (0.005 to 0.67)	.62
Intraocular pressure† (mm Hg)	12.7 (5.2)	14.4 (8.9)	.15
Number of glaucoma medications†	0.95 (1.3)	0.96 (1.3)	.99
Mean deviation* (dB)	-16.31 (-27.5 to -9.84) (N = 14)	-12.69 (-20.71 to -0.56) (N = 11)	.34
Cup-to-disc ratio*	0.85 (0.7 to 0.95)	0.7 (0.5 to 0.85)	.28

* Data presented as median (IQR); P-value was obtained from Mann-Whitney U test.

† Data presented as mean (standard deviation); P-value was obtained from Student’s t-test.

The mean (SD) number of 5-FU injections was 1.87 (2.1) in statin users and 1.21 (1.7) in statin nonusers. Multivariable Poisson regression analysis of the number of 5-FU injections indicated that statin users had a significantly higher number of 5-FU injections at 24 months [unadjusted incidence rate ratio (IRR) = 1.55, 95% CI: 1.19–2.03, P = .001; adjusted IRR = 1.59, 95% CI: 1.08–2.08, P = .014).

The mean (SD) number of bleb-needling procedures was 0.66 (1.0) in statin users and 0.32 (0.7) in statin nonusers, with a significantly higher number of bleb-needling procedures performed in statin users than in nonusers. (unadjusted IRR = 2.09, 95% CI: 1.29–3.39, P = .003 and adjusted IRR = 1.91, 95% CI: 1.06–3.45, P = .031).

Regarding bleb morphology, the bleb extent and height at 3 months post-operation according to IBAGS grading showed no significant difference between both groups (extent P = .86 and height P = .19). Statin users showed greater vascularity than nonusers (P < .001). Representative images of bleb vascularity grades of V2 to V4 according to IBAGS grading are shown in S1 Fig.

In order to control for the effects of age, an additional analysis involving 2:1 age matching analysis was performed. The data from 80 statin nonusers and 40 users showed the same results. The 24-month cumulative failure probability was significantly higher in statin users (77.5%) than in nonusers (62.5%) (P = .048). The Cox proportional-hazards model revealed that the hazard ratio for statin users was 1.44 (95% CI: 0.92–2.24, P = .11). Similarly, the multivariable Poisson regression analysis also showed a significantly higher number of 5-FU injections and bleb-needling procedures in statin users (IRR = 1.76, 95% CI: 1.31–2.37, P <0.001; IRR = 1.93, 95% CI: 1.14–3.27, P = .015, respectively).

Discussion

Our study revealed that statin use significantly increased the hazard risk of trabeculectomy failure and significantly increased the failure rate at 24 months. There were no statistically significant differences in the mean IOP, visual acuity, or number of medications between statin users and nonusers at 24 months. Statin users, however, underwent significantly more 5-FU injections and bleb-needling procedures than nonusers.

The present study identified higher trabeculectomy failure rates, both for cumulative failure and for failure at 24 months, than other clinical trials [14, 15]. Our cumulative probability of failure during 2 years of follow-up was 78.7% in statin users and 60.4% in nonusers. By contrast, in the Tube Versus Trabeculectomy Study, Gedde et al. reported a 46.9% failure rate in their trabeculectomy group during 5 years of follow-up [16]. Our higher failure rate may be explained by the retrospective nature of our study, which precluded assessment of washed-out IOPs and allowed only medicated IOP values to be obtained. Since mean preoperative IOPs are lower when using only medicated IOP data, a greater number of eyes failed to achieve a 20% IOP reduction from baseline in our study.

Inadequate IOP reduction was the most common cause of failure in both groups, though more commonly in statin users. The success of trabeculectomy at reducing IOP is linked to an effective interruption of the wound-healing response in order to maintain patency of the new filtration pathway. In a simplistic sense, wound healing after trabeculectomy may be understood by dividing the process into three distinct phases, inflammatory, proliferative, and remodeling phases [4, 17]. Previous studies have suggested that statin use may be linked to an acceleration of this wound-healing process [18] and to excessive fibrosis formation [10, 11], which may explain the higher failure rate in the statin group observed in our study.

The inflammatory phase of wound healing is characterized by an influx of inflammatory cells, cytokines, and growth factors. In the early stages, this phase is controlled by immediate leakage of blood components from incised vascular tissue. Several chemical factors, including thrombin, thromboxane A₂, platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), interleukin (IL) -1, transforming growth factor (TGF-β), and interferon-γ (IFN-γ) are also secreted, triggering the onset of the next phase [19–21]. Previous studies have shown that statins induced proinflammatory responses by increasing the number of T lymphocytes secreting IFN-γ and by stimulating the secretion of IL-1ß, a member of the IL-1 family of cytokines, by monocytes. Moreover, studies have demonstrated that caspase-1, which is required for the processing and secretion of IL-1ß, was activated by statins [22, 23].

