Iris Anatomy and Pupil Responses

Iris Anatomy: General Features

• Mobile diaphragm, ~12mm diameter.

• Pupil: hole near the center (slightly inferior-nasal).

• Decentration of >0.5mm is abnormal (ectopic pupils).

• Only internal structure that can be visualized.

• Arises behind the limbus (corneo-scleral junction).

• Continuation forward of ciliary body tissue.

• Anterior to the crystalline lens.

• Divides the globe into anterior and posterior chambers.

• Collarette divides the iris into pupillary and ciliary portions.

• Thinnest at the iris root (~0.5 mm) - potential site of tearing (iridodialysis).

• Thickest near the collarette (site of the minor arterial circle).

Iris – Tearing at the thin iris root

• Iridodialysis: tearing at the thin iris root.

• Can also tear blood vessels.

Iris - General Features: Anterior Surface

• Unique appearance.

• Highly pigmented and appears colored.

• Crypts of Fuch: Large pit-like depressions; Holes in Anterior Border Layer (ABL) through which aqueous can pass.

• Peripheral crypts.

• Contraction folds (most obvious when pupil dilated).

Iris - General Features: Pupil Ruff and Posterior Surface

• Pupillary ruff arises from the posterior pigmented epithelium.

• Iris posterior surface is dark brown/black (heavy pigmentation) - numerous folds.

• Schwalbe’s folds: radial contraction furrows in the pupillary zone and structural folds throughout ciliary zone.

Iris - General Features

• Posterior epithelial layers are highly pigmented to make the iris opaque.

• Iris color depends on pigmentation in the anterior border layer (ABL) and stroma, connective tissue density in the ABL, and degree of light scatter in the stroma.

• Blue and green appearance results from Tyndall scattering of light in the stroma (Rayleigh scattering).

Iris – Colour Inheritance

• Inherited from parents.

• Brown (B) is more dominant than blue (b).

Iris - General Features: Heterochromia

• Iris heterochromia: iris color varies between eyes or within one iris.

• Result of relative excess or lack of melanin (genetic or acquired).

  • Horner’s syndrome: decreased pigment in affected eye.

  • Prostaglandins can increase iris pigment.

Iris - General Features: Naevi

• Freckles/naevi: hyperpigmented regions of the iris.

• Clusters of melanocytes within stroma.

• Can increase with age and sunlight exposure.

• Can be benign or become melanomas; monitor for changes.

Iris - General Features: Coloboma

• Colobomas: missing piece of tissue in a structure.

• Notches, gaps, keyhole appearance.

• Due to incomplete closure of the optic/choroidal fissure during embryological development.

• Inferior location.

• Prevalence ~0.01%, relatively rare.

• Effect on vision can be minimal or eye can be blind.

Iris - General Features: Aniridia

• Aniridia: absence of iris - arises from disease, surgery, or embryologically (PAX6 gene mutation).

• Genetic cause often affects other structures, leading to poor vision.

Iris - Ultrastructure

• Four main layers:

  1. Anterior border layer

  2. Stroma and sphincter muscle

  3. Anterior epithelium and dilator muscle

  4. Posterior pigmented epithelium

Anterior Border Layer (ABL)

• Surface layer of the iris; thin condensation of stromal tissue.

• Facilitates light absorption and allows aqueous to penetrate stroma.

• Consists of interwoven collagen fibers interspersed with Fuchs’ crypts.

• Fibroblasts concentrated at the surface with melanocytes below.

• Variable numbers of melanocytes.

• No large blood vessels.

• Anterior epithelium present at birth but replaced by fibroblasts after ~2 years.

• Varies in thickness; thicker in brown irises.

• Absent or very thin at iris crypts; thickest at collarette.

• Freckles are accumulations of pigmented melanocytes in the ABL.

• Capillaries sometimes seen in ABL - rubeosis iritis, often associated with chronic retinal ischemia.

Iris Stroma

• Same structural elements as ABL (melanocytes and fibroblasts) but in a much looser form.

• Contains collagen bundles with spaces, scattered melanocytes and fibroblasts.

• Includes the sphincter muscle, blood vessels (bulk of the layer), nerves, and immune cells.

• Looser, sponge-like structure consistent with pupil constriction and dilation.

