Iris and Pupil

Iris

Thin circular structure anterior to the lens

Diaphragm of optical system

Iris collarette

Thickest part, 1.5mm from margin

Divides the iris into pupillary and ciliary zone

Pupillary zone

Encircles the pupil

Collarette to margin

Ciliary zone

Collarette to iris root

Iris root

Thinnest, joins iris to anterior aspect of the CB

Blunt trauma will cause root to tear

Iridodialysis

Separation of iris root from CB

Anterior chamber

In front of the iris

Posterior chamber

Behind the iris

Aqueous flow

Flow posterior to anterior chamber through pupil with no resistance under normal conditions

Anterior border layer of the iris

Thin condensation of the stroma on the iris surface

Composed of fibroblasts, pigmented melanocytes, and collagen fibrils

Absent in iris crypts

Forms iris processes at iris root

Iris crypts

Indentations in the iris

Iris furrows

Ridges between the crypts

Iris process

Extensions of anterior border layer into TM

Do not impede aq outflow

Iris stroma

Bulk layer

Contains the sphincter and non-fenestrated blood vessels

Composed of pigmented and nonpigmented cells, collagen fibrils, and ground substance

Continuous with CB stroma

Clump cells in iris stroma

Pigmented cells

Altered macrophages and melanocytes

Nonpigmented cells in iris stroma

Fibroblasts, lymphocytes, mast cells

Iris sphincter muscle

Smooth muscle cells joined by tight junctions

Parasympathetic innervation, M3 receptors → Miosis

Branches of major circle of the iris

Arteries encircled by collagen to prevent kinking

Collagen is continuous with stroma collagen

Minor circle of the iris

Incomplete circular vessel in region of collarette

Anterior epithelium

Contains myoepithelial cells which make up the dilator muscle

Continuous with pigmented epithelium of CB

Myoepithelial cells

Apical portion is pigmented cuboidal joined by tight junctions and desmosomes

Basal portion is elongated contractile smooth muscle processes that extend into stroma

Dilator muscle

4-5 layers of contractile tissue that extends from root to stroma below center of sphincter

Arranged radially

Sympathetic innervation → Mydriasis

Posterior epithelium

Single layer of pigmented cells joined by tight junctions and desmosomes

Part of BAB

Continuous with nonpigmented epithelium of the CB

How are the anterior and posterior iris epithelial layers connected?

Positioned apex to apex, joined by microvilli and desmosomes

Pupillary ruff

Posterior epithelium curling around to the anterior surface at the pupillary margin

Liberated pigments

When the iris bows backward, the posterior epithelium interacts with zonules → Pigment is sheared off

Causes pigment dispersion syndrome or pigmentary glaucoma (pigment clogs TM)

Krukenberg spindle

Triangle shape of liberated pigments on the cornea

Shape is caused by convection currents of aq

Pupillary block

Too much irido-lenticular contact → Iris bows forward from high IOP pushing outward → Root blocks TM → Angle closure glaucoma

Laser peripheral iridotomy

Treatment for angle closure glaucoma

Hole in thinnest part of iris → At crypts near root

What does iris color depend on?

• CT amount and arrangement in anterior border layer and stroma

• Melanocyte density

• Melanocyte pigment density

Blue iris

Results from light scatter caused by collagen arrangement

Collagen scatters what kind of wavelength?

Short

The longer the wavelength…

The smaller the amount of light scatter

Dark brown eyes

Anterior surface is usually very smooth and thicker

Harder dilation and iridotomy

Other colored eyes

Caused by light absorption by pigment within the melanocytes

Pupil diamater

Varies between 1 and 9mm

Influence by light levels

Pupil

Dynamic aperture that regulates retinal illumination

Slightly nasal and inferior to iris center

Pupil functions

• Control of retinal illumination

• Reduce optical abberations

• Depth of focus

Control of retinal illumination

Retina has range of 12 log units that allow vision in all kinds of environments

Pupil movement accounts for 1.5 log units of that range → 30 fold change in retinal illumination in half a second

Light vs dark eyes in controlling light let into the retina

Dark eyes with a smaller pupil are more efficient at limiting light

Light eyes with a smaller pupil much less efficient because more light can get through the iris

Reduction in optical abberations

Peripheral rays not in focus → Iris blocks them from getting into retina

Smaller pupils reduce degree of chromatic and spherical aberrations

Smaller pupil → Better VA

Pupil size in lasik

If pupil is larger than treatment area, more chances for glare

Smaller pupil is better for Sx,

Larger pupil more likely to report halos/glare after Sx

Depth of focus

Smaller pupil increases depth of focus → Increase VA

Pinhole effect

Small pupil increases depth of focus by…

• Reducing light scatter

• Limiting entrance of peripheral rays

• Decreases blue circle size

Vizz/Aceclinide

Cholinergic muscarinic agonist → Miosis for about 10 hours

Increase VA, has a lower risk for retinal detachment because it is more inclined to bind the sphincter compared to CB

Red eyes is a side effect

Near triad response

Converge, accommodate, pupil constriction

Any of these 3 functions can be selectively abolished or elicited without affecting the others

