The first key finding is that the routine data set underestimated the true prevalence of sight impairment – with just 7% of the population with sight impairment in 2016 being known to the register. The underestimation of blindness by CVI figures in England and Wales is something that has received considerable attention to date [7, 11,12,13]. A recent publication by Olvera-Barrios et al. suggested that under-registration might be as high as 84% in patients with diabetic retinopathy at the largest center for ophthalmic treatment in England [14]. Closer inspection of this paper and its data suggests that this figure is likely an over-estimate – since certification is not typically offered as the point of care and the researchers may not have fully factored in the challenge with fluctuating vision in patients with treatable eye conditions such as diabetes. Bourkiza et al. comment on this in their paper which looked specifically at certification in people with diabetes in East London stating that “a single measure of vision should never to used to assess certification” [15].

Figure 1a, b illustrates why CVI figures (TTBWA) will underestimate blindness.

a CVI rates do not equate to blindness rates. b CVI – tip of the low vision iceberg.

Teoh et al. recently reported on “Temporal changes in the epidemiology of childhood severe visual impairment and blindness in the UK” [16]. To do this, comparison was made between findings from two national population-based epidemiological studies of incident childhood severe visual impairment and blindness in the UK [17, 18]. The studies used identical methods – one was conducted in 2000 and the second in 2015. Key findings were that the overall annual and cumulative incidence rates remained broadly stable and mortality in children diagnosed in infancy declined. Together these pieces of information might lead us to deduce that the prevalence of childhood blindness may have increased – which were findings from analysis of registration and certification data [19, 20].

If incidence stays the same but mortality decreases, prevalence may increase unless recovery improves (Fig. 2). The incidence of blindness in children has remained constant, mortality has decreased, there has been little change in recovery in the particular conditions that impact in children and so prevalence may have increased [21].

The Epidemiologist’s Bathtub.