“Is Early Blindness Protective of Psychosis or Are We Turning a Blind Eye to the Lack of Statistical Power?”, 2020-03-31 ():
…we performed a nationwide population register-based study based on data from the Danish National Patient Register (containing diagnostic information regarding blindness) and the Danish Psychiatric Central Research Register (containing diagnostic information regarding mental disorders) to examine whether early blindness is protective of development of psychotic disorders.
…In the birth cohort consisting of 2,500 332 individuals followed for a total of 47.5 million person-years, we identified 460 cases of early blindness, 10 440 cases of schizophrenia, and 19 716 cases of psychotic disorder. Based on the 8,100 person-years of follow-up for the individuals with early blindness (and the incidence rates of the mental disorders reported above), we would expect to find 1–2 cases of schizophrenia and 3–4 cases of psychotic disorder if there was no protective effect of early blindness. We found <5 with schizophrenia and <5 with any psychotic illness (numbers <5 are not publishable according to Danish legislation—due to the risk of identification of individuals). This means that we can neither confirm nor refute the hypothesis that early blindness is protective of psychosis, due to insufficient statistical power.
To achieve sufficient power, we would need an even larger cohort and/or longer follow-up time—but how much larger/longer? We estimated the required sample size, depending on how protective early blindness could be. Specifically, based on nationwide Danish register data from Pedersen et al, we assumed a lifetime cumulative incidence rate for schizophrenia of ~2% (Figure 1, green line), and ~4% for psychotic disorders (Figure 1, blue line). If early blindness (estimated cumulative incidence = 460/2.500.332 = 0.000184) was to completely abolish the risk of schizophrenia (hazard ratio ~0.00), statistically-significant results would require a cohort of ~3,000 000 individuals long enough to reach the assumed cumulative incidence, and if early blindness would only halve the risk (hazard ratio ~0.5), a cohort of at least 11,000 000 individuals would be required. Substantially larger cohorts would be required to investigate a protective effect of early cortical blindness, specifically, as this is a much more rare condition than early blindness in general. Such numbers could potentially be achieved by combining data from several nationwide registers with long follow-up.
In conclusion, the results of this study, which is by far the largest of its kind with its follow-up time of 47.5 million person-years, show that the hypothesis of early blindness being protective of psychosis is currently untestable using Danish register data.
[Keywords: schizophrenia, blindness, vision, cortical blindness, congenital blindness, psychotic disorders]