Description of the condition
Cataract, an opacification of the crystalline lens in the eye, can be caused by many factors including the natural aging process, metabolic abnormalities, nutritional disorders, chronic ocular inflammation, and trauma. The three types of cataract are classified according to the location of the opacity: cortical, nuclear sclerosis, and posterior subcapsular. Cataract formation or acceleration also can occur after intraocular surgery, especially following vitrectomy, a surgical technique for removing the vitreous that is used in the treatment of disorders that affect the posterior segment of the eye. Vitrectomy causes progression of nuclear sclerotic cataracts.
Vitrectomy was first developed by Machemer in 1971 (Machemer 1971). It is a microsurgical technique in which specialized instruments and techniques are used to gain access to the vitreous cavity and retina. During vitrectomy surgery, three small incisions, each approximately 1.4 mm in length, are made in the eye in order to place instruments: a vitreous cutter, a fiberoptic light source to illuminate the inside of the eye, and an infusion cannula to maintain proper intraocular pressure during the surgery. During the past 40 years, advances in surgical technique and instrumentation have made vitrectomy a common surgical procedure for posterior segment disorders. Vitrectomy is indicated for numerous ocular conditions including vitreous loss in cataract surgery, subluxation of the lens, malignant glaucoma, dense pupillary membranes, non-clearing vitreous hemorrhage due to diabetic retinopathy or vein occlusions, retinal detachment, macular hole, macular pucker, vitreo-macular traction, and endophthalmitis.
Although vitrectomy has revolutionized the treatment of posterior segment disorders and improved visual outcomes in patients with retinal diseases requiring surgical intervention, vitrectomy also is associated with co-morbidities that may compromise visual acuity such as retinal detachment, corneal decompensation, and cataract formation or progression in phakic eyes (Benson 1988). The type of cataract that forms or accelerates after vitrectomy is nuclear sclerotic cataract. Cataract formation or progression is believed to be the most common complication associated with vitrectomy. In fact, in many eyes undergoing vitrectomy, the lens is removed at the same time. Often, the nuclear sclerotic cataracts that develop after vitrectomy limit visual acuity outcomes to a degree that would result in surgical removal of the lens in an otherwise 'normal' eye.
The exact pathogenesis of cataract formation or acceleration after vitrectomy is unknown. Older studies have suggested that light toxicity, oxidation of lens proteins, use of intraocular gas, and length of operative time may be causative factors (Cherfan 1991; de Bustros 1988; Ogura 1991). Newer research suggests that vitrectomy surgery increases oxygen tension within the eye; oxygen exposure has been linked with progressive nuclear sclerotic cataract formation (Holekamp 2005; Palmquist 1988).
Although cataract progression is common after vitrectomy, only a few prospective studies have evaluated this occurrence. Do and Hawkins performed a review (unpublished) of the pertinent literature in the PubMed database published from 1966 through 2005. A total of 51 studies were found. The majority of published studies on cataract progression after vitrectomy were retrospective analyses. The reported incidence of cataract was highly variable, from 6% to 100% of cases, depending upon the condition that prompted vitrectomy, duration of follow-up, and the method used to monitor development of cataract. These retrospective studies are limited by the non-uniformity of the lens grading system used or the absence of a description of the lens grading system in the published report.
The Vitrectomy for Macular Hole Study, a randomized controlled trial that evaluated vitrectomy for the treatment of macular holes, retrospectively examined the incidence of cataract development among 74 eyes of patients that participated in the study (Cheng 2001). Investigators used a scoring system similar to the Lens Opacities Classification System II, which contains five grading categories for nuclear and posterior subcapsular opacities. Although duration of surgery did not increase the risk for cataract progression, vitrectomy itself was a risk factor for cataract acceleration; 60/74 (81%) eyes in the surgery cohort had nuclear sclerotic cataract progression at six months of follow-up, compared to only 13/74 (18%) fellow eyes in the control group. By two years, 100% of eyes in the surgery cohort had cataract progression, compared to 8% of control eyes. Similarly, Cherfan and colleagues retrospectively reviewed 100 eyes after vitrectomy for idiopathic macular pucker (Cherfan 1991). After an average follow-up of 29 months (range six to 99 months), 80/100 (80%) eyes in the vitrectomy group and only 24/100 (24%) fellow eyes had developed a visually significant nuclear sclerotic cataract or had undergone cataract extraction.
