Strengths and limitations

Strengths of our study include the use of a large nationally representative sample, separation of near and distance visual impairment and the assessment of multiple potential confounders and mediators of the relationship between sensory impairment and common mental illness.

There are also a number of limitations. First, causation cannot be inferred since the APMS data are cross-sectional and there is no means to determine the temporality of the data. This is particularly relevant to the mediation analysis. Although we have assumed that depression and anxiety do not directly cause sensory impairment, it could be that they reduce motivation to address medical causes of sensory impairment early on and facilitate its development. Stress and mental illness may even predispose to sensory impairment.^{Reference Zheng, Bokman, Lam, Christ, Swenor and West35–Reference Lin, Lin, Liu, Weng, Lin and Lin37} Social isolation might also reduce opportunity to obtain help for sensory impairment. Hence, reverse causation cannot be ruled out in our study, and a longitudinal analysis is needed to confirm our findings.

The CIS-R, despite being a valid measure, gives an indication of symptomatology only and cannot be as accurate as a professional diagnostic interview in ascertaining the presence of common mental illness. Measurement of the exposure was by self-report only, although self-reported sensory impairment has been shown to be a reasonably accurate measure.^{Reference Sindhusake, Mitchell, Smith, Golding, Newall and Hartley38,Reference Whillans and Nazroo39} Measurement of having received a diagnosis and treatment was also limited to self-report data. Some potential confounders were not measured, such as tinnitus.^{Reference Matthews40} This might have led to an overestimation of the association.

The analyses exploring the treatment gap may have included too few participants in each group to have the power to detect a clinically significant effect. This is one possible reason that we found people with sensory impairment to be more likely to have received a diagnosis of mental illness but not treatment. Including people with hearing aids might have selected proportionately more people who are better able to seek healthcare into the exposed group, reducing the likelihood of finding a treatment gap. The data we have used in assessing receipt of diagnosis and treatment may also be prone to recall bias, as it depended on participants recounting events over the past year.^{Reference McManus, Bebbington, Jenkins and Brugha18} Some of the participants would have been living with dementia; they may have had particular difficulties recalling diagnosis and treatment, but are at increased risk of experiencing anxiety and depression.^{Reference Jorm41} This would be expected to artificially inflate the treatment gap for people with sensory impairment; however, this was not seen in our study.

The APMS only covers those living in private households and excludes groups such as prisoners and people in nursing homes. The older population who live in communal or institutional settings are more likely to have worse mental health and dementia than older people who live in private households.^{Reference McManus, Bebbington, Jenkins and Brugha18,Reference Livingston, Barber, Marston, Rapaport, Livingston and Cousins42} As a large proportion of the population with sensory impairment are older this may have led to a slight underestimation of the association between sensory impairment and CIS-R score. It has, however, been estimated that those not living in private households represent less than 2% of the UK population.^{Reference McManus, Bebbington, Jenkins and Brugha18}

It is also possible that people with sensory impairment may have been more prone to exclusion from the APMS than the general population. This may have led to an underestimate of the effect size if the most socially disadvantaged people with sensory impairment could not participate. We did not distinguish between degrees of impairment, whether impairment was well-corrected with aids or recently acquired and life-long impairments. The results between these groups could be different.^{Reference Du Feu and Fergusson29}

Implications

Clinicians working with people with sensory impairments need to be alert to the possibility of comorbid mental illness so that timely interventions can be provided where appropriate. This is especially true in younger adults, for whom the association appears to be more pronounced. Treatment for clinically significant mental illness should be offered when it is detected, regardless of comorbidities, particularly given our finding that people with sensory impairment might be more likely to receive a diagnosis. Depression and anxiety should not be seen as inevitable or untreatable consequences of sensory impairment. Integrating mental and physical healthcare in sensory-impairment specific interventions has been shown to be effective in reducing the incidence of depressive disorders in a low vision clinic.^{Reference Rovner, Casten, Hegel, Massof, Leiby and Ho43} There is also evidence that auditory rehabilitation helps people with hearing impairment to adapt to this condition.^{Reference Blazer and Tucci44} Our findings also highlight the importance of preventing and correcting sensory impairment, which is often possible, to preserve psychological well-being.^{Reference Rishi, Rishi, Maitray, Agarwal, Nair and Gopalakrishnan45} Finally, clinicians should be aware of specialist mental health services for culturally deaf individuals.

It appears that further research into the mediators of common mental illness in people with sensory impairment is also required. Social functioning may be a potentially modifiable target for the prevention of mental illness in people with sensory impairment. Sensory impairment severe enough to interfere with social functioning in particular should alert clinicians to the need to consider the impact on the patient's mental health.