Recent Classification of Hearing Loss in Adults
Severity Classification by Decibel Thresholds
The American College of Medical Genetics and Genomics (ACMG) 2022 guideline establishes the most authoritative current classification system, defining hearing loss severity across six distinct categories based on pure tone average thresholds. 1, 2
The ACMG classification defines:
- Slight hearing loss: 16-25 dB 1, 2
- Mild hearing loss: 26-40 dB 1, 2
- Moderate hearing loss: 41-55 dB 1, 2
- Moderately severe hearing loss: 56-70 dB 1, 2
- Severe hearing loss: 71-90 dB 1, 2
- Profound hearing loss: ≥91 dB 1, 2
Normal hearing is defined as ≤20 dB HL. 2
Key Clinical Application Points
- Apply the classification to the better-hearing ear to accurately reflect functional hearing status, not the worse ear. 2
- The ACMG system is more granular than older three-tier systems, particularly in distinguishing "slight" hearing loss (16-25 dB) as a separate category and separating "moderately severe" (56-70 dB) from "severe" (71-90 dB), which provides more precise staging for treatment decisions and genetic counseling. 2
- Slight hearing loss (16-25 dB) may go unrecognized by patients but still impacts communication in challenging listening environments. 2
Alternative Classification Systems
The American Academy of Pediatrics uses a simplified three-tier system that omits the "slight" and "moderately severe" categories, defining only mild (20-40 dB), moderate (40-70 dB), and severe (70-95 dB) hearing loss. 2 However, the ACMG six-category system should be prioritized as it provides superior clinical granularity for adult hearing loss management. 1, 2
Etiologic Classification
Age-Related Hearing Loss (Presbycusis)
The American Academy of Otolaryngology-Head and Neck Surgery 2024 guideline defines age-related hearing loss as progressive bilateral sensorineural hearing loss associated with aging in persons ≥50 years old. 1
Presbycusis characteristics include:
- Bilateral and symmetric presentation (audiometric results within 15 dB for pure tone average between ears, with word recognition score difference ≤15% between ears). 1
- Gradual and progressive course, as opposed to sudden-onset hearing loss. 1, 3
- High-frequency hearing loss pattern that increases with age, rising more rapidly in men than women. 1
- Primary pathology is hair cell degeneration starting at the basal turn of the cochlea, rather than strial atrophy as previously thought. 1
Conductive Hearing Loss
Conductive hearing loss occurs when pathology in the external auditory canal or middle ear blocks sound conduction to the inner ear. 3 Common causes include cerumen impaction, foreign bodies, tympanic membrane perforation, middle ear effusion, cholesteatoma, ossicular discontinuity/fixation, and otosclerosis. 3 The prognosis is generally good with appropriate intervention, as many causes are medically or surgically correctable. 3
Sensorineural Hearing Loss
Sensorineural hearing loss involves dysfunction of the inner ear, cochlear nerve, or central auditory pathways, resulting from damage to hair cells in the cochlea, auditory nerve, or central auditory centers. 3 Beyond presbycusis, this includes noise-induced hearing loss, ototoxic medication exposure, genetic causes (particularly GJB2/connexin 26 mutations accounting for 15-40% of cases), and mitochondrial hearing loss. 1
Mixed Hearing Loss
Mixed hearing loss combines elements of both conductive and sensorineural hearing loss, involving dysfunction in both the sound conduction pathway and the sensorineural components. 3
Critical Clinical Distinctions
Asymmetric vs. Symmetric Hearing Loss
Asymmetric sensorineural hearing loss requires immediate specialized evaluation as it may indicate serious underlying pathology such as vestibular schwannoma or meningioma. 3 Patients with significant asymmetric hearing loss should be referred urgently to otolaryngology. 3
Hidden Hearing Loss (Cochlear Synaptopathy)
Recent research identifies a phenomenon where moderate noise exposure causes irreversible degeneration of auditory nerve synapses without affecting outer hair cells or absolute hearing thresholds. 1 This "hidden hearing loss" manifests as difficulty understanding speech in noise despite normal audiograms, particularly affecting 5-15% of adults seeking audiologic help with normal hearing thresholds. 1
Epidemiologic Context
Age-related hearing loss affects 1 in 3 adults aged 65-74 and nearly 50% of those ≥75 years. 1 The prevalence doubles with each decade of life, affecting more than 60% by age 70 and 80% by age 85. 1 African Americans have lower risk compared to white and Hispanic populations, possibly due to protective melanin expression in the stria vascularis. 1