During the late 1990s, the Submacular Surgery Trials (SST) were initiated to evaluate surgical removal of subfoveal choroidal neovascularization (CNV) compared with observation in patients with age-related macular degeneration (AMD) (SST Group N and Group B), ocular histoplasmosis syndrome (OHS) (SST Group H), and idiopathic CNV (SST Group H) (SST 2004a; SST 2004b; SST 2004c). In these three randomized controlled trials, visually significant cataract was defined as either cataract surgery or lens opacity reported by the SST ophthalmologist to be sufficient to reduce visual acuity by two or more lines in a normal eye. Among the AMD participants in the SST Group N study, 80% of eyes assigned to vitrectomy and surgical removal of their subfoveal CNV developed visually significant cataracts at two years of follow-up. Sixty per cent of eyes had undergone cataract surgery by their last follow-up examination two to four years after enrollment. Among the OHS participants in the SST Group H study, 39% of eyes assigned to vitrectomy developed visually significant cataracts, among which 24% underwent cataract removal. The difference between eyes with AMD and eyes with OHS developing post-vitrectomy cataract is likely due to the median age of the patients. Patients under the age of 50 years are less likely to develop post-surgical accelerated nuclear sclerosis (Melberg 1995). Data from the SST provide the largest and most complete follow-up of eyes undergoing vitrectomy that are at high risk for developing visually significant post-surgical nuclear sclerotic cataracts.
Presentation and diagnosis
Patients who develop post-vitrectomy cataract present with decreased visual acuity despite anatomic or functional success (or both) of the vitrectomy surgery. Individuals who have undergone vitrectomy may have lower levels of baseline (pre-cataract) visual acuity due to the underlying nature of their retinal pathology; therefore patients with post-vitrectomy cataract are more likely to present with poorer vision than individuals with typical senile cataracts. Diagnosis is made with ocular examination using slit-lamp biomicroscopy.
Description of the intervention
Cataract surgery, typically using phacoemulsification and intraocular lens implantation, is commonly recommended for individuals with visually significant lens opacities. Two features of post-vitrectomy nuclear sclerosis make affected lenses especially challenging for cataract surgeons to remove. The nucleus tends to be harder than in age-related nuclear sclerosis, requiring longer phacoemulsification time during the procedure. Also, the absence of vitreous in the posterior segment allows for more mobility of the posterior capsule, increasing the risk of capsular rupture. Thus, surgery for post-vitrectomy nuclear sclerotic cataract may have a higher incidence of complications, although evidence from comparative studies is lacking (Ahfat 2003; Biro 2002).
How the intervention might work
Patients who develop cataract after vitrectomy may undergo cataract extraction; however, visual acuity and other outcomes after cataract surgery may be poor due to the underlying retinal disorder. Most patients who have vitrectomy surgery have serious underlying problems, as indicated by the reasons for vitrectomy. Furthermore, eyes with post-vitrectomy cataract are at risk of complications that affect all eyes that undergo cataract surgery such as endophthalmitis, cystoid macular edema, etc. Thus, vision is often already impaired before cataract surgery and may remain impaired after cataract surgery. Although cataract surgery in a normal eye typically improves vision, the visual prognosis after surgery for post-vitrectomy cataract is uncertain. It likely depends on the success of treatment for the retinal disorder and avoidance of complications during cataract surgery.
Why it is important to do this review
The incidence of cataract formation after vitrectomy varies widely and has been reported to be between 6% and 100%. The majority of published studies confirm that a high rate of cataract formation occurs, but few data are available on visual acuity outcomes after cataract removal. The retinal problem that led to vitrectomy may progress or recur. However, peer-reviewed data on outcomes after surgery for post-vitrectomy cataract are scarce. Even in situations in which cataract formation is not due to vitrectomy, visual impairment can still exist despite cataract extraction. The Los Angeles Latino Eye Study (Barañano 2007) published visual acuity outcomes after cataract extraction in adult Latinos and reported that 41% of eyes had visual impairment (defined as a best-corrected visual acuity of 20/40 Snellen equivalent or less). Age-related macular degeneration and diabetic retinopathy accounted for approximately 57% of retinal pathology after cataract extraction. In addition, in eyes that have undergone vitrectomy surgery the absence of vitreous in the posterior segment allows for more mobility of the posterior capsule, increasing the risk of capsular rupture. Surgery for post-vitrectomy nuclear sclerotic cataract may have a higher incidence of complications. A systematic review of outcomes from controlled clinical trials would provide information for adequate counseling of patients and for guiding ophthalmologists' recommendations.