725Bull World Health Organ 2019;97:725–728 | doi: http://dx.doi.org/10.2471/BLT.19.230367

Perspectives

Hearing impairment negatively affects well-being and is a major contributor to years lived with disability.1,2 The World Health Organization (WHO) estimates that 466 million people were living with disabling hearing impairment in 2018 and this estimate is projected to rise to 630 million by 2030 and to over 900 mil-lion by 2050.3 However, these projec-tions are based on a hearing impairment classification that does not fully reflect the provisions of the International classification of functioning, disability and health for assessing all forms of functional impairments.4 Here we make the case for a review of the concept of disabling hearing loss adopted by WHO after the recommendation of the Global Burden of Disease (GBD) Expert Group on Hearing Impairment in 2008.

The need for an independent clas-sification system for all impairments and disabilities as a complement to the well-established International statisti-cal classification of diseases and related health problems, was first suggested in 1976 by the World Health Assembly. As a result, in 1980 WHO developed the International classification of impair-ments, disabilities and handicaps.5 One of the key features of this system was the use of qualifiers such as mild, moderate, severe and profound to distinguish vari-ous levels of observed or measured de-viations outside of the range considered for normal functioning for any health condition. This categorization has been reinforced in the subsequent revisions to the system, such as the International Classification of Functioning, Disability and Health, and accompanied with de-scriptions of typical problems encoun-tered in daily activities at various levels of severity.4 The classification, notably, does not use the term disabling, as it recognizes the needs of all persons with functional impairments for appropriate intervention.

WHO’s first classification for hear-ing impairment dates to 1986 and is based on the recommendations of an expert group established by WHO spe-cifically for this purpose; the classifica-tion has been modified several times since then. The current one is shown in Table 1; it is based on the version published in 1991, which remarked that persons with average pure-tone audi-ometry of 15–20 decibels (dB) hearing level may experience hearing problems, and those with unilateral hearing losses may experience hearing problems even if the better ear was normal.6 However, currently only adults (≥ 15 years) with a permanent unaided hearing impairment above 40 dB hearing level in the better ear and children (from birth to 14 years of age) with > 30 dB hearing level are regarded as having a disabling hearing impairment.3 This classification raised three major concerns. First, the pre-scribed threshold for normal hearing of 25 dB hearing level is not in agreement with several reports in the literature on the functional experience of persons with slight or mild hearing impairment (< 25 dB hearing level). Second, there is no scientific or rational basis for the uneven steps between the various grades of severity. Third, and more crucially, the definition of disabling hearing impair-ment excludes all persons with unilateral hearing impairment of any severity and those with mild bilateral hearing impair-ment, which is not consistent with the International classification of function-ing, disability and health.

In 2008, the GBD Expert Group on Hearing Loss addressed these concerns by reviewing the WHO classification for hearing impairment and other data inputs for the GBD study 2010.7 This assignment resulted in a proposal for a revised classification (Table 2). The limit for normal hearing was re-duced from 25 to 20 dB hearing level, a

separate category for unilateral hearing impairment was introduced and the six categories for bilateral hearing impair-ment were differentiated consistently by 15-dB steps. The decision to reduce the limit for normal hearing was informed by the extensive clinical experience of this expert group as well as available evidence in the literature at the time. While threshold of normal hearing set at 15 dB could be found in the literature, often resulting in an additional category of slight impairment (16–25 dB hearing level),8 the expert group opted not to deviate from the International classifica-tion of functioning, disability and health lowest generic qualifier of mild impair-ment, but reduced the lower limit from 25 to 20 dB hearing level. This decision was reinforced by the fact that persons with hearing sensitivity < 20 dB rarely benefit from or require amplification devices, and was in agreement with the remarks by the WHO expert group in 1991.6 For example, one survey showed that only 2.7% of a total of 556 026 hear-ing aids dispensed over an undisclosed period were for persons with < 20 dB hearing loss.8 Perhaps, most notable was the predominant use of this threshold in sweep test audiometry, especially in school-aged children.9

