3.1.9 Pupils reactions

Afferent Pathway

Nerve that carries information towards the CNS

• RGC

• ONH

• Optic Chiasm

• Optic tract

• Pretectal Nucleus

• EWN

Occipital lobe is responsible for the eyes

Efferent Pathway

Nerve that carries information away from the CNS

Two pathways;

• Sympathetic - Dilator Muscle - Mydriasis - fight or flight

• Parasympathetic - Sphincter Muscle - Miosis - rest & digest

Sympathetic branch of Efferent Pathway

• Postero-lateral hypothalamus (spinal cord)

• Over the Apex of the lung

• Superior cervical ganglion

• Travels along the Ophthalmic division of the 5th neve (trigeminal)

• Long ciliary nerve

• Dilator muscle

Parasympathetic branch of Efferent Pathway

• EWN

• CN3 - oculomotor

• Ciliary Ganglion

• Short ciliary nerve

• Iris sphincter muscle

• Pupil miosis

What are Cholinergic receptors (muscarinic) stimulated by?

• Acetylcholine - PS

What are adrenergic receptors (alpha-1) stimulated by?

• Noradrenaline - S

Sphincter muscle

Pupil size is mainly determined by the contraction & relaxation of the sphincter muscle

• It is a thin circumferential ring of smooth muscle fibres

• The sphincter muscle responds to signals from the short ciliary nerve

  • This constricts the pupil

• The sphincter muscle is innervated by cholinergic parasympathetic fibres

Dilator Muscle

The dilator muscle has a secondary effect on pupil size

• The dilator muscle responds to signals coming from the long ciliary nerve

  • This dilates the pupil

• The dilator muscle is innervated by adrenergic sympathetic fibres

• The Sphincter & Dilator muscles are antagonistic muscles

How does the pupil Dilate in dark lighting?

1. The sphincter muscle relaxes

• The sphincter muscle has the biggest effect on pupil size

2. The dilator muscle contracts

How does the pupil constrict in bright lighting?

1. The sphincter muscle contracts

• The sphincter muscle has the biggest effect on pupil size

2. The dilator muscle relaxes

What will a lesion at the Optic nerve cause?

• Likely to cause RAPD

• Light Shone into the Affected Eye will Cause:

  • A reduced direct response

  • A reduced consensual response

• Light Shone into the Non-Affected Eye will Cause:

  • o A normal direct response

  • A normal consensual response

Damage to PS pathway

DILATED PUPIL + Poor response to light

Adie’s tonic pupil; Damage to the ciliary ganglion or postganglionic fibres of the short ciliary nerve

Arygll Robertson pupil; lesion around EWN

Adie’s Tonic Pupil

• Affected eye is dilated

• Pupil reacts poorly to light (poor direct & consensual response)

• Near reaction is strong, slow & tonic

• When the patient re-fixates at a distance, the pupil re-dilates very slowly

• Maximum response to Pilocarpine

Damage to Sympathetic branch of Efferent pathway

• Affects dilator = pupil miosis

  • aka Horner’s syndrome

Horner’s syndrome

Common Aetiology;

• Lung Cancer

• Pancoast Tumour

Signs;

• Ptosis

• Anhidrosis

• Miosis

• Pupils react NORMALLY TO LIGHT & NEAR

Arygll Robertson Pupil

Damage to PS pathway - light reflex pathway affected - accommodation is still functional

Usually due to neurosyphilis - presumed

can be due to DM/Alcoholism

Signs

• Pupils both small

• Respond poorly to light or not at all

• Normal response to near unlike Adie’s pupil (accommodation)

In OM why do we test the vertical gaze?

• It won’t help us detect any EOM dysfunction but it can tell us if Px has a V pattern or A exo/eso

• A exo - greater in primary gaze compared to upwards

• A eso - greater in upwards gaze compared to primary gaze

• V exo - greater in upwards gaze compared to primary - most common pattern in strabismus

• V eso - greater in primary gaze compared to upwards

+ve RAPD

• Pupils dilate on swinging flashlight test

Duane’s retraction syndrome

• Mainly unilateral more common in LE and Females

• Absent or abnormal 6th nerve - LR muscle affected