Hearing loss, and why the type decides the treatment

Hearing loss is usually discussed as a single thing that varies in degree —
mild, moderate, severe. The more useful division is by type, because the
two main types have different causes, different treatments, and very different
prospects for recovery.

Conductive loss

Sound is not reaching the inner ear. Something in the outer or middle ear — the
canal, the eardrum, the small bones behind it — is blocking or failing to
transmit it.

Common causes are earwax, fluid behind the eardrum after an infection, a
perforated eardrum, or otosclerosis, where one of the middle-ear bones
stiffens.

The important thing about conductive loss is that it is frequently
treatable, sometimes trivially. Wax is removed; fluid drains or is drained;
a stiffened bone can be replaced surgically. Hearing often returns.

Sensorineural loss

The inner ear or the auditory nerve itself is damaged. This is the more common
type in adults and, with current medicine, it is generally permanent.

The hair cells of the cochlea convert vibration into nerve signals. Humans do
not regenerate them. Once they are gone, they are gone.

Causes include ageing, long exposure to loud noise, certain medicines
(some antibiotics and chemotherapy drugs are ototoxic), meningitis, head
trauma, and heredity.

Both types can occur together, which is called mixed hearing loss.

Noise is the preventable one

Most sensorineural loss from noise is avoidable, and most of it is
cumulative — the damage from years of moderately loud exposure adds up, and
each increment is permanent.

Two practical rules:

• Sound above about 85 decibels starts to cause damage with sustained
  exposure. That is roughly heavy traffic, or a busy restaurant.
• The safe duration halves for every 3 dB louder. Eight hours at 85 dB is
  comparable to four at 88, two at 91, and so on. A concert at 100 dB reaches
  the day's safe dose in minutes.

Ringing or muffled hearing after a loud event is not a harmless after-effect.
It is a sign that damage occurred.

What helps

• Hearing aids amplify, which works when there are still hair cells to
  stimulate. Modern ones do far more than make everything louder — they shape
  the amplification to the specific frequencies lost.
• Cochlear implants bypass the hair cells and stimulate the auditory nerve
  directly, for severe to profound loss where amplification is no longer
  enough.
• Treating the cause, for conductive loss.

Why waiting is costly

Untreated hearing loss in adults is associated with social withdrawal,
depression, and faster cognitive decline. The likely mechanism is not mysterious:
conversation becomes effortful, so people have fewer of them.

The practical consequence is that hearing aids are worth fitting earlier than
most people accept them. The average delay between noticing loss and doing
something about it runs to years, and the adaptation is harder the longer the
brain has gone without the input.