2Measuring the burden — grades, prevalence & DALYs

FWhy definitions come first

It is tempting to skip past definitions to the dramatic figures, but the figures are the definitions. When one source says 466 million people have disabling hearing loss and another says 1.5 billion have hearing loss, they are not contradicting each other — they are using different cut-offs. A clinician who cannot say which definition a number rests on cannot use the number safely. So we begin with the measuring instruments.

FTGrading severity

Hearing is graded by the quietest sound a person can detect — their threshold— averaged across the speech frequencies and expressed in decibels of hearing level (dB HL). The higher the threshold, the worse the hearing. The WHO World Report on Hearing sets out a ladder of grades from normal through mild, moderate, moderately severe, severe and profound, to complete loss. Each grade carries an everyday meaning — the difference between struggling with a whisper and being unable to hear a shout.[2021]

These bands are where cochlear implantation lives: candidacy has traditionally centred on the severe-to-profound range, where hearing aids can no longer deliver usable speech. Knowing the grade boundaries is therefore not just an epidemiological nicety — it is the first filter that decides who even enters the implant pathway.

TThe better ear, and the disabling threshold

Population figures are almost always quoted for the better-hearing ear, because that is the ear that governs everyday function. The WHO counts loss as disablingwhen the better ear's threshold reaches a defined level — about 35 dB in adults. The bar is set lower for children, because a developing child needs better hearing than an adult to acquire spoken language; a degree of loss an adult would shrug off can derail a child's speech. This single asymmetry explains much of why paediatric programmes intervene at thresholds that would not trigger action in an adult.

FTPrevalence vs incidence

Two different questions hide behind the word “how many.” Prevalence is how many people have hearing loss at a point in time — the stock. Incidence is how many people newly acquire it over a period — the flow. They answer different needs: prevalence tells a health system how many hearing aids, implants and audiologists it needs now; incidence tells it whether prevention is working and how fast the problem is growing.

For a congenital condition, a particularly useful flow measure is the birth prevalence — cases per thousand live births. The familiar figure that permanent childhood hearing loss affects on the order of one to two per thousand newborns is a birth-prevalence statement, and it is the number that justifies universal newborn screening (Module 9).

TCPutting a number on disability

Hearing loss rarely kills, so a measure that counts only deaths makes it disappear. The breakthrough was the disability-adjusted life year (DALY) and its component the year lived with disability (YLD), which assign a weight to time spent in less than full health. With these instruments, the chronic burden of a non-fatal condition can finally be compared on the same axis as cancer or heart disease.

When that comparison is made, hearing loss is revealed to be the third-largest cause of years lived with disability in the world — ahead of conditions that command far more attention and funding. That single re-ranking, more than any prevalence headline, is what put hearing on the global health agenda.[2021]

FWhy figures disagree

Armed with these tools, the apparent contradictions between sources dissolve. Numbers differ because they use different severity cut-offs (any loss vs disabling loss), measure the better ear or both, count adults, children or all ages, and rely on self-report or audiometry — and self-reported surveys systematically under-count mild loss. The right response to a hearing-loss statistic is never to accept or reject it, but to ask which of these choices it embodies.

With the measuring instruments in hand, we can now read the figures honestly — beginning with the burden closest to home: the scale of hearing loss in India (Module 3).