Generally, hyperacusis symptoms can be traced back to physical trauma such as a blow to the head or exposure to traumatic noise. With that said, any damage to the mechanisms of the inner ear can result in hyperacusis symptoms. Although scientists are not yet clear on what specifically causes hyperacusis, it's suspected that it may be a disorder of the auditory nerve.
In other words, hyperacusis has clear physical symptoms and a suspected physical cause.
Misophonia, on the other hand, is entirely psychological. It may be due to trauma associated with a particular noise or a symptom of an underlying mental illness. Although the anxiety, panic, or anger accompanying an episode of misophonia can manifest certain physical symptoms, it's generally accepted that misophonia is a psychological condition rather than a physical one.
Another noteworthy difference between the two is how long an individual episode lasts. With misophonia, symptoms rarely last for more than an hour or two. Hyperacusis, meanwhile, can linger for days, sometimes weeks or even months after exposure.
Misophonia generally results in a fully-body response—in many cases, exposure to a triggering noise could result in a panic attack. Hyperacusis rarely manifests in any meaningful way outside the ears. Hyperacusis is also distinct in that the condition can be present in only one ear rather than both.
Finally, as one might expect, the two conditions tend to be treated differently. Hyperacusis is usually best addressed through a prescription hearing aid or sound therapy to help 'reprogram' one's auditory system. It's a similar principle to tinnitus treatments in that you're conditioning your brain to either ignore the sound or regard it as mundane.
For misophonia sufferers, counseling is generally the best approach. Speaking to a trained therapist can help one determine exactly why a sound causes such an extreme emotional response. From there, treatment often consists of cognitive behavioral therapy paired with sound therapy.
Before we wrap things up, there's one last thing we need to say. Neither misophonia nor hyperacusis is somehow more 'legitimate' than the other. Both are genuinely debilitating and diagnosable conditions.
And if you look down on someone with misophonia because it's 'all in their head,' then you've completely missed the point of this piece.