Maximum Steepness of Slopes in Hearing-Threshold Curves.

The maximum steepness of slopes in pathological hearing-threshold curves has been determined in 41 patients having purely sensorineural hearing loss with extremely steep slopes. Below 500 Hz, a slope steepness of maximally 70–90 dB per octave was found while in the octave range from 2 500 to 5 000 Hz, the steepness could amount to 300–350 dB per octave, or, in some single cases, to still higher values. Thus, for an increase in frequency by a factor of 2, i.e. by 1 octave, the increase in slope steepness corresponded to roughly 75 dB per octave, possibly with a somewhat higher increase in steepness in the middle frequency range and a smaller increase in the higher frequency range.The patients' hearing impairments in this specific material showed a variety of etiologies, such as bacterial meningitis, viral infections, hereditary deafness and noise trauma by light and heavy weapons. In a number of cases, the hearing impairment could not be linked to a specific causes.The different etiological groups show a tendency to have their sloped in different frequency regions. There is also some diversity between the groups regarding monaurality or binaural symmetry of hearing loss. In the individual case, however, the audiometric steep-slope pattern turned out not to be specific enough to allow safe concludions with regard to the cause of the hearing impairment.A considerable number of the cases with slope centers below 1 000 Hz had suffered hearing impairment since very early childhood, while in nearly all of the cases with slope centers above 1 500 Hz the onset of their hearing loss was in adulthood.Preliminary studies of pitch discrimination for pure tones in cases from two of the etiological groups with slope centers ≤1 000 Hz indicated practically the same pitch discrimination ability in the damaged steep-slope range as in the range of normal hearing below the slope range.In the discussion, results from studies on other slope phenomena of the hearing system are briefly reviewed in relation to the results from the pathological cases in the present study.