Explaining the revised eCSC calculation

Explaining the revised eCSC calculation

In 2022, in collaboration with Aravind Eye Care System, the Tamil Nadu State Blindness Control Society (under the Department of Health and Family Welfare, Government of Tamil Nadu, India), conducted eight RAABs across the state of Tamil Nadu to develop district-specific plans for enhancing eye care services.

In the process of calculating eCSC, we reviewed the definition proposed by WHO in a 2021 Lancet Global Health article. We noted that definition included only people with bilateral cataract, either operated or unoperated, and did not include those with cataract in only one eye (either operated or unoperated). In other words, the prevailing definition excluded people with unilateral cataract who had best-corrected visual acuity (BCVA) worse than 6/12 in the other eye due to other conditions (e.g. corneal opacity, phthisis, posterior segment disease, globe/CNS abnormalities, diabetic retinopathy, glaucoma, etc.). 

In summary, the following groups were excluded:

  • Persons with operated cataract and Presenting VA ≥6/12 in one eye and the other eye with a non-cataract condition and BCVA <6/12 should be part of ‘a’ (classified as “ua” in table below)
     
  • Persons with operated cataract and any Presenting VA in one eye and the other eye with a non-cataract condition with BCVA <6/12 should be part of ‘x’ (classified as “ux” in table below)
     
  • Persons with BCVA <6/12 in one eye due to cataract and BCVA <6/12 in the other eye due to non-cataract conditions should be part of ‘z’ (classified as “uz” in table below)

To determine the impact of this on eCSC, we analysed the full dataset from the eight RAABs, to compare the 2021 definition with a revised one that included unilateral cataract patients.

The eCSC estimates given by the two approaches are shown in Table 1. For a cataract surgical threshold of 6/12, eCSC was 40.6% using the “2021 definition” and 40.3% using the “2022 definition”. Although the impact in this instance was minimal, the indicator is no longer inadvertently excluding groups of people who have benefited from, or could potentially benefit from, cataract surgery.

Table 1: Comparison of 2021 and 2022 eCSC calculations using data from eight RAABs in Tamil Nadu, India, 2022

  a b x y z ua ux uz a+b x+y+z eCSC (%)
2021 defintion 1,834 5,379 3,023 7,400 7,353 - - - 7,213 17,776 40.6
  a b x y z ua ux uz a+b+ua x+y+z+ux+uz eCSC (%)
2022 definition 1,834 5,379 3,023 7,400 7,353 104 236 140 7,317 18,152 40.3

People with unilateral cataract newly included are represented in columns ‘ua, ux, and uz’.

The update to the calculation will not systematically increase or decrease eCSC estimates from RAABs or other surveys.

The new definition was agreed at a WHO Technical Working Group meeting in July 2022 and can be referenced in a recent Lancet Global Health article reporting eCSC estimates from RAAB data in 55 countries.

Recommended calculation method for eCSC (at the 6/12 threshold)

(a + b) / (x + y + z) * 100

a = individuals with unilateral operated cataract attaining PVA ≥6/12 in the operated eye, who have BCVA <6/12 with cataract as the main cause of vision impairment or blindness in the other eye

b = individuals with bilateral operated cataract attaining PVA ≥6/12 in at least one eye

x = individuals with unilateral operated cataract (regardless of visual acuity in the operated eye), who have BCVA <6/12 with cataract as the main cause of vision impairment or blindness in the other eye

y = individuals with bilateral operated cataract, regardless of visual acuity

z = individuals (without cataract surgery in either eye) with BCVA <6/12 in both eyes with cataract as the main cause of vision impairment or blindness in one or both eyes

Text with strikethrough has been removed from the previous definition and text in bold has been added to the previous definition.

A lady has her vision screened by an examiner holding a mobile telephone

RAAB data collection in Tamil Nadu, India, 2022. (Via Lions Aravind Institute of Community Ophthalmology.)