pupil

what controls the pupil

A: The pupil is controlled by both sympathetic and parasympathetic nervous fibers.

How is pupillary dilation mediated?

Pupillary dilation - 3-neuron sympathetic pathway

1st order central neuron

Starts: hypothalamus & ↓ cervical spinal cord (C8-T2), (ciliospinal center of Budge)

2nd (preganglionic)

neuron exits spinal cord & travels → cervical sympathetic chain → synapses superior cervical ganglion

3rd (postganglionic)

neuron enters the cranium via internal carotid artery → orbit & innervates iris dilator muscles & Müller's muscle in eyelids.

Pupillary dilation - three-neuron sympathetic pathway

1st order central neuron

Starts: hypothalamus & ↓ cervical spinal cord (C8-T2), (ciliospinal center of Budge)

A: 2nd (preganglionic) neuron exits spinal cord & travels → cervical sympathetic chain → passes over the pulmonary apex & synapses superior cervical ganglion near mandible & carotid artery bifurcation.

A: 3rd (postganglionic) neuron enters the cranium via internal carotid artery → trigeminal nerve, → orbit & innervates iris dilator muscles & Müller's muscle in eyelids.

Pupillary Constriction

A: Pupillary constriction is controlled by parasympathetic fibers traveling with the oculomotor nerve (cranial nerve III).

What is spasmus nutans and when does it occur?

A: Spasmus nutans is an acquired form of nystagmus seen in children, usually within the first 2 years of life.

: The triad of spasmus nutans includes:

1) Nystagmus (involuntary eye movement),

2) Head bobbing,

3) Torticollis (twisting of the neck).

: Head bobbing and torticollis are thought to be compensatory mechanisms to improve vision by reducing the nystagmus' frequency and asymmetry.

Ocular Motor Apraxia (OMA)

1. A: OMA is a condition where there is a defect or absence of voluntary eye movement control. Children with OMA struggle to move their eyes in a specific direction, especially horizontally.

   A: To compensate, they use a head thrust to track objects, as they cannot initiate horizontal eye movements. Vertical eye movements typically remain unaffected.

Q: Describe the pupillary light reflex pathway.

A: 1) Light hits the retina, initiating signals in retinal ganglion cells.

2) Signals travel through the visual pathway.

3) Some axons diverge to the pretectal nucleus, relaying to the Edinger-Westphal nucleus (EWN)

4) EWN sends signals via the third cranial nerve, through the ciliary ganglion to the sphincter muscle of the iris, causing pupil constriction.

Describe the sympathetic pathway for pupil dilation.

A: 1) Originates in the hypothalamus.

2) First-order neuron descends to the ciliospinal center of Budge (T1-T2).

3) Second-order neuron exits spinal cord, enters sympathetic chain, terminates in superior cervical ganglion.

4) Third-order neuron travels along internal carotid artery, joins ophthalmic nerve, enters orbit to innervate dilator muscle of the iris.

Oculocardiac Reflex (OCR)

OCR is triggered by stimulation of the vagus nerve due to traction on the extraocular muscles (EOMs), often seen in pediatric fractures, causing a reduction in heart rate (bradycardia)

Q: Describe the oculocardiac reflex (OCR) pathway.

A: 1) Afferent limb: Trigeminal nerve (CN V) carries signals from stretch receptors via ciliary nerves to ciliary ganglion, then to trigeminal nucleus.

2) Efferent limb: CNS processes information, communicates with vagus nerve (CN X) motor nucleus.

3) Vagal impulses travel to sinoatrial node, causing bradycardia.

 

what is the Near reflex

The near reflex (or near response) occurs when focusing on a close object.

1. Accommodation:

2. Convergence:

3. Pupillary Constriction:

The neurons carry on to the visual cortex from LGB, optic radiations = area 19 - visual cortex

• area 19 - interprets near object.

• Signals → via descending fibers → thru internal capsule.

• fibers synapse in the EW nucleus of midbrain.

• Parasympathetic fibers (via CN III and ciliary ganglion) cause:

  • Accommodation (lens thickens).

  • Pupillary constriction.

• Signals to MR muscles of both eyes = convergence.

pupil size depends on

• Age

• Hippus

• Light intensity

• Accommodation

• Drugs

• Pharmaceutical

• Recreational

• Psychosensory

• Attraction

• Fear

How do you asses pupil

• Observation

• Shape

• Size (look for anisocoria)

• Direct light reflex

• Consensual light reflex

• Swinging flashlight test

• Accommodative reflex

• Look for other abnormalities

• Eyelids

• Eye position

what are the main pupil abnormalities

• Anisocoria

• Mydriatic pupil

• Miosed pupil(s)

• Irregular pupils

• Trauma

• Iris tumours

• Coloboma

• Posterior adhesions to the lens (Synechiae)

Causes of a mydriatic pupil

Failure to constrict……..?

