Table 2. Synopsis of the causes and clinical features of hearing impairment, with differential diagnoses for each hearing impairment syndrome.

	Conductive hearing loss	Sensory hearing loss	Neural hearing loss	Central hearing loss
Cause	acoustic-mechanical disturbance of sound conduction in the external auditory canal, across the tympanic membrane, or in the ossicular chain	dysfunction of the hair cells or their synaptic connections to the cochlear nerve; if the outer hair cells are affected, loss of cochlear amplification and thus of recruitment of intermediate intensities blurring of frequency resolution reduction of temporal resolution	cochlear nerve dysfunction delayed impulse conduction disturbed neural encoding of the acoustic signal	dysfunction of the audtiory pathway or auditory cortex (processing of bilateral auditory stimuli, synchronization, signal modulation, recognition, noise suppression)
Clinical features	If the cause is in the external auditory canal: reduced sound intensity (sound is perceived as soft) If the cause is in the tympanic membrane or ossicular chain: altered sound frequency and intensity (high and low tones may be either softer or louder)	loss of intensity and dynamics soft noises or speech may be perceived as either too soft or too loud often, distorted perception	similar to sensory hearing loss, but usually unilateral speech perception worse than tone perception	there may be no disturbance of tone perception impaired rapid speech processing impairment of sound localization, poor understanding of speech with superimposed noise, impairment of auditory memory
Differential diagnosis	Acute: blockage by cerumen tubular catarrh tympanic effusion traumatic eardrum perforation acute otitis media or externa Permanent: canal stenosis/atresia defect of eardrum or ossicular chain due to chronic purulent infection of the mucosa cholesteatoma malformation otosclerosis tympanosclerosis	Acute: idiopathic sudden sensorineural hearing loss acute noise-induced trauma blast trauma explosion trauma bacterial/viral labyrinthitis Hereditary/permanent:: hereditary hearing impairment presbycusis noise-induced hearing impairment toxic (incl, drug-induced) hearing impairment idiopathic chronic progressive hearing impairment drug side effects lasting sequelae of infections and sudden hearing loss	acoustic neuroma (= vestibular schwannoma) other tumors of the petrous bone or cerebellopontine angle (meningioma, chordoma, chondrosarcoma) compression syndrome	infarction hemorrhage tumor multiple sclerosis auditory processing disorder
Audiological testing	tuning-fork test whispering test test of hearing at a distance pure-tone audiogram impedance audiometry	tuning-fork test whispering test test of hearing at a distance pure-tone audiogram speech audiogram otoacoustic emissions	pure-tone audiogram speech audiogram supraliminal tests auditory fatigue tests electric response audiometry	test of hearing at a distance pure-tone audiogram speech audiogram supraliminal tests auditory fatigue tests electric response audiometry