A separate category for unilateral hearing impairment was introduced in line with the extensive literature on the functional, educational, psychological or social impact of this type of hearing impairment in all age groups.10 This category also reflected the International classification of functioning, disability and health specific provisions for hear-ing problems associated with localiza-tion and lateralization, particularly in difficult listening situations. The GBD expert group proposed six categories for bilateral hearing impairment, differenti-ated consistently by 15-dB steps, since scientific or clinical evidence was lack-

Hearing loss grades and the International classification of functioning, disability and healthBolajoko O Olusanya,a Adrian C Davisb & Howard J Hoffmanc

a Centre for Healthy Start Initiative, 286A Corporation Drive, Dolphin Estate, Ikoyi, Lagos, Nigeria.b The Ear Institute, University College London, London, England.c Division of Scientific Programs, National Institute on Deafness and Other Communication Disorders, Bethesda, United States of America.Correspondence to Bolajoko O Olusanya (email: bolajoko.olusanya@uclmail.net).(Submitted: 20 January 2019 – Revised version received: 11 April 2019 – Accepted: 13 April 2019 – Published online: 3 September 2019 )

726 Bull World Health Organ 2019;97:725–728| doi: http://dx.doi.org/10.2471/BLT.19.230367

PerspectivesInternational classification for hearing loss Bolajoko O Olusanya et al.

Table 1. WHO’s Grades of hearing impairment

Grade of impair-ment

Corresponding audiometric ISO valuea,b

Performance Recommendations Comments added to the previous classification

0: no impairment 25 dB or better No or very slight hearing problems. Able to hear whispers

None 20 dB also recommended. People with 15 – 20 dB levels may experience hearing problems. People with unilateral hearing losses may experience hearing problems even if better ear normal

1: slight impairment

26–40 dB Able to hear and repeat words spoken in normal voice at 1 m

Counselling. Hearing aids may be needed

Some difficulty in hearing but can usually hear normal level of conversation

2: moderate impairment

41–60 dB Able to hear and repeat words using raised voice at 1 m

Hearing aids usually recommended

None

3: severe impairment

61–80 dB Able to hear some words when shouted into better ear

Hearing aids needed. If no hearing aids available, lip-reading should be taught

Discrepancies between pure-tone thresholds and speech discrimination score should be noted

4: profound impairment including deafness

81 dB or greater Unable to hear and understand even a shouted voice

Hearing aids may help in understanding words. Additional rehabilitation needed. Lip-reading and sometimes signing essential

Spoken speech distorted, the degree depending on the age at which hearing was lost

dB: decibel; Hz: Hertz; ISO: International Organization for Standardization; m: meter; WHO: World Health Organization.a In the better ear.b Average of 500, 1000, 2000 and 4000 Hz.

Notes: Disabling hearing loss refers to hearing loss greater than 40 dB in the better hearing ear in adults (Grades 2, 3 and 4) and greater than 30 dB in the better hearing ear in children.Source: WHO.6

Table 2. Grades of hearing impairment as recommended by the Global Burden of Disease Expert Group on Hearing Loss

Category Pure-tone audiometrya,b Hearing experience in a quiet environment

Hearing experience in a noisy environment

Normal hearing −10.0 to 4.9 dB hearing level Excellent hearing Good hearing5.0 to 19.9 dB hearing level Good hearing Rarely have difficulty in following/

taking part in a conversationMild hearing loss 20.0 to 34.9 dB hearing level Does not have problems hearing

what is saidMay have real difficulty following/taking part in a conversation

Moderate hearing loss 35.0 to 49.9 dB hearing level May have difficulty hearing a normal voice

Has difficulty hearing and taking part in conversation

Moderately severe hearing loss

50.0 to 64.9 dB hearing level Can hear loud speech Has great difficulty hearing and taking part in conversation

Severe hearing loss 65.0 to 79.9 dB hearing level Can hear loud speech directly in one’s ear

Has very great difficulty hearing and taking part in conversation

Profound hearing loss 80.0 to 94.9 dB hearing level Has great difficult hearing Cannot hear any speechComplete or total hearing loss

95.0 dB hearing level or greater Profoundly deaf, hears no speech or loud sounds

Cannot hear any speech or sound

Unilateral < 20.0 dB hearing level in the better ear, 35.0 dB hearing level or greater in the worse ear

Does not have problems unless sound is near poorer hearing ear

May have real difficulty following/taking part in a conversation

dB: decibel; Hz:Hertza In the better ear.b Average of 500, 1000, 2000 and 4000 Hz.

