## 4 Discussion
### Aggregate Analysis  
All seven articles that solely assessed DPOAEs supported their clinical value as a predictive prognostic indicator for hearing improvement in cases of ISSNHL. Four of five articles that evaluated the use of both DPOAEs and TEOAEs supported the clinical use of both types of OAEs as a predictive prognostic indicator for hearing recovery. The fifth article that evaluated both DPOAEs and TEOAEs determined TEOAEs to be an objective, efficient, and sensitive diagnostic tool for the prediction of hearing improvement in ISSNHL; however, their evidence was not significant enough to support the use of DPOAEs. One of two articles that solely evaluated the use of TEOAEs supported their clinical value as a predictive prognostic tool for hearing recovery; however, the second article noted their findings to be insignificant. Finally, [PERSON] et al. evaluated the use of DPOAEs, TEOAEs, and SOAEs, and while they supported their use as an indicator of inner ear functional status, the authors did not support their use in predicting hearing improvement, specifically for cases of LFSNHL [25]. The seven articles that solely assessed DPOAEs came to the following conclusions: the relationship between treatment-induced change in hearing threshold is reflected in DPOAE SNR changes, specifically at frequencies of 8000 and 10,000 Hz, therefore, this physiologic measure may aid in predicting hearing outcome for individuals with ISSNHL at those frequencies [25]; DPOAE amplitude is largest at f2 frequencies of 3031 and 4812 Hz for patients with the most significant hearing improvement; and a significant association between DPOAEs measured before and after treatment in predicting prognosis was determined [35]. [PERSON] et al. found that the functional state of OHCs is relatively spared in ears with an initial mild-to-moderate hearing loss, and recovery of measurable DPOAEs is not related to hearing improvement, whereas functional improvement of OHCs is important for recovery in those with an initial moderately-severe to profound hearing loss [4]. The authors noted that, for those with an initial moderately-severe hearing loss, changes of DPOAE sum values positively correlated with hearing improvement; although presence of initial DPOAE responses indicated good prognosis, absence did not always indicate poor prognosis. The rate of improvement for patients with flat audiometric configurations and larger increases in DPOAE amplitudes within the first three days displayed faster recovery [16; 17]. On the basis of this finding, the authors developed a mathematical model to compare treatment options and predict the curve of improvement for patients with ISSNHL within a specific time frame. In summary, both articles supported the value of DPOAEs as a predictive prognostic indicator [16; 17]. [PERSON] et al. concluded that patients with intact DPOAEs had a significant improvement in their PTA in conjunction with steroid therapy, whereas the majority of patients with absent DPOAEs had no hearing recovery, despite treatment [33]. The authors suggested that DPOAEs may be a useful prognostic tool as they are positively associated with hearing improvement in ISSNHL cases. [PERSON] et al. supported this finding as ISSNHL patients with significant hearing recovery tended to have present DPOAEs, whereas, when hearing recovery was not complete, DPOAEs did not reappear [34]. Overall, all seven articles that solely assessed DPOAEs determined them to be a valuable tool for predicting hearing outcome for ISSNHL patients. Four of the five articles that supported the use of both DPOAEs and TEOAEs came to the following conclusions: [PERSON] found that monitoring TEOAE and DPOAE measures in patients with ISSNHL throughout and after treatment provides an understanding of the recovery course of OHC function in relation to subjective hearing improvement; however, their findings also revealed instances where OAEs were surprisingly large in the presence of poor pure tone thresholds [29]. [PERSON] et al.