Permanent visual loss in eyes with keratoconus

Keratoconus is normally considered to be a benign condition. In early disease visual rehabilitation can be achieved with glasses or contact lenses. However, over time, scarring or contact lens intolerance can develop, and an estimated 21% of patients eventually require keratoplasty Lass et al. (1990). Because the risk of permanent visual loss is unknown, we have attempted to estimate the prevalence of blindness associated with keratoconus. We searched the electronic healthcare records of Moorfields Eye Hospital for the period January 2005–September 2011 using keywords keratoconus and blind. The total of search results with keratoconus was recorded, and the medical records of patients who satisfied both criteria were reviewed. The inclusion criterion was blindness in an eye with pre-existing keratoconus. Blindness was defined according to the World Health Organization criterion of a corrected distance visual acuity (CDVA) of <3/60. Additional data recorded were demographics, ocular and medical comorbidity, surgical intervention and history of trauma. For statistical analysis, Snellen acuity was converted to LogMAR equivalents. LogMAR equivalents for count fingers (CF), hand movements (HM), light perception (LP) and no light perception (NLP) were 1.85, 2.30, 2.70 and 3.00, respectively Schulze-Bonsel et al. (2006). Patients with an unrelated cause for their blindness (e.g. retinitis pigmentosa) were excluded. A total of 6399 patients with keratoconus attended during the study period. The records of 77 patients were reviewed, of which 58 were excluded because the acuity was ≥3/60 (n = 36) or because there was an unassociated blinding condition (n = 22). The remaining 23 eyes of 19 patients (median follow-up, 8.1 years; range, 0.2–37.7 years) had a median final CDVA of 1/60 (range, 1/60 to NLP) with 11 eyes NLP (Table 1). Corneal transplantation had been performed in 9 (47.4%) eyes of the 19 patients (median number of grafts, 1; range, 1–7). During the study period, 394 (6.2%) of the 6399 patients received a transplant. Ocular trauma was the most common cause for severe visual loss (seven eyes, two following assault and five following accidental injury), all of which were NLP. Atopic keratoconjunctivitis (six patients) was the commonest associated risk factor. Four eyes were blind following the complications of intraocular surgery (one endophthalmitis, one pseudophakic retinal detachment, two irreversible allograft failure). Ten eyes (six patients) were functionally blind due to advanced keratoconus, including four patients with Down syndrome or other learning difficulties, who could not tolerate visual correction and in whom keratoplasty was declined. During the study period, the prevalence of severe irreversible visual loss attributable to keratoconus or its management was estimated to be 0.20% of (12798) eyes, with 0.16% of (6399) patients functionally blind in both eyes. The principal limitations of this study are the uncertainty of the denominator used to estimate the prevalence of keratoconus-related blindness. We studied a hospital-based population but community optometrists manage an unknown proportion of patients with mild keratoconus, and therefore, the prevalence we report is likely to be an overestimate. Also, CDVA could not be searched electronically and ‘blind’ was adopted as a proxy value. However, we have confirmed that severe irreversible visual loss is a rare consequence of keratoconus. In this group, traumatic globe rupture, usually in men, was the most common cause accounting for one-third of cases. Interestingly, only 2 of 6 ruptured globes occurred in eyes that had a keratoplasty, although this is a well-documented risk factor Steinberg et al. (2011). Poor vision and weak sclera or corneal tissue may increase the risk of globe rupture in keratoconus following accidental trauma. This study also identified a small group of patients who remain functionally blind from keratoconus because contact lenses or lamellar or penetrating keratoplasty was not considered feasible, or because surgery was declined. Although long-term graft survival can be achieved in patients with Down syndrome, Wroblewski et al. (2006) there is a wide range of disability, and surgery may carry a prohibitively high risk of complications. Early intervention with collagen cross-linking may preserve reasonable unaided vision in this patient group.