iris and pupil

Basic function of the iris and pupil

Admit and regulate light entering the eye, and affect quality of the retinal image.

Anisocoria

A detectable difference in pupil diameter (>0.5 mm).

Hippus

Normal, rhythmic oscillation of pupil diameter (±2 mm); larger/faster with increased light.

Aperture stop of the eye

The iris.

Field stop of the eye

The iris limits peripheral rays, affecting the field of view.

Depth of field

Object space (range where objects appear in focus).

Depth of focus

Image space (retinal focus range).

Effect of large pupil on image quality

Large pupil → ↑ aberration (↓ image quality).

Effect of small pupil on image quality

Small pupil → ↓ aberration, ↑ diffraction (still better than aberration).

Normal pupil size range

2-8 mm (bright light = small, dim light = large).

Pupil response to light intensity

Diameter decreases ~1 mm for each log unit increase in intensity.

Faster pupil response

Constriction is faster; dilation is slower.

Normal decentration of the pupil

~0.5 mm nasally. >0.5 mm = ectopic pupil.

Five layers of the iris

1. Anterior border layer

2. Stroma

3. Muscular layer

4. Anterior pigmented epithelium

5. Posterior pigmented epithelium

anterior border layer

Crypts of Fuchs, iris processes

Stroma

collagen, vessels, melanocytes, clump cells

Muscular layer

sphincter + dilator

Anterior pigmented epithelium

dilator origin

Posterior pigmented epithelium

light absorption

Thinnest and weakest part of the iris

Iris root → most common trauma site.

Collarette

Landmark dividing pupillary (sphincter) and ciliary (dilator) regions; site of minor arterial circle.

Contraction folds

Ridges/valleys from dilation; can block trabecular meshwork → ↑ IOP.

Major source of iris blood supply

Long posterior ciliary artery → Major arterial circle (MAC).

Minor arterial circle

Incomplete circle at collarette; supplies sphincter + dilator.

Vessel adaptation to pupil size changes

Straighten during constriction, curve during dilation (provides slack).

Venous drainage of iris

Through ciliary processes → vortex vein.

Two iris muscles

Sphincter + dilator

sphincter

circular → constriction (miosis)

dilator

radial myoepithelium → dilation (mydriasis)

Constriction

sphincter contracts OR dilator relaxes

Dilation

dilator contracts OR sphincter relaxes.

which branch of the autonomic NS controls the sphincter?

parasympathetic

What is the effect of parasympathetic stimulation on the pupil?

miosis (pupil constriction)

Which neurotransmitter is released by parasympathetic nerves controlling the sphincter?

Ach

Which type of receptor does acetylcholine bind to in the iris sphincter muscle?

muscarinic receptors

Where does the parasympathetic signal for miosis originate?

Edinger-Westphal (EW) nucleus

After the EW nucleus, where does the signal travel next?

along the oculomotor nerve (CN III) --> ciliary ganglion

What happens at the ciliary ganglion?

Parasympathetic fibers synapse (relay)

Which nerves carry the signal from the ciliary ganglion to the iris sphincter?

Short ciliary nerves

What muscle do the short ciliary nerves stimulate for pupil constriction?

Sphincter pupillae muscle

Which branch of the autonomic nervous system controls the iris dilator muscle?

sympathetic

What is the effect of sympathetic stimulation on the pupil?

mydriasis (pupil dilation)

Which neurotransmitter is released by sympathetic nerves to stimulate the dilator muscle?

NE

Which type of receptor does norepinephrine bind to in the dilator muscle?

α-adrenergic receptors

Where does the sympathetic pathway to the eye begin?

brainstem

After the brainstem, where does the sympathetic signal travel?

spinal cord at the T1 level

From the spinal cord (T1), where does the signal go?

sympathetic trunk

Which ganglion does the signal synapse in?

superior cervical ganglion

After synapsing in the superior cervical ganglion, where does the signal travel?

Along the sympathetic plexus (around the internal carotid artery)

How does the sympathetic signal reach the eye from the sympathetic plexus?

via short ciliary nerves

Which muscle is activated by the sympathetic system to dilate the pupil?

Dilator pupillae muscle

synechia

iris adheres to other structures

Anterior synechia

iris adheres to cornea

Posterior synechia

iris adheres to lens capsule

What is iris bombe?

360* posterior synechia --> traps aqueous & iris bows forward

iris bombe treatment

iridectomy --> polke holes in iris to shunt aqueous to anterior chamber

What is heterochromia?

Different iris colors (congenital or acquired)

What is pigmentary dispersion glaucoma?

Pigment from posterior epithelium clogs trabecular meshwork → ↑ IOP.

What is anisocoria?

Pupillary asymmetry >0.5 mm.

Direct reflex?

Constriction of illuminated pupil.

Consensual reflex?

Constriction of opposite pupil.

Swinging flashlight test detects what?

Afferent pupillary defect (Marcus Gunn pupil).

Adie's pupil?

Dilated pupil, poor/absent light reflex, sluggish constriction.

Argyll Robertson pupil?

No light reflex but accommodation intact ("prostitute's pupil").

Horner's syndrome triad?

Ptosis, miosis, anhidrosis (sympathetic lesion).

types of drugs that constrict pupil (miosis)?

sphincter stimulators and dilator blockers

Which drug(s) stimulate the iris sphincter muscle?

Pilocarpine, Physostigmine

Pilocarpine and physostigmine act on which autonomic system?

parasympathetic

Which drug inhibits the iris dilator muscle?

Dapiprazole

Dapiprazole acts on which autonomic system?

sypathetic

Which drug inhibits the iris sphincter muscle?

Tropicamide

Tropicamide acts on which autonomic system?

Parasympathetic

Which drug stimulates the iris dilator muscle?

Phenylephrine

Phenylephrine acts on which autonomic system?

sympathetic

types of drugs that dilate pupil (mydriasis)?

dilator stimulators, sphincter blockers

What forms iris stroma?

Neural crest cells (2nd wave of migration).

What forms iris epithelium & muscles?

Neural ectoderm (rim of optic cup).

When do muscles develop?

Sphincter ~4 months gestation; dilator ~6 months.

When does iris color develop?

Postnatal → gradual melanin accumulation.

What is the last iris structure to form?

The pupil.

Aniridia

total absence of iris.

Coloboma

failure of closure → missing iris sector.

Hypoplasia

reduced cell formation.

Ectopic pupil

mispositioned pupil.

Persistent pupillary membrane

strands across pupil from incomplete vascular regression.