Sphincter Pupillae Muscle

• Located in the pupillary portion of the iris (stroma).

• Smooth muscle: 0.75 – 1 mm wide and 0.1 to 0.17 mm thick.

• Encircles the pupil.

• Contraction causes pupil miosis.

• Innervated by the parasympathetic system.

• Supported by collagen and connective tissue of the stroma.

• Muscle cells are spindle-shaped and closely packed.

• Innervated by post-ganglionic parasympathetic fibers of short ciliary nerves from ciliary ganglion.

• Clump cells near the sphincter muscle clean up stray melanin, increase with age.

Dilator Pupillae Muscle

• Thin myoepithelial layer: from iris root to sphincter.

• Single layer ~ 12.5 µm thick, continuous with anterior pigment epithelium.

• Apical epithelial portion and basal muscular portion (4 µm thick) comprise spindle-shaped processes (7 µm wide, 60 µm long).

• Non-myelinated post-ganglionic sympathetic neurones terminate near muscle processes.

• Peripherally attaches to the ciliary body.

• Muscle spurs project into the sphincter and stroma.

Posterior Pigmented Epithelium

• Derived from the internal layer of the optic cup.

• Height of 36 to 55 µm and width of 16 to 25 µm.

• Densely packed with melanin granules.

• Becomes unpigmented in the periphery (iris root).

• Continuous with non-pigmented epithelium of the ciliary body.

• Curls around the pupillary margin to produce pupillary ruff.

• Makes the iris opaque.

• Areas missing this layer transilluminate.

• Ocular albinos have a melanin deficiency.

Iris: Blood Supply

• Arteries are branches of the major arterial circle (in the CB near the iris root).

• Minor arterial circle at the collarette.

• Blood vessels located in the stroma; vessels of ciliary zone radially arranged.

• Vessels of pupillary zone more regular.

• Venous channels follow the arterial ones.

• Blood vessels have a corkscrew shape to cope with changes in the iris structure.

• Iris blood vessels are non-fenestrated but have overlapping endothelial cells connected by tight junctions (zonulae occludens), creating a blood-aqueous barrier.

• Sheath around endothelial cells consists of connective tissue and pericytes.

Iris Innervation: Parasympathetic and Sympathetic

• Sphincter pupillae receives parasympathetic innervation from the midbrain via the 3rd (oculomotor) cranial nerve.

• Dilator pupillae receives sympathetic innervation.

  • Originates in the posterior hypothalamus, descends the brain stem to synapse in the cilio-spinal center of Budge.

  • Postganglionic fibers follow the internal carotid artery into the cranial cavity and travel with the 1st division of the trigeminal nerve.

Pathology: Iritis

• Iritis or anterior uveitis is the inflammation of the iris tissue caused by trauma, infection, or autoimmune conditions.

• Leaky iris blood vessels breach the blood-aqueous barrier.

• Results in cells and flare in the anterior chamber, cloudy vision, pain, and light sensitivity.

Pathology: Pigment Dispersion Syndrome

• Pigment granules from the posterior surface float around the eye and deposit on the iris, lens, posterior cornea, and trabecular meshwork.

• Can block the meshwork, decrease aqueous outflow, and cause glaucoma, resulting in high intraocular pressure and damage to retinal cells.

Pathology: Essential/ Progressive Iris Atrophy

• Very rare, progressive disorder with iris atrophy (degeneration).

• Polycoria (more than one opening in the iris) and corectopia (displacement of the pupil).

• Ectropion uveae (iris pigment epithelium on the anterior surface of the iris).

Pathology: Iridocorneal Endothelial Syndrome (ICE)

• Irregular corneal endothelium, corneal edema, iris atrophy, and secondary angle-closure glaucoma.

• Altered corneal endothelium migrates posteriorly onto the trabecular meshwork and onto the peripheral iris.

• Forms peripheral anterior synechiae (PAS).

Surgery: Iridectomy

• Some forms of glaucoma are caused by aqueous being trapped behind the iris.

• A peripheral iridectomy can be performed to aid aqueous flow from the posterior to anterior chambers.

• A wedge-shape full-thickness piece of the iris is removed from the iris edge, usually superiorly.

Iridology

• A technique used to diagnose systemic diseases through observation of the topography of the iris.

