NAL vs DSL in Children with Severe-to-Profound Hearing Loss
Ching et al (2015) report on prescribed and measured gain, as prescribed by NAL-NL1 and DSL v5, using the Phonak Naida V SP hearing aid. Sixteen children (aged 7 to 17 years) with severe-to-profound sensorineural hearing loss (SNHL) participated. The Speech Intelligibility Index (SII) and estimated loudness of the fittings were calculated with input loudness levels of 50 (low), 65 (medium), and 80 dB (high) SPL. Of note, NAL aims to maximize speech intelligibility, whereas DSL aims to normalize loudness. Ching et al report that in general, given a specific audiogram for children, DSL prescribed significantly higher overall gain than did NAL (adult DSL targets are approximately 7 dB less than targets prescribed for children).
The authors report that for low and medium input levels, all NAL fittings were within 5 dB RMS of prescribed targets. However, 33 percent of DSL fittings varied by more than 5 dB RMS on average. For high input levels, NAL (for 60 percent of ears) and DSL (for 43 percent of ears) varied more than 5 dB RMS from target. On average, greater variation occurred as hearing loss increased. The authors report on average, SII was higher for DSL than NAL with low level input, and at medium and high level inputs the SIIs were equivalent, despite the greater loudness of DSL. With regard to Fit-to-Target, NAL fittings were generally within 3.2 dB of target, and DSL fittings were within 4.96 dB.
Ching et al note that for children with moderately severe to profound SNHL, NAL and DSL prescribed substantially different overall gain, thus impacting SII and loudness. With respect to SII and loudness, DSL and NAL prescriptions “there may be benefits for increasing gains” as prescribed by NAL-NL1 for low input levels to increase audibility...(and at)…medium and high input levels…”NAL and DSL are comparable with respect to SII. At medium to high level inputs, NAL was generally “closer to normal loudness” than DSL.
Ching et al state that “hearing aids set according to NAL-NL1 are less likely to result in loudness discomfort or potential threshold shifts due to hearing aid use….” Further, they report for sounds between 250 to 2000 Hz, DSL and NAL closely matched their targets. However, at 4000 Hz, NAL was generally closely matched, while DSL targets were “underachieved.” The authors concluded “Although the two prescriptions differ markedly in estimated loudness, they resulted in comparable predicted speech intelligibility for medium and high input levels.”
For More Information, References, and Recommendations
Ching TYC, Dillon H, Hou S, Zhang V, Day J, Crowe K, Marnane V, Street L, Burns L, Van Buynder P, Flynn C, Thomson J. (2013) A Randomized Controlled Comparison of NAL and DSL Prescriptions for Young Children—Hearing Aid Characteristics and Performance Outcomes at Three Years of Age. International Journal of Audiology 52:S17–S28
Ching TYC, Quar TK, Johnson EE, Newall P, Sharma M. (2015) Comparing NAL-NL1 and DSL v5 in Hearing Aids Fit to Children with Severe or Profound Hearing Loss: Goodness of Fit-to-Targets, Impacts on Predicted Loudness and Speech Intelligibility. Journal of the American Academy of Audiology 26:260–274.
Ching TYC, Scollie SD, Dillon H, Seewald R. (2010) A Cross-Over, Double-Blind Comparison of the NAL-NL1 and the DSL v4.1 Prescriptions for Children with Mild-to-Moderately Severe Hearing Loss. International Journal of Audiology 49:S4–S15.