Alayne Morrel

Background:  It has been noted in several psychiatric studies that there is an absence of reported patients with schizophrenia who had comorbid blindness.¹⁻⁶  This spurred multiple studies ranging from surveying state hospitals to studies encompassing entire populations, such as Australia and Denmark.⁴ ⁷ ⁸ If a correlation exists, it would change our understanding of the pathophysiology of schizophrenia.

Methods: The primary data sources used were Google Scholar and the Texas A&M library database. Search terms included, but were not limited to, “schizophrenia and blindness” and “psychosis and blindness.”

Results:  An Australian study performed a survey of ~500,000 state records and found none of the 66 children with cortical blindness developed schizophrenia or any other psychotic condition.⁷ A group in Denmark surveyed the national health registers, and based on the 8100 person-years of follow-up for the individuals with early blindness, expected to find 1-2 cases of schizophrenia and 3-4 cases of psychotic disorder if there was no protective effect of early blindness. Their search returned “<5” with schizophrenia and “<5” with any psychotic illness, a result the national health register returns if the numbers are too few (to protect their privacy).⁸ A rodent study treated pregnant mice with MAM (methylazoxymethanol acetate) to simulate schizophrenia in the pups, then performed bilateral enucleation.The rats displayed elevated dopamine neuron population activity, deficits in prepulse inhibition of startle, and hypersensitivity to psychomotor stimulants.⁹

Conclusions:  Neither the Australian nor Danish studies found strong evidence of comorbid congenital blindness and schizophrenia, but both were underpowered. The Danish team estimated a sample size of 3 million would be needed to confirm complete protection, and 11 million to confer a 50% reduction in risk.⁸ Importantly, the type and timing of blindness seem critical.¹⁰ Reported protection seems limited to early, especially cortical, blindness; acquired blindness does not show this effect.

Proposed mechanisms focus on visual processing and cortical reorganization. One theory uses Bayesian prediction error minimization, suggesting the absence of visual input leads to more stable internal models and reduced sensory conflict, lowering psychosis risk.¹¹ Another model suggests early blindness leads to “cognitive strengthening,” or reallocating brain resources to executive functions, and “cognitive constraint,” or limiting excessive neuroplasticity associated with schizophrenia.¹²  Future research must use larger cohorts and delineate the types of blindness conferring protection. If a protective effect exists, it could open new avenues for understanding schizophrenia and developing early interventions.

Works Cited

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9. Perez SM, Elam HB, McCoy AM, Boley AM, Eassa NE, Lodge DJ. Congenital blindness does not protect against a schizophrenia-related phenotype in rodents. Schizophrenia Research. 2023;258:1-8. doi:https://doi.org/10.1016/j.schres.2023.06.007
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