Pupil Abnormalities Flashcards
Constriction
There are circular muscles in the iris that cause pupil constriction. They are stimulated by the parasympathetic nervous system using acetylcholine as a neurotransmitter. The fibres of the parasympathetic system innervating the eye travel along the oculomotor (third cranial) nerve.
Dilation
The dilator muscles of the pupil arranged like spokes on a bicycle wheel travelling straight from the inside to the outside of the iris. They are stimulated by the sympathetic nervous system using adrenalin as a neurotransmitter.
Dilation
The dilator muscles of the pupil arranged like spokes on a bicycle wheel travelling straight from the inside to the outside of the iris. They are stimulated by the sympathetic nervous system using adrenalin as a neurotransmitter.
Causes of mydriasis (dilated pupil)
Third nerve palsy
Holmes-Adie syndrome
Raised intracranial pressure
Congenital
Trauma
Stimulants such as cocaine
Anticholinergic
causes of miosis (constricted pupil)
Horners syndrome
Cluster headaches
Argyll-Robertson pupil (in neurosyphilis)
Opiates
Nicotine
Pilocarpine
Third Nerve Palsy
A third nerve palsy causes:
Ptosis (drooping upper eyelid)
Dilated non-reactive pupil
Divergent strabismus (squint) in the affected eye. It causes a “down and out” position of the eye.
The third cranial nerve is the oculomotor nerve. It supplies all of the extraocular muscles except the lateral rectus and superior oblique. Therefore when these muscles are no longer getting signals from the oculomotor nerve, the eyes moves outward and downward due to the effects of the lateral rectus and superior oblique still functioning without resistance.
It also supplies the levator palpebrae superioris, which is responsible for lifting the upper eyelid. Therefore third nerve palsy causes a ptosis.
The oculomotor nerve also contains parasympathetic fibres that innervate the sphincter muscle of the iris. Therefore third nerve palsy causes a dilated fixed pupil.
The oculomotor nerve travels directly from the brainstem to the eye in a straight line. It travels through the cavernous sinus and close to the posterior communicating artery. Therefore, cavernous sinus thrombosis and a posterior communicating artery aneurysm can cause compression of the nerve and a third nerve palsy.
Causes of a Third Nerve Palsy
Third nerve palsy can be idiopathic, without a clear cause.
A third nerve palsy with sparing of the pupil suggests a microvascular cause as the parasympathetic fibres are spared. This may be due to:
Diabetes
Hypertension
Ischaemia
A full third nerve palsy is caused by compression of the nerve, including the parasympathetic fibres. This is called a “surgical third” due to the physical compression:
Idiopathic
Tumour
Trauma
Cavernous sinus thrombosis
Posterior communicating artery aneurysm
Raised intracranial pressure
Horner Syndrome
Ptosis
Miosis
Anhidrosis (loss of sweating)
They may also have enopthalmos, which is a sunken eye. Light and accommodation reflexes are not affected.
Horner syndrome is caused by damage to the sympathetic nervous system supplying the face.
The journey of the sympathetic nerves to the head is relevant for the causes of Horner syndrome. The sympathetic nerves arise from the spinal cord in the chest. These are pre-ganglionic nerves. They then enter into the sympathetic ganglion at the base of the neck and exit as post-ganglionic nerves. These post-ganglionic nerves then travel to the head, running alongside the internal carotid artery.
The location of the Horner syndrome can be determined by the anhidrosis. Central lesions cause anhidrosis of the arm and trunk as well as the face. Pre-ganglionic lesions cause anhidrosis of the face. Post-ganglionic lesions do not cause anhidrosis.
The causes can be remembered as the 4 Ss, 4 Ts and 4 Cs. S for Sentral, T for Torso (pre-ganglionic) and C for Cervical (post-ganglionic).
Central lesions (4 Ss):
S – Stroke
S – Multiple Sclerosis
S – Swelling (tumours)
S – Syringomyelia (cyst in the spinal cord)
Pre-ganglionic lesions (4 Ts):
T – Tumour (Pancoast’s tumour)
T – Trauma
T – Thyroidectomy
T – Top rib (a cervical rib growing above the first rib above the clavicle)
Post-ganglionic lesion (4 Cs):
C – Carotid aneurysm
C – Carotid artery dissection
C – Cavernous sinus thrombosis
C – Cluster headache
Congenital Horner syndrome is associated with heterochromia, which is a difference in the colour of the iris on the affected side.
Cocaine eye drops can be used to test for Horner syndrome. Cocaine acts on the eye to stop noradrenalin re-uptake at the neuromuscular junction. This causes a normal eye to dilate because there is more noradrenalin stimulating the dilator muscles of the iris. In Horner syndrome, the nerves are not releasing noradrenalin to start with so blocking re-uptake does not make a difference and there is no reaction of the pupil.
Alternatively, a low concentration adrenalin eye drop (0.1%) won’t dilate a normal pupil but will dilate a Horner syndrome pupil.
Holmes Adie Pupil
A Holmes Adie pupil is a unilateral dilated pupil that is sluggish to react to light with slow dilation of the pupil following constriction. Over time the pupil will get smaller. This is caused by damage to the post-ganglionic parasympathetic fibres. The exact cause is unknown but may be viral.
Holmes Adie Syndrome is where there is a Holmes Adie pupil with absent ankle and knee reflexes.
Argyll Robertson Pupil
An Argyll-Robertson pupil is a specific finding in neurosyphilis. It is a constricted pupil that accommodates when focusing on a near object but does not react to light. They are often irregularly shaped. It is commonly called “prostitutes pupil” due to the relation to neurosyphilis and because “it accommodates but does not react“.
