I’m mildly skeptical that blindness prevents schizophrenia

(Low confidence, written in a hurry.)

I was (randomly) trying to make sense of schizophrenia the other day, and there’s a piece of the puzzle that just seems not to fit with everything else: namely, the claim that being congenitally blind (or becoming blind in the first 5-6 years of life) prevents schizophrenia.

Random example of the claim that I’m disputing. You can find it all over the place.

If that’s true, then, as the saying goes, “I notice that I am confused”. In terms of how schizophrenia seems to work at a low level (note that I’m very much not an expert), I can’t currently make any sense of how congenital blindness would prevent schizophrenia from developing. There are papers that discuss this topic and propose explanations, but they all seem to be grasping at straws.

So purely on priors, I find myself skeptical of whether early blindness actually reduces the risk of schizophrenia. So I spent a couple hours looking into it this weekend. Here’s what I found out, or you can just skip to the conclusion at the bottom.

Claims & Evidence

The claim is often stated as: “there are no reported cases of people with schizophrenia who were born blind or who developed blindness shortly after birth”. This claim seems to be false. There was one person that turned up in Morgan et al. 2018 (which I’ll return to below), and better yet, Leivada & Boeckx 2014 dug up a handful of case reports—see that paper for details.

So then Leivada & Boeckx 2014 narrowed the hypothesis to: congenital cortical blindness is protective against schizophrenia, congenital peripheral blindness is not. (Terminology in footnote.^[1]) That seems like an odd and unjustified direction for people to have pivoted. The problem is: congenital cortical blindness is even more uncommon than congenital blindness overall—that paper says 18% of congenitally blind people are cortically blind, while the data in Morgan et al. 2018 implies that it’s ≈10%. If the authors only had a handful of total reports of congenitally-blind schizophrenics in the first place, and if only 1 out of 5-10 congenitally-blind people are more specifically cortically-blind, then we really shouldn’t be shocked that none of that handful of documented cases is cortically-blind. That seems like the kind of thing that could easily happen by coincidence.

OK, leaving aside the false “it has never happened” claim, and the unjustified “cortical blindness” topic, we can retreat to a weaker form of the hypothesis: comorbidity of schizophrenia and congenital-blindness has happened, but it happens much less often than if they were statistically independent. Do we have evidence on this claim?

The best kind of evidence would be a population-wide systematic study. Unfortunately, the null hypothesis (schizophrenia and congenital-blindness are independent) has them co-occurring at a rate of 0.72% × 0.03% = 2 in a million. So you need a really big population to get good evidence on this hypothesis. Morgan et al. 2018 grabbed data from Western Australia, but it turned out (in my assessment) that they didn’t have enough data to come to a conclusion one way or the other.^[2] Jefsen et al. 2020 grabbed Danish data and found the same (non) conclusion.

Well, so much for that plan! Alternatively, we can look at how many published case reports of congenitally-blind schizophrenics there are, and compare it to how many we would have normally expected. If there are many fewer than expected, that’s evidence that blindness protects against schizophrenia. See Silverstein et al. 2013 for a nicely-laid-out argument that this is the case.

Again, if schizophrenia and congenital-blindness were independent, something like 0.72% × 0.03% = 2 in a million people would have it, i.e. 650 people in the USA. But the number of case reports is (they say, although see above) 0. I’ll admit it: 650 is a lot more than zero. But not everyone with schizophrenia gets properly diagnosed. (I think the 0.72% is total incidence, not how many people are diagnosed, but I could be misreading, here’s the source.) And there’s a much much bigger problem: not every psychiatrist, when treating a blind schizophrenic person, will publish a case report about them! Y’know, people are busy! Jeez!!

So, what’s the probability that any given blind schizophrenic patient will wind up immortalized in the literature as a case report? “Much less than 1 in 620” does not immediately strike me as implausible. I’m not an expert, of course. I don’t know how easy and routine it is for psychiatrists & psychologists to publish case reports. Maybe every neighborhood therapist is just publishing case reports left and right, whenever they have a spare moment. Beats me! I tried to look up how many psychology case reports get published per year, but couldn’t easily figure it out, please comment if you know.

What about unpublished anecdotes? Well, as soon as I looked, I did in fact quickly find a couple people on the internet offering anecdotal reports of congenitally-blind schizophrenics—quora example, reddit example. Granted: people sometimes lie on the internet! But for what it’s worth, the quora person claims to be a “DeafBlind Tech Trainer, ASL Interpreter, CVI Consultant” and claims to have met at least three blind schizophrenics “including congenitally blind people”, and this person has a bunch of other quora posts that seem pretty good and serious and consistent with their claimed identity—it’s not just a trolling account, or if it is, it’s an unusually high-effort trolling account!

Conclusion

My tentative conclusion is that, if I have a good way to think about schizophrenia, and everything hangs together well, except that it can’t explain why congenital / early blindness is protective against schizophrenia, I think I should stick with the theory and declare that congenital / early blindness is not actually protective against schizophrenia after all!

To be perfectly clear, if I had two equally-good-in-every-way theories of schizophrenia, and Theory A predicted that congenital blindness was somewhat protective against schizophrenia, and Theory B didn’t, I would consider that a point in favor of Theory A. It would just be a pretty small point in favor of Theory A.

^
If I understand correctly, “cortical blindness” means that you’re blind because there’s something wrong with your occipital lobe, and “peripheral blindness” means you’re blind because there’s something wrong with your eye, retina, etc.
^
This is my assessment. The authors definitely didn’t put it that way. As far as I can tell, the authors did no statistical analysis whatsoever; the only nod to statistics is a throwaway comment that their study is “possibly underpowered” (!). OK fine, let me get out my calculator. If I’m doing the math right, they expected to find 2.5 people in the early-peripherally-blind schizophrenia group and actually found 1, which is a p-value of 0.29; and they expected 0.264 people in the early-cortically-blind schizophrenia group and actually found 0, which is p-value of 0.77. If that’s right, then consider me underwhelmed! But please someone double-check my math!