Source: Global Burden of Disease Expert Group on Hearing Loss.7

727Bull World Health Organ 2019;97:725–728| doi: http://dx.doi.org/10.2471/BLT.19.230367

PerspectivesInternational classification for hearing lossBolajoko O Olusanya et al.

ing to support the variable steps between the grades in the WHO classifications. The choice of 15-dB was intended to reflect the minimum shift in pure-tone audiometry thresholds that is typically considered as clinically and functionally significant, especially in occupational noise surveillance.11

The concept of disabling hearing impairment also needed to be appro-priately expanded to include both uni-lateral and bilateral hearing impairment outside the range for normal hearing. This was to ensure that the persons with such impairment were not discrimi-nated against in accessing appropriate interventions. Calibrating the degree of difficulty in percentage terms across the various categories as suggested in the International classification of func-tioning, disability and health is difficult. However, by definition, a condition is said to be disabling if it causes someone to have an illness, injury or condition that makes it difficult to do the things that other people do. As a result of the expanded concept of disabling hearing impairment, it was no longer critical to have a separate classification for children (Table 1).

However, the term disabling is nei-ther recommended by International clas-sification of functioning, disability and health nor routinely applied for other impairments. Therefore, discontinuing such description might be more appro-priate from a practical, human rights and equity perspective. Conceiving an impairment or disability that is not disabling in all contextual factors is chal-lenging. However, discontinuing the use of the term should not prevent a public health policy that prioritizes those for whom hearing devices (hearing aids and cochlear implants) would provide

more benefit, especially in low-resource settings. The mere recognition of those with mild hearing impairment could be advantageous, especially for school children who may benefit from simple but effective intervention such as pref-erential seating in the classroom. Such recognition would also help parents and carers to have better understanding of the communication challenges children with mild hearing impairment face in their daily life.

Classifications based solely on pure-tone audiometry have limitations.8 For instance, while measurement of a behavioural pure-tone audiometry tests the entire auditory pathway, it does not localize to any one segment along the auditory pathway. Hence, this measurement does not provide specific information on the status of the central auditory nervous system and, therefore, may offer only limited insight into au-ditory function in real-world settings. Therefore, persons with normal pure-tone audiometry thresholds can report having significant difficulties in hear-ing. Concerns also exist regarding the various qualifiers attached to pure-tone audiometry thresholds such as normal hearing or mild hearing impairment, as these qualifiers do not always preclude some degree of functional impairments.8 As a result, the proposed pure-tone au-diometry categories should be comple-mented with descriptions of functional performance that are not offensive or derogatory in any given cultural context and that reflect activity limitation or participation restriction, particularly in quiet and noisy environments. These categories should also include possible indications for rehabilitation with assis-tive devices. Moreover, the International classification of functioning, disability

and health allows that, wherever pos-sible, the person whose level of function-ing is being classified (or the person’s advocate) should have the opportunity to participate and to challenge or con-firm the appropriateness of the category being used, and the assessment assigned.

The proposed classification was first reported in the 2010 GBD study to generate prevalence estimates and dis-ability weights for hearing impairment, and has been adopted in all subsequent GBD publications.2 A recent study spe-cifically investigated the extent to which the revised pure-tone audiometry classi-fication correlated with communication deficits based on functional measures of speech communication in the adult population.12 The analysis showed good validity for the proposed classification system, based on the evidence from relatively large population and clinical studies. The analysis also established sig-nificant changes in functional commu-nication as the classification progresses from slight/mild through severe grades.

