By Emma Young

Various visual impairments and abnormalities, such as unusual eye movement patterns, blink rates and retinal problems, are more common than usual in people diagnosed with schizophrenia, suggesting these issues may contribute to the development of the condition. Yet paradoxically, since the 1950s, there have also been intriguing hints that people who are blind from birth or an early age are less likely to develop schizophrenia and other kinds of psychoses, suggesting blindness can act as a protective factor against the illness.

Before now, findings – mostly from case-study type research – suggested that cortical blindness (resulting from abnormalities in the occipital cortex of the brain, rather than the eyes) may even be completely protective. As far as the authors of a new study are aware, not a single case of schizophrenia has ever been reported in someone who is cortically blind.

“Note that most authors are cautious to add that ‘absence of evidence is not evidence of absence’,” Vera Morgan at the University of Western Australia told me. But a total of zero documented cases among such people to date is striking.

Morgan is the lead author of the first large-scale population study investigating this phenomenon, published in Schizophrenia Research, and it provides further evidence that blindness really does reduce – or even eliminate – the risk of developing schizophrenia.

Morgan and her colleagues used Western Australian state-wide registers to estimate the prevalence of cases of schizophrenia among people with congenital and early peripheral or cortical blindness. (“Early” was defined as starting before the age of six.) Cortical blindness can be inherited but it can also be caused by stroke-like events or brain trauma. Among the causes of peripheral blindness are glaucoma and congenital lens malformations.

The data were from a broader project to investigate risk factors for schizophrenia and related illnesses in a group of almost half a million people born in Western Australia between 1980 and 2001 (making them between 14 and 35 years old at the time of the analysis).

Of the entire sample, 9,120 had been diagnosed with some kind a psychotic illness (including schizophrenia), while 1,870 had a diagnosis of schizophrenia specifically. This latter figure is close to the 0.5 per cent prevalence rate in the wider population for people in this age range (given that some of the cohort were still children when the data was analysed, more cases of schizophrenia would be expected to emerge with time.)

There were 66 cases of cortical blindness and 613 of peripheral blindness in the sample. Eight of the people with peripheral blindness had developed a psychotic illness, and just a few had developed schizophrenia. However, among the group with cortical blindness, the figure was zero.

For the peripheral blindness group, this represents a lower-than-normal prevalence rate of 0.2 per cent, while for the cortical blindness group, it was obviously zero.

As congenital or early cortical blindness in particular is rare, even a total group size of almost half a million people might not be enough for clear conclusions about the impacts on schizophrenia risk to be drawn. But as the researchers write, the results do “lend confidence” to findings from previous studies that early blindness reduces the risk of a psychotic illness, and that early cortical blindness is apparently completely protective.

What might explain this protective effect?

Early blindness leads to clear enhancements in other areas of brain functioning – in sound perception, auditory attention, memory, language and the construction of subjective experience – all of which are often found not to be working well in people with schizophrenia, the researchers note. “It appears that early compensatory developmental neuroplasticity … creates advantages to blind people in the very functions that are impaired in schizophrenia,” they write.

Why might cortical rather than peripheral blindness be particularly protective?

Blindness due to eye abnormalities does impact brain areas and pathways normally involved in vision. But the two types of blindness don’t affect the same structures. These differing impacts on connectivity between various brain regions may account for the extra protection afforded by cortical blindness – though quite how is not clear.

The protective phenomenon now “warrants careful, clinical investigation,” the researchers write. Determining the underlying mechanisms will surely shed light on the complex causes of schizophrenia and – as they note – such work may also indicate targets for new early intervention or even prevention strategies, perhaps involving a focus on sensory perceptions.

—Congenital blindness is protective for schizophrenia and other psychotic illness. A whole-population study

Emma Young (@EmmaELYoung) is Staff Writer at BPS Research Digest