• Each organ is represented in a specific sector of the iris.

• No anatomical or physiological evidence to support the claims.

Pupil - General Features

• Dark circular opening in the center of the iris; the aperture of the eye’s light imaging system.

• Variable size, displaced slightly down and nasally.

• Normal pupil diameter ~ 3-4 mm; age changes.

• Pupil constriction is called miosis; pupil dilation is called mydriasis.

• Anisocoria: difference in pupil size. ~30% of the population have anisocoria of ≤ 0.3 mm (physiologic, normal).

Factors Affecting Pupil Size

• Stimulus characteristics: light intensity, spectral composition, target distance.

• Individual variability: age, gender, iris color, refractive error, psychological effects.

• Disease: diabetes and pupil miosis.

Pupil – Age Effects

• Pupil size decreases with age.

• Senile pupil miosis occurs due to relative atrophy of the iris dilator compared to the iris sphincter.

Pupil Functions

• Light regulation: pupil area is proportional to retinal illumination.

• Depth of focus: particularly important for near objects.

• Reduce aberrations: optimize visual acuity.

Pupil – Size Varies with Lighting Level

• Normal dynamic range of 2-8 mm.

• Can change retinal illumination by a factor of 1.2 log units.

Pupillary Reflexes

• Light reflex: controls the diameter of the pupil in response to light intensity.

• Near reflex: accompanies accommodation and convergence.

• Darkness reflex: response to interruption of light at retina.

• Psychosensory reflex: higher order centers, sensory stimuli.

Light Reflex

• Direct and consensual; bilateral and symmetrical.

• Mediated via subcortical visual pathway, EW nuclei.

Characteristics of Pupil Light Response

• Latency of ~250 - 350 ms with irregular oscillations.

• Change in pupil size is directly proportional to light intensity.

• Pupillary reactions are suppressed by light adaptation and return with dark adaptation.

Near Reflex

• Near triad: accommodation, convergence, and constriction of the pupil.

• Function: to focus near images onto the retina and maintain single clear vision.

• Longer latency than the light reflex.

Darkness Reflex

• Occurs when interruption of light stimuli.

• Longer latency (300-400 ms) than the light reflex (250 - 350 ms).

• Includes active contraction of dilator and inhibition of sphincter.

Psycho-Sensory Reflex

• Pupil dilatation following stimulation of sensory nerve or strong physical stimulus.

• Pathways include cortical activity to the hypothalamus.

Other Pupil Reflexes

• Vestibular Stimulation Reflex: pupil dilation as sympathetic fibers innervating the iris traverse the middle ear.

• Sleep Reflex: pupil constriction immediately before REM sleep.

Tests for Abnormality

• Assess pupil size, shape, equality in bright and dim conditions.

• Direct and consensual light reflexes; swinging flashlight tests; near response.

Alternating/ Swinging Flash Light

• Compare strength of direct and consensual response in the two eyes.

• RAPDs do not cause anisocoria

Measurement of Pupils

• Pupillometers use infrared illumination to visualize the pupil and visible lighting to stimulate the pupil.

Pupil Cycle Time

• Slit beam on biomicroscope turned horizontal; normal value 600-900 ms.

• Prolonged with optic neuritis and optic neuropathy.

Abnormal Responses: 3rd Nerve Palsy

• Complete or partial.

• Problem with the innervation to the iris sphincter muscle.

Abnormal Responses: Horner’s Syndrome

• Unilateral miosis and subtle ptosis.

• Problem with the sympathetic innervation to the iris dilator.

Abnormal Responses: Adie’s Tonic Pupil

• Relatively common anisocoria with delayed direct and consensual light reflex.

• Involves damage and denervation of the parasympathetic postganglionic supply to sphincter pupillae.

Abnormal Responses: Argyll Robertson Pupil

• Bilateral, irregular, miotic eccentric pupils; the light reflex is absent, near reflex is preserved.

• Lesion is in the midbrain at the EW nucleus.

Abnormal Responses: RAPD- Marcus Gunn pupil

• One eye’s direct and consensual responses are less due to afferent pathway problem.

Comparison of Iris Muscles

Comparison of Pupil Reflexes

Comparison of Pupil Abnormalities