We recommend that WHO consid-ers adopting this revised classification to ensure that future WHO estimates of hearing impairment are aligned with those published periodically in the GBD study and other epidemiological data. Doing so would also ensure that no persons with functional hearing impairment, regardless of the severity, are unduly placed at a disadvantage compared to those with other impair-ments within the framework of the In-ternational classification of functioning, disability and health, especially under the disability-inclusive global agenda of the sustainable development goals. ■

Competing interests: None declared.

References1. Olusanya BO, Neumann KJ, Saunders JE. The global burden of disabling

hearing impairment: a call to action. Bull World Health Organ. 2014 May 1;92(5):367–73. doi: http://dx.doi.org/10.2471/BLT.13.128728 PMID: 24839326

2. James SL, Abate D, Abate KH, Abay SM, Abbafati C, Abbasi N, et al.; GBD 2017 Disease and Injury Incidence and Prevalence Collaborators. Global, regional, and national incidence, prevalence, and years lived with disability for 354 diseases and injuries for 195 countries and territories, 1990-2017: a systematic analysis for the Global Burden of Disease Study 2017. Lancet. 2018 11 10;392(10159):1789–858. doi: http://dx.doi.org/10.1016/S0140-6736(18)32279-7 PMID: 30496104

3. Addressing the rising prevalence of hearing loss. Geneva: World Health Organization; 2018. Available from: http://www.who.int/pbd/deafness/estimates/en/ [cited 2019 Apr 1].

4. International classification of functioning, disability and health. Geneva: World Health Organization; 2001. Available from: https://www.who.int/classifications/icf/en/ [cited 2019 Apr 1].

5. International classification of impairments, disabilities and handicaps (ICIDH): a manual of classification relating to the consequences of diseases. Geneva: World Health Organization; 1980. Available from: https://apps.who.int/iris/bitstream/handle/10665/41003/9241541261_eng.pdf?sequence=1 [cited 2019 Apr 1].

6. Report of the informal working group on prevention of deafness and hearing impairment programme planning, Geneva, 18-21 June 1991. Geneva: World Health Organization; 1991. Available from: http://www.who.int/iris/handle/10665/58839 [cited 2019 Apr 1].

728 Bull World Health Organ 2019;97:725–728| doi: http://dx.doi.org/10.2471/BLT.19.230367

PerspectivesInternational classification for hearing loss Bolajoko O Olusanya et al.

7. Stevens G, Flaxman S, Brunskill E, Mascarenhas M, Mathers CD, Finucane M; Global Burden of Disease Hearing Loss Expert Group. Global and regional hearing impairment prevalence: an analysis of 42 studies in 29 countries. Eur J Public Health. 2013 Feb;23(1):146–52. doi: http://dx.doi.org/10.1093/eurpub/ckr176 PMID: 22197756

8. Clark JG. Uses and abuses of hearing loss classification. ASHA. 1981 Jul;23(7):493–500. PMID: 7052898

9. Bamford J, Fortnum H, Bristow K, Smith J, Vamvakas G, Davies L, et al. Current practice, accuracy, effectiveness and cost-effectiveness of the school entry hearing screen. Health Technol Assess. 2007 Aug;11(32):1–168, iii–iv. doi: http://dx.doi.org/10.3310/hta11320 PMID: 17683682

10. Tharpe AM. Unilateral and mild bilateral hearing loss in children: past and current perspectives. Trends Amplif. 2008 Mar;12(1):7–15. doi: http://dx.doi.org/10.1177/1084713807304668 PMID: 18270174

11. National Institute of Occupational Safety and Health (NIOSH). publication 98-126: Available at: http://www.cdc.gov/niosh/docs/98-126/pdfs/98-126.pdf [cited 2019 April 1].

12. Humes LE. The World Health Organization’s hearing-impairment grading system: an evaluation for unaided communication in age-related hearing loss. Int J Audiol. 2019 Jan;58(1):12–20. doi: http://dx.doi.org/10.1080/14992027.2018.1518598 PMID: 30318941

CorrigendumIn: Health workforce burn out. Bull World Health Organ. 2019 Sep 1;97(9):585–86

on page 479, third column, the first sentence should read as follows:

“…(Canada), with Australia and Ireland reporting proportions comparable to those in Canada.”