In the proliferative phase, several processes are stimulated, including re-epithelialization, angiogenesis, and fibroblast activity. PDGF and TGF-β are key profibrogenic cytokines implicated in the recruitment and differentiation of fibroblasts into myofibroblasts. Akershoek et al. [24] demonstrated an acceleration of the healing process in full-thickness burn wounds in patients receiving atorvastatin treatment. These authors hypothesized that the shortened healing time in atorvastatin users was related to an earlier transition from the inflammatory to the proliferative phase of wound healing and to a faster resolution of myofibroblast presence in wounds.

During the proliferative phase, angiogenesis occurs concurrently with release of VEGF from platelets and macrophages [4, 17]. Numerous studies have demonstrated effects of statins on angiogenesis, including upregulation of endothelial nitric oxide synthase, stimulation of the intracellular serine/threonine kinase Akt signaling pathway in endothelial cells, and promotion of the migration and differentiation of endothelial progenitor cells [25–27]. In 2012, a histopathology study demonstrated that topical simvastatin accelerated wound healing in diabetic mice via promotion of angiogenesis and lymphangiogenesis [18], which supported previous in vitro studies showing the marked angiogenic effects of statins on vascular endothelial cells [28, 29]. In addition, statins have been reported to augment VEGF expression in osteoblastic cells and to stimulate retinal microvasculature angiogenesis [30, 31]. The presence of VEGF in trabeculectomy blebs has also been shown to be associated with blood vessel growth and fibrosis [32], a finding that clarified the role of anti-VEGF therapies during wound-healing modulation after glaucoma-filtering surgery [33]. In our study, statin use increased the number of 5-FU injections that were required. Since 5-FU injections were performed when blebs were highly vascularized, we postulate that the higher number of 5-FU injections in the statin group may reflect augmentation of VEGF expression by this class of drugs.

In 2012, Xu and colleagues [34] demonstrated that statin use was associated with interstitial lung abnormalities, increasing a patient’s susceptibility to interstitial lung disease. Lungs from mice treated with statins and bleomycin were shown to have increased lung fibrosis and collagen deposition compared to lungs of mice exposed to bleomycin alone. The statin and bleomycin group also had significant increases in expression of IL-1β and IL-18 in bronchoalveolar lavage fluid and in lung tissue, which was correlated with increased caspase-1 activation. Furthermore, another study described pulmonary side effects from statins, including the development of hypersensitivity pneumonitis and fibrotic nonspecific interstitial pneumonia [10]. Although similar studies have yet to be conducted in the human eye, these findings suggest that there may be a correlation between statin use and fibrosis formation after trabeculectomy.

While these previous studies have demonstrated possible negative effects of statin use on bleb outcomes, other studies have contradicted our findings by demonstrating that statins may actually inhibit postoperative scarring after glaucoma-filtering surgery. In 2008, Mayer-ter-Venh et al. [35] reported that lovastatin inhibited TGF-β-induced myofibroblast transdifferentiation in human tenon fibroblasts, which is crucial for activation of fibroblasts in the proliferative phase and for proper extracellular matrix replacement during the remodeling phase [36, 37]. In addition, Park and colleagues [38] demonstrated lovastatin’s antifibrotic effect by showing reduced myofibroblast transdifferentiation after glaucoma-filtering surgery in a rabbit model.

We hypothesize that the contradictory findings in these prior studies may be related to the types of statins used and to different routes of administration. Moreover, wound healing is known to be a multifactorial and complicated process in which individuals may mount responses differently. Another possible explanation is that statins may have biphasic, dose-dependent effects on angiogenesis, promoting the response at lower doses and inhibiting it at higher doses, similar to what has been proposed for endothelial cell proliferation, migration, differentiation, and VEGF release [26, 39]. Therefore, different or unknown doses of statins in our study and others might have influenced these processes differently. The relationships between the types and dosages of statins used and the wound-healing process still need to be explored in future studies.

It is important to note that this study had limitations. First, since it was a retrospective study, we could not perform objective assessments of bleb morphology, such as bleb imaging, which would have helped to verify the presence of fibrosis. Second, the type and dosage of statins used by patients was not systematically recorded. Third, the study was conducted in a uniformly Thai population, and therefore, its applicability to other ethnicities, especially non-Asian ethnicities, needs to be validated. Fourth, the majority of our study patients had advanced glaucoma. The results may not, therefore, be generalizable to patients with other degrees of glaucoma severity. Additionally, bleb grading, 5-FU injections, and bleb-needling procedures were performed at doctors’ discretion. It should be noted, however, that all the doctors who were making these decisions and performing these procedures were glaucoma specialists practicing under the same guidelines outlined by our institution. Lastly, we were unable to evaluate the duration of topical antiglaucoma medication usage before glaucoma-filtering surgeries, which has been shown to have a deleterious effect on trabeculectomy outcomes [40]. In the future, prospective studies using bleb imaging to better understand the effect of statin use on scar formation following trabeculectomy are warranted.

In conclusion, this study demonstrated that oral statin was associated with higher rates of trabeculectomy failure. Statin use also increased the number of 5-FU injections and bleb-needling procedures required in the post-trabeculectomy period. Although further investigation is necessary, this study suggests that there may be a link between oral statin use and postoperative scarring after glaucoma-filtering surgery.