iris

**Anatomy** - The iris is the anterior-most part of the uveal tract (uvea = iris + ciliary body + choroid). It sits between the anterior chamber and posterior chamber, in front of the lens. - Layers (anterior → posterior): anterior border layer, stroma (vessels, melanocytes), sphincter and dilator muscles, and a posterior pigmented epithelium (continuous with the retinal pigment epithelium/ciliary body epithelium; derived from neuroectoderm of the optic cup). - The iris root attaches to the ciliary body. It forms the boundary of the iridocorneal (filtration) angle, where the trabecular meshwork drains aqueous humor into the canal of Schlemm.

**Muscles and innervation (high-yield)** - Sphincter pupillae: circular fibers. Parasympathetic pathway: Edinger-Westphal nucleus → CN III → ciliary ganglion → short ciliary nerves → M3 muscarinic receptors → miosis. - Dilator pupillae: radial fibers. Sympathetic pathway: hypothalamus → intermediolateral cell column (C8-T2, ciliospinal center) → superior cervical ganglion → long ciliary nerves → α1 receptors → mydriasis. - Pupillary light reflex: afferent = CN II; efferent = CN III. Light in one eye causes a direct and a consensual response (pretectal nucleus → bilateral Edinger-Westphal nuclei).

**Embryology** - The iris stroma comes from neural crest (mesenchyme). The muscles and the posterior epithelium come from neuroectoderm (optic cup). The iris sphincter and dilator muscles are unusual because they are of neuroectodermal origin.

**Eye color** - Determined by melanin amount and distribution in the anterior stroma. Brown eyes have abundant melanin. Blue eyes have little stromal melanin, and the color is due to light scattering. All irises have a pigmented posterior epithelium.

**Clinical correlations** - Horner syndrome (sympathetic lesion): ptosis, miosis, anhidrosis (± enophthalmos). The affected pupil is smaller and shows dilation lag. - CN III palsy: "down and out" eye, ptosis, and a blown (dilated, fixed) pupil. Compressive lesions (e.g., posterior communicating artery aneurysm, uncal herniation) affect the superficial parasympathetic fibers early. Ischemic (diabetic) lesions often spare the pupil. - Adie (tonic) pupil: damage to the ciliary ganglion or postganglionic fibers. The pupil is dilated and reacts poorly to light but shows light-near dissociation and slow accommodation. Cholinergic supersensitivity responds to dilute pilocarpine. - Argyll Robertson pupil (neurosyphilis): "accommodates but doesn't react" (prostitute's pupil), with light-near dissociation. - Relative afferent pupillary defect (Marcus Gunn): optic nerve or severe retinal disease. Shown by the swinging-flashlight test (paradoxical dilation when light is swung to the affected eye). - Acute angle-closure glaucoma: the iris bows forward (iris bombé) and blocks the trabecular meshwork. Features: severe eye pain, halos around lights, a mid-dilated fixed pupil, and a red eye with a hazy cornea. Treatments include acetazolamide, topical β-blockers, α-agonists, pilocarpine (miosis), and definitive laser peripheral iridotomy. Avoid mydriatics and anticholinergics. - Iritis/anterior uveitis: pain, photophobia, ciliary flush, and cells and flare in the anterior chamber. Associated with HLA-B27 diseases (ankylosing spondylitis, reactive arthritis, IBD, psoriatic arthritis), JIA, sarcoidosis, and Behçet disease. Complications include synechiae (posterior synechiae leading to an irregular pupil). - Lisch nodules: pigmented iris hamartomas seen in neurofibromatosis type 1 (NF1). They are asymptomatic and appear on slit-lamp exam. - Brushfield spots: speckled white/gray spots on the peripheral iris in Down syndrome (trisomy 21). - Aniridia: congenital absence or hypoplasia of the iris. Associated with WAGR syndrome (Wilms tumor, Aniridia, Genitourinary anomalies, Intellectual disability [formerly "mental retardation"]) from a deletion at 11p13 (PAX6 and WT1 genes). - Heterochromia iridis: different colored irises. Causes include congenital, Horner syndrome (congenital), Waardenburg syndrome, trauma, and inflammation. - Kayser-Fleischer rings: copper deposition in Descemet membrane at the corneal periphery (not the iris itself) in Wilson disease. They are often confused with iris findings. - Floppy iris syndrome (IFIS): associated with α1-antagonists (especially tamsulosin), which complicates cataract surgery. - Drug effects on the pupil: Mydriasis: atropine and other anticholinergics, sympathomimetics (cocaine, amphetamines, phenylephrine), LSD, and others. Miosis: opioids (pinpoint pupils), organophosphates (cholinesterase inhibitors), pilocarpine, and clonidine. Latanoprost (a prostaglandin analog) can increase iris pigmentation (brown discoloration) as an adverse effect. - Iris neovascularization (rubeosis iridis): caused by retinal ischemia (proliferative diabetic retinopathy, central retinal vein occlusion), leading to neovascular glaucoma.