IIIrd N palsy

Holmes-Adie Tonic pupil

Dorsal midbrain syndrome

Acute glaucoma

Trauma

Pharmacological accident

Hutchinson pupil (coma)

IIIrd N palsy

Compressive

Aneurysm: Junction of Posterior communicating artery (PCA) and internal carotid artery

Associated with other EOM defects

Accommodation affected

Aberrant regeneration

Holmes-Adie Tonic Pupil

• Lesion in the ciliary ganglion

  • Bacterial/ viral infection

• Mostly affects women (30s-40s)

• No response to direct/consensual light reflex

• Accommodative response impaired - N reduced

  • Due to denervation and supersensitivity

• an enlarged pupil that constricts slowly in bright light:

• WILL REACT TO 0.125% PILOCARPINE

• or METACHOLINE CHLORIDE (2.5%)

• Possible reduction in knee jerk reflex

Dorsal midbrain syndrome - bilateral light near dissociation

Pupillary light-near dissociation, lid retraction, convergence-retraction nystagmus, reduced upgaze, eye misalignment

Causes

Tumors, strokes, hydrocephalus, head trauma, multiple sclerosis, A/V malformation

pharmacological dilation

causes of miosis

Failure to dilate…….?

Uniocular

Horner’s syndrome

Anterior segment inflammation

Binocular

Argyll Robertson

Convergence/Accommodative spasm

Horners syndrome

Characteristics: - PAREDINE - order neurone TEST & COCAINE

• PTOSIS, MIOSIS, ANHYDROSIS

• Additional characteristics

• Heterochromia (congenital cases)

• Apparent enophthalmos

• Can be associated with contralateral IVth N (nuclear/fascicular) or VIth N (cavernous sinus)

Anisocoria increses in dim light

• Cocaine (2-4%) dilates a nomal pupil but not a Horner’s !

• 1st (central), 2nd (Preganglionic) or 3rd (Post-ganglionic)order neuron lesion differential diagnosis

• Differential diagnosis test: PAREDRINE (1% Hydroxyamphetamine) drops will fully dilate the pupil if 1st or 2nd order neuron lesion, but subnormal dilation if 3rd neuron.

Second Order neuron lesion- Pancoast’s tumour

• can lead to horners

Argyll Robertson

• Usually bilateral miotic pupils

• May be asymmetrical

• Poor dilation in the dark and to mydriatics

• Light-Near dissociation

• NO response to light

• Responds to near target

• Hallmark of neurosyphilis

• Site of lesion: Region of the Sylvian aqueduct in the rostral midbrain

Relative Afferent Pupillary Defect (RAPD)(Marcus Gunn Pupil)

• Lesion - afferent pathway

• Retina, optic nerve or anterior visual pathway

• Diagnosed by the swinging flashlight test

• The affected pupil will dilate instead of constrict when the light is transferred from the normal eye to the abnormal eye.

• Direct response<consensual response

Afferent Pupillary Defect(Marcus Gunn Pupil)

Some of the conditions that exhibit RAPD are:

Optic Neuropathy

Extensive retinal damage (Central retinal artery/vein occlusion, marked retinal detachment)

TED – optic nerve compression

Amaurotic pupil- “Blind Eye”

Mild RAPD- amblyopia

Light Near Dissociation

Better pupillary responses to a “near”accommodative target than to a “light”

• Conditions include:

• Optic neuropathy

• Adies Tonic Pupil

• Argyll Robertson

• Parinaud’s syndrome (dorsal midbrain syndrome)

Rare Pupil disorders

Springing pupil-pupil dilation associated with migraines

Tadpole pupils-Sectoral pupil dilation

Midbrain corectopia- eccentric or oval pupil associated with rostral midbrain disease

parasympathetic pathway - constriction

Parasympathetic Pathway

• Retinal Ganglion Cells: Afferent signals originate here.

• Synapse in Pretectal Nucleus: Axons diverge from the optic tract to the superior colliculus.

• Edinger-Westphal Nucleus: Bilateral relay of information.

• Third Nerve Pathway: Preganglionic fibers synapse in the ciliary ganglion.

• Iris Sphincter Activation: Postganglionic fibers constrict the pupil.