Dive Pacific 171 Oct- Nov 2019

divenewzealand31206

DIVEMEDICINE

The fascinating problem of inner

ear decompression sickness

By Professor Simon Mitchell, University of Auckland

In this incident a diver was evacuated for recompression, but flew home sooner than DAN

advised and, unfortunately, the symptoms returned and persisted for some time. Could this have

been avoided if the diver had delayed his flight home?

Decompression sickness

(DCS) is the well-known

diving disorder caused by

bubbles formed from inert

gas (usually nitrogen) that we

have breathed and absorbed

during a dive.

Everyone who has done an entry

level dive course knows the

story. We absorb nitrogen into

blood and tissues from the air we

breathe during a dive. The deeper

we go the more nitrogen we can

absorb, and the longer we stay at

depth the more nitrogen is taken

up. Most dives are performed

according to time and depth

guidelines that allow us to make

a direct ascent to the surface

at the end of the dive (so-called

“no-decompression diving”) but if

we exceed certain depth and time

limits then we have to ascend

more slowly and make “decompression

stops” to allow time for

nitrogen or other inert gases we

may breathe to be eliminated

from the body.

What dive tables and computers

attempt to do in prescribing

no-decompression time limits, or

in prescribing a pattern of decompression

stops during ascent, is to

prevent the pressure of dissolved

gas in our bodies from exceeding

the surrounding pressure (a

condition known as supersaturation)

by more than is considered

safe. Supersaturation of dissolved

gas in our bodies is the primary

driver for bubbles to form, so by

controlling supersaturation we

control bubble formation and

lower the risk of DCS – that’s the

theory anyway!

Nothing is simple about DCS.

Bubbles can form in tissue

themselves, or in the blood

(typically the venous blood

because it drains from the tissues

where all the nitrogen has

accumulated).

Organs can be affected

Many organs can be affected,

producing confusing patterns of

symptoms of varying severity.

Some symptoms are caused

by those bubbles that form in

tissues, and others are thought

more likely related to bubbles that

have formed in the blood. Some

organs can be affected by both.

The inner ear is a tiny organ

that lies in the bone deep to

the ear. It is responsible for

processing the neural signals of

hearing and balance. It is easy

to understand how something

going wrong with it can produce

very unpleasant and potentially

disabling symptoms, like intractable

vertigo, nausea, vomiting

and deafness.

Why now?

Why write about it now? Well, I

was reminded of the issue this

month in processing the next

issue of Diving and Hyperbaric

Medicine to be published at the

end of September. In It there is

an article by a group from Malta

describing their DCS caseload

since the late 80s. Around 1990

about 5% of their DCS cases had

inner ear symptoms but by 2017

this had risen to 50%, an extraordinary

increase [1].

Our DCS case numbers are low

across the board in New Zealand,

…The inner ear is tiny, but it contains several relatively

large pools of fluid that can absorb and eliminate gas from

the blood – but only through the sensitive neural tissues.

These pools of fluid have no blood supply of their own…

but we too are seeing more inner

ear cases. The obvious question

is why? And the answer is that

it probably reflects the nature

of the diving going on. Basically,

with the increasing popularity of

technical diving and rebreather

use we are going deeper.

Going deeper

To understand why depth can

be a particular issue for inner

ear DCS we need to consider its

fascinating pathophysiology.

Here it can get a little complicated.

The inner ear is one of

those organs we believe can be

injured by bubbles forming in the

organ itself, and also by bubbles

reaching it in the blood.

Let’s start with bubbles forming

in the inner ear itself. That seems

like a fairly simple concept. If the

inner ear tissue became excessively

supersaturated with inert

gas then bubbles could form,

disrupt the inner ear function,

and cause symptoms. This has

often been seen during decompressions

from very deep dives

(I’m thinking typically more than

100m).

54 Dive New Zealand | Dive Pacific

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