43. Eye

Question 1

Which cranial nerves supply the

extraocular muscles?

Answer 1

Remember ‘LR6SO4’.

> Lateral rectus muscle is supplied by the sixth cranial nerve
(abducens
nerve).

> Superior oblique muscle
is supplied by the fourth cranial nerve
(trochlear
nerve).

> A ll of the other extraocular muscles 
medial rectus, superior rectus,
inferior rectus and inferior oblique) 
are supplied by the third cranial nerve
(oculomotor nerve).

Question 2

What determines intraocular
pressure (IOP)?

normally?

Answer 2

Intraocular pressure is normally less than 15 mmHg.

The effects of external
pressure and the pressure exerted by intraocular contents (aqueous humour,
choroidal blood volume and vitreous humour) determine the IOP.

Question 3

What are the causes of raised

IOP?

Answer 3

1
> Increased extrinsic pressure, 
e.g. retrobulbar haematoma, 
orbital compression 
in the prone position.

2
> Increased aqueous humour,

e.g. open-angle glaucoma (angle between iris and cornea remains patent but partially obstructed trabecular meshwork),

closed-angle glaucoma (anterior bulging of the iris
closing the drainage angle, trabecular meshwork remains patent)

3
> Sulphur hexafluoride injection,
e.g. during retinal surgery
SF6 may be injected into the vitreous.
Nitrous oxide interacts with SF6 by increasing
the volume of the gas and therefore may increase IOP.

4
> Increased choroidal blood volume,
e.g. gravitational effects of head down position, hypoxia, hypercarbia, hypertension.

Question 4

Describe the effects on IOP of
the commonly used anaesthetic
drugs.

Answer 4

1
> Induction agents –

all agents except for ketamine lower IOP.

2
> Volatiles – all lower IOP.

3
> N2O – has no effect on IOP unless used in combination with surgical use
of SF6 during vitreo-retinal surgery.

4
> Neuromuscular blocking agents –
non-depolarising relaxants
slightly lower IOP through reduction in extraocular muscle tension;

suxamethonium causes a transient rise in IOP.

Question 5

What is the oculocardiac reflex and how can it be prevented?

Answer 5

Oculocardiac reflex describes the
bradycardia and
or asystole

observed as a result of

traction on the
extraocular muscles or

extrinsic compression of the
eye.

The reflex pathway involves the trigeminal nerve and the vagus nerve.

It is more pronounced in children.
It may be obtunded by administration of
anticholinergics such as atropine.