The GiveWell Blog

Surgeries performed vs. cases of blindness prevented

We’ve written before about the possibility that surgeries to correct blindness are extremely cost effective. While summarizing the evidence of effectiveness for trachoma interventions, we’ve learned more and it’s clear that equating surgeries performed with cases of blindness prevented is plain wrong.

I read Trachoma: an overview, a literature review of the evidence of effectiveness for the SAFE Strategy, the WHO-recommended approach to trachoma control.

Matthew J. Burton, the author, reviews each of the four components of the strategy, including surgery. He writes (Pg 109, in the PDF version):

There are about 10 million people with trachomatous trichiasis (TT) worldwide who are at increased risk of developing irreversible blinding corneal opacification (CO). Surgical correction of TT probably reduces the risk of progressive CO and blindness. The indications for TT surgery vary between control programmes. Some advocate early surgery when one or more lashes touch the eye, whereas others practice epilation until more severe TT develops. As the progression of TT can be quite swift in some people, where access to ophthalmic services is limited, surgery for mild disease is a logical approach.

A major problem limiting the effectiveness of surgery is the recurrence of trichiasis following surgery, which can be as high as 40– 60%…. There can be a small improvement in vision following surgery of about a line of Snellen visual acuity.

The quote tells me that:

  1. Surgeries are performed for people who are at-risk of becoming blind, but not yet blind. The review doesn’t specify the probability that they’ll become blind.
  2. Surgeries are not performed on those already blind — trachoma-caused blindness is irreversible.
  3. There’s significant chance of recurrence, so performing a surgery is not the same as preventing the patient from ever having the condition again.
  4. There is some vision improvement for those with TT and low vision, but it’s extremely small. (According to Wikipedia, the Snellen visual acuity chart is the eye chart we’re all used to at the doctor’s office and one line is not much.

We don’t have the data to make a reasonable estimate of the cost per case of blindness prevented. We’ve tried unsuccessfully to find the percentage of those with TT who eventually become blind.

Assuming that it costs ~$20-60 to perform one surgery, and assuming 50% recurrence and that 50% of people with TT become blind, the cost per blindness averted would be $80-240. But, assuming 50% recurrence and that only 5% of people with TT become blind, the cost per blindness averted would be well over $1,000.

Addendum added by Holden: Adding a little more context on the ratio between TT infection and blindness. Estimates of total blindness due to trachoma vary a lot – this PLoS paper (table 3) puts the number around 3 million while this WHO report puts it closer to 1 million in the same year (37 million total people blind worldwide, 3.6% due to trachoma). Assuming constant prevalence of both TT and blindness would imply that TT turns to full-blown blindness 10-30% of the time, which in turn implies (using the 50% “recurrence risk” figure) that there’s 1 case of full-blown blindness averted for every 6-20 successful surgeries.

Surgeries have other benefits too, and with all the layers of uncertainty about prevalence, the 6-20 range could be off by a lot in either direction. It seems safe, though, to agree with the top-line statement that equating surgeries with “blindness prevented” would substantially overstate what you’re getting for your dollar.