Dexamethasone and IGF-1 have been shown in human clinical trials to be very effective in SSHL.
I'm not aware of any studies showing that anything is "very effective" for SSHL. Even the evidence for steroids is mixed. Thanks for posting the specific paper. Reading it and others, IGF-1 looks like it may have some promise, but the paper you cite does not prove it is "very effective" - nor do any others that I have found. Regarding the cited paper:
For the primary outcome: "The primary outcome was the proportion of patients showing hearing improvement (10 dB or greater in puretone average hearing thresholds) at 8 weeks after treatment. In the Dex group, 53.6% (95% CI, 39.7–67.0%) of patients showed hearing improvement at 8 weeks after treatment, whereas in the IGF-1 group, 66.7% (95% CI, 52.9–78.6%) of patients showed hearing improvement (Table 4).
The null hypothesis for the primary outcome was not rejected (P = 0.109). However, a trend was observed in the higher proportion of patients in the IGF-1 group showing complete or marked recovery (30 dB or greater in pure-tone average hearing thresholds) over that in the Dex group (Table 2)." (emphasis added)
So there was no overall effect. Now the part about the "complete or marked" recovery is interesting. Table 2 does not show this. It's really not clear what they intended. Table 4 shows that 12/53 on Dex had complete or marked recovery compared to 16/53 for IGF-1. But they don't provide any test information. Given that the difference between 29/53 and 36/53 was not significant, it seems unlikely the difference between 12 and 16 is. Maybe they meant figure 2. But it doesn't show it either. It does show a divergent trend, but that is always < 10 dB which is the usual definition of clinically significant. So the average difference is not clinically significant even if it is statistically significant. None of the other outcomes show a significant difference. So, it is a promising statement that doesn't seem to be supported by any of the tables/figures in the paper
It does appear to have a better safety profile though that is also a bit weird given that they say: "However, topical IGF-1 therapy requires surgical procedures and causes uncomfortable symptoms associated with the local application." Given that, IGF-1 seems riskier. Particularly since "In addition, spontaneous recovery of hearing occurs in 30 to 60% of patients with SSHL [5,26-28]." This is the big issue with showing anything is effective for SSHL. It typically has unknown eitology, which makes knowing the right treatment hard, and frequently resolves (at least partially) on its own.
Regarding the surgical procedure: "After tympanostomy under local anesthesia with 1% lidocaine, the hydrogel (which contained 300 μg of mecasermin) was placed in the round window niche of the middle ear; a single application was used." So this is not an injection. This also raises the issue that while the patients are randomized, it is not placebo controlled. (it's also somewhat surprising they got good hearing results after ear surgery.)
It's also the case that there was a surprising statistical difference in aural fullness in favor of the IGF-1 group. If aural fullness is some symptom of damage, the Dex group may be worse of in some ways not otherwise measured. While no where near significant, the pattern of pre-baseline improvement also favored IGF-1.
I also wonder about the patient selection. To be eligible, steroids had to be unsuccessful: "they showed less than 30 dB hearing improvement in the mean hearing level, based on pure-tone audiometry (PTA) at five tested frequencies (0.25 kHz, 0.5 kHz, 1.0 kHz, 2.0 kHz, and 4.0 kHz) after more than 7 days of systemic corticosteroid treatment." So, though it wasn't IT steroids, oral steroids had already proved somewhat ineffective for this group. This may mean that even It steroids are more likely to be ineffective (and remember anyone who was helped by systemic steroids is not eligible for the trial.
Finally, here's the authors' conclusion: "We performed a randomized, controlled clinical trial of topical IGF-1 therapy in patients with SSHL refractory to systemic corticosteroids and compared this treatment to intratympanic corticosteroid therapy. Present results suggest the possibility that IGF-1 is superior to intratympanic Dex therapy, but the current study design failed to confirm this possibility. The positive effect of topical IGF-1 application on hearing levels and its favorable safety profile suggest utility for topical IGF-1 therapy as a salvage treatment for SSHL."
You have to love the tortured language "present results suggest the possibility..." They clearly want to say they found what they were looking for but of course "the current study design failed to confirm this possibility". It may well be the case that IGF-1 is an appropriate salvage treatment, and it seems like there is enough promise for people to keep exploring this. But it is hardly the case that it has been shown to be very effective in treating SSHL. Even if it is effective for SSHL, that doesn't mean it is effective more generally. It would be interesting to know if it is effective for some causes of SSHL.
In some follow-up work (
https://www.ncbi.nlm.nih.gov/pubmed/26739948) they do regression analysis and find a statistically significant but clinically insigificant 5.9 dB improvement for their IGF-1 hydrogel. This is smaller than the effect for having a < 90 dB (vs > 90 dB) hearing loss (16.67 dB improvement), being < 60 years old (10.58) and < 14 days to treatment (7.20). They note that one possible reason for their results is the different method of administration and that hydrogel steroids might work better than IT steroids, but they don't test that.
Ito's group clearly wants to find something here. And it makes sense because, just like a company, they are testing a hydrogel they created: "To provide a new therapeutic option for SSHL,
we developed a topical insulin-like growth factor-1 (IGF-1) therapy in which
gelatin hydrogels impregnated with recombinant human IGF-1 are applied to the
middle ear." (emphasis added) Given this, it is important to pay attention to the details because whether conscious or not, they will spin things in a positive way.
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