Arise from different cell groups within the oculomotor nucleus and travel by separate fibers, synchronized by supranuclear connections

Pupillary light response

3 major divisions that produce pupil constriction

Afferent, Interneuron, Efferent

(eye, midbrain, eye)

Afferent division of the pupillary light response

Consists of retinal input from photoreceptors, bipolar neurons, and ganglions

Rods

Primarily in peripheral retina, mediates the pupil light reflex in dim light

More sensitive to blue light than red → Constrict more with blue

Cones

Concentrated in fovea, mediates transient pupil light reflex in bright light

Pupil will dilate after initial constriction

Sensitive to both blue and red → Have similar response in initial constriction but will stay more constricted with blue after because of ganglion photoreceptors

Ganglion cell photoreceptors

Subset of ganglion cells that express melanopsin, provide a sustained constricted pupil in bright conditions

Slow to start and stop signaling, can signal without input from rods or cones

Most sensitive to blue light → Stay constrict more with blue after initial constriction

Signal to suprachiasmatic nucleus and sets circadian rhythm

Melanopsin

Photopigment contained in ganglion cell photoreceptors

Provide midbrain path for pupil light response

Wavelength sensitivity profile (Purkinje’s Shift)

Whatever is seen visually is what your eye is most sensitive to

Seeing dark light → Rods take over → Sensitive to short wavelength

Seeing bright light → Cones take over → Sensitive to long wavelength

Other characteristics of the stimulus light after the pupillary light reflex

• Long duration stimuli elicit more and sustained constriction but escape can occur

• Large area stimuli elicit more constriction from spatial summation

• Central and foveal stimuli elicit more constriction compared to peripheral

• Cannot track flickering stimuli greater than 4 Hz → Will stay constricted if more because CB muscle and neurons cannot keep up

Pupillary escape

Dilate slightly with extended light stimulus

Pretectal olivary nucleus

Retinal ganglion cells OU convey light info via synapses to interneurons here

Nasal fibers cross, temporal stay ipsilateral

Swinging light test

Finds defects in afferent signaling by swinging light between eyes

Relative Afferent Pupillary Defect (RAPD)

Afferent signaling is damaged

Light into bad eye → Dilation OU

Light into good eye → Constriction OU

Causes of RAPD

• Asymmetric glaucoma, or any asymmetric ON disease

• CRAO

• Sphenoid wing meningioma, tumor compresses ON

Interneuron division of the pupillary light reflex

Neurons in pretectal olivary nucleus send crossed and uncrossed fibers by way of the posterior commissure to each Edinger Westphal

Afferent signals are distributed equally OU → Equal response among pupils

Neurons fire at a linear rate to intensity of given light → Brighter light causes more constriction

What can block the impulse pathway from the pretectal neurons to Edinger Westphal?

Lesions in dorsal midbrain (tumors)

Encephalitis (brain inflammation, tertiary syphilis)

Causes light near dissociation

Light near dissociation

Pupil won’t constrict in light but will for accommodation

Neurons coming from cortex intact, neurons from pretectal not

Efferent division of the pupillary light response

Neurons in Edinger Westphal send preganglionic axons into R and L fascicles of CNIII to join motor axons

Fascicles exit the midbrain through subarachnoid space → Cavernous sinus → Synapse in ciliary ganglion → Short ciliary nerves → Sphincter

Adie’s tonic pupil

Denervation of the postganglionic parasympathetic supply to the sphincter and ciliary muscle

Ciliary ganglion is presumed site of denervation

Affects young women and is unilateral to start

Adie’s acute stage

Consensual light reflex is absent because efferent is affected

Shine light in effected eye → Constriction in healthy only

Adie’s chronic stage

Pupil may become small from sphincter scarring

Anisocoria is seen better in dark than in bright (acute)

“Little old Adie”

Pilocarpine in Adie eye

Will cause constriction because of denervation hypersensitivity

Aneurysm of the posterior communicating artery

Causes compression of CNIII and results in sphincter palsy since PNS rides on outside of nerve

What 2 processes lead to dilation?

1. Relax sphincter from supranuclear inhibition of Edinger Westphal

2. Contraction of dilatory through sympathetic activation of alpha1 receptors

Why won’t an eye dilate with phenylephrine alone?

The sphincter is VERY strong, PNS needs to be completely knocked out to fully dilate

Pupil during sleep

Inhibition of Edinger Westphal is inactive → Miotic

CNIII sympathetic pathway

Hypothalamus → Cervicothoracic level of C7-T2 (ciliospinal center of Budge) → Over lung apex → Superior cervical ganglion at carotid bifurcation → Cavernous sinus → Abducens → Trigeminal → Long ciliary nerves → Dilator

Horner’s Syndrome

Loss of sympathetic innervation to pupil and Muller’s muscle

Ptosis and miosis

Anisocoria in Horner’s vs. Adie’s

Adie: Worse in bright light

Horner: Worse in dark light

Physiological anisocoria

Benign inequality in pupil size that can vary

Sizes are within 1mm, same in dark and light

Observed in 1/5 of population