Idiopathic sudden sensorineural hearing loss (SSNHL) has been long known since its first documentation in 1944 [1]. It is not rare with a prevalence of 5 to 20 per 100,000 people in the United States [2]. However, its pathophysiology and causes have not been fully determined. The recovery rates varied according to the investigators. Among the prognostic factors is the severity of initial hearing loss [234567]. If the severity of initial hearing loss is high, the recovery rate is low. Seventy-decibel (dB) hearing is considered as a significantly difficult level for verbal communication, and a patient usually needs hearing aid. Even with the hearing device, he/she may encounter unsatisfactory understanding. If one cannot recover the 70-dB threshold, he/she may be separated from the verbal society. Therefore, a 70-dB loss has been regarded as a level considering cochlear implantation in many studies. There have been little studies focusing on severe to profound SSNHL. The present study was aimed at investigating the recovery pattern from a high degree SSNHL (≥70 dB). It was also aimed to determine a proper time to consider using hearing interventions such as hearing aid or cochlear implantation.

Age, sex, onset time of treatment and follow-up period were comparable among 4 groups (Table 1). We looked at recovery time by conducting long-term follow-up of 46 patients (Fig. 1). Changes in average hearing level in all the 4 groups showed the highest recovery during the initial 3 weeks after treatment. Slight recovery was observed between 3 weeks and 3 months. There was almost no recovery after 3 months. We examined the association between hearing gain in each time period and final hearing results using Pearson correlation analysis (Table 2). Compared to a later time period, initial 3 weeks' recovery showed the strongest strong correlation with the final recovery level. This means that most of the recovery is presented already at the initial 3 weeks. In other words, the initial 3 weeks' recovery predicts the final outcome.

Considering these results, we evaluated a more large number of SSNHL with high degree hearing loss at 3 weeks (Fig. 2). The results show that if the initial hearing level is bad, it is unlikely to achieve complete recovery and likely to have a higher portion of no improvement. In particular, in the ≥100 dB group, no patient showed complete recovery, and the majority showed no improvement. There were significant differences in the rate of complete recovery among the 70 dB group and 90 dB group. Partial recoveries and slight improvements were however comparable among 70 dB, 80 dB, 90 dB groups. Evenstill ≥ 100 dB group showed significant less portion of partial recovery and slightly improvement compared to other groups. The overall recovery rate was 61.02% in the 70 dB group and 10% in the ≥100 dB group. The results showed that as the initial hearing level got worse, the recovery rate got lower. This is consistent with previous studies that reported initial hearing loss is a predictive factor for hearing recovery.

The level of 3-week hearing gain by a 10-dB degree difference in each group was also studied (Fig. 3). The mean hearing gain was 23.85 dB in the 70 dB group, 23.94 dB in the 80 dB group, 19.03 dB in the 90 dB group and 6.61 dB in the ≥100 dB group. The average hearing gain was 18.36 dB overall. The mean hearing gain of the other groups was higher than that of the ≥100 dB group with statistical significance.

There are several factors affecting the prognosis of SSNHL. Currently known prognostic factors include initial degree of hearing loss, time from onset to treatment, age, accompaniment of vertigo and tinnitus, and type of hearing loss [234567].

A no correlation between the degree of hearing loss and prognosis has been claimed [9]. However, many investigators agree that the initial degree of hearing loss is an important determinant of prognosis. Wilson et al. [10] suggested that a hearing loss of 40 dB or less could be recovered regardless of whether the treatment was performed or not. Sheehy [11] also noted that an initial hearing loss of 45 dB or less yielded a satisfactory prognosis. On the contrary, the severer the initial hearing loss, the worse the prognosis [12]. Liu et al. [13] reported that hyperbaric oxygen effectively resulted in a good to fair recovery in 72% of patients with an initial hearing level of over 91 dB (technically meaning deafness). On the other hand, corticosteroids were without effects when the initial hearing loss was 90 dB or more [10]. According to Hong et al. [7], among 34 patients who had an initial hearing loss of over 100 dB, none experienced a complete recovery, with only two showing a partial recovery. In the present study, we examined four groups of patients with SSNHL with different degrees of initial hearing loss from 70 dB to ≥100 dB. The recovery rate was the highest in the 70 dB group (61%), and 53.9% in the 80 dB group, 38.3% in the 90 dB group, and 10% in the ≥100 dB group. These results support the previous notion that the recovery rate decreases as a function of the severity of initial hearing loss.

Ito et al. [14] reported that the hearing recovery during the initial 1-2 weeks has a good correlation with the final outcome. The recovery was seen mostly within 3 weeks in our study. The recovery at 3 weeks was positively correlated with final hearing gain. It is assumed that the early recovery rate predict the final outcome in severe to profound SSNHL also. There was a slight recovery during the period between 3 weeks to 3 months. The recovery was rarely seen thereafter. Only one patient in the 70 dB group and one patient in the 90 dB group showed a partial recovery among the 46 patients. We suggest that since recoveries after 3 months are rare, hearing intervention should be considered after this time period. We have previously reported that SSNHL occurring in the only hearing ear also shows rare recovery after 3 months [8]. Accordingly, early hearing intervention as cochlear implantation should be considered to restore patient's verbal communication.

In conclusion, the level of initial hearing loss and the recovery rate in the initial 3 weeks are prognostic factors in the high degree of SSNHL. A recovery is rare after 3 months. An early hearing intervention such as hearing aid should be considered especially with a loss over 100 dB. If severe to profound SSNHL occurs in the only hearing ear, cochlear implantation may be considered as early as 3 months to restore the patient's verbal communication.