Crystalline Lens, Retina, Choroid & Posterior Chamber

Crystalline Lens, Retina, Choroid & Posterior Chamber

Quick Answer: The posterior segment houses the crystalline lens (the eye's ~+19.00 D accommodation mechanism), the retina (which contains the macula, fovea, and the photoreceptors that define visual acuity), the choroid (the vascular supply to the outer retina), and the posterior chamber (the aqueous-filled space behind the iris). Posterior segment disease—cataract, pseudophakia, aphakia, diabetic retinopathy, macular degeneration—changes what a contact lens can and cannot achieve for a given patient.

Although contact lenses rest on the anterior segment, the CLRE candidate must understand posterior segment anatomy and pathology because best-corrected acuity with a contact lens cannot exceed the retinal potential. A perfectly fit lens on a diseased retina yields disappointing acuity, and some posterior conditions (aphakia, aniseikonia, pseudophakic anisometropia) are direct contact lens indications.

Crystalline Lens

The crystalline lens is a biconvex, avascular structure suspended behind the iris by the zonules of Zinn (attached to the ciliary body). It contributes ~+19.00 D to the eye's total ~+60.00 D and is the organ of accommodation.

Accommodation: ciliary muscle contracts → zonules relax → lens assumes a more spherical shape → anterior surface curvature increases → power increases. Presbyopia (loss of accommodation) begins in the early 40s; multifocal and bifocal contact lenses are designed around this mechanism.

Cataract, Pseudophakia, and Aphakia

• Cataract — any opacity of the crystalline lens. Visual impact depends on type (nuclear sclerotic, cortical, posterior subcapsular). Contact lens fitting is generally deferred until cataract surgery is complete because refractive error will change.
• Pseudophakia — status post cataract extraction with intraocular lens (IOL) implantation. The IOL is usually monofocal; residual refractive error can be corrected with contact lenses. Anisometropia between eyes (different IOL powers, different axial lengths) is a classic contact lens indication to avoid image-size disparity (aniseikonia) that spectacles would produce.
• Aphakia — no crystalline lens and no IOL. Aphakic eyes are extremely hyperopic (~+10 to +14 D). An aphakic contact lens is the optical correction of choice when secondary IOL implantation is contraindicated (e.g., pediatric traumatic cataract, capsule compromise). Spectacles create ~25% image magnification and significant aniseikonia; contact lenses produce ~7% magnification—much better tolerated.

Retina

The retina is the neurosensory tissue lining the posterior two-thirds of the eye. It converts light into neural signals carried by the optic nerve.

Rods — ~120 million; peak density 20° from fovea; contain rhodopsin; scotopic vision; high sensitivity, low acuity. Cones — ~6 million; concentrated at the fovea; three pigments (long-, medium-, short-wavelength); photopic vision; high acuity, color perception.

Retinal Abnormalities Relevant to Contact Lens Candidacy

• Diabetic retinopathy — microvascular disease causing macular edema, hemorrhages, neovascularization. Diabetic patients also have reduced corneal sensitivity and delayed epithelial healing, which raises the risk of neurotrophic ulcers with contact lens wear. Fitting requires high-Dk/t materials and shorter wearing schedules.
• Age-related macular degeneration (AMD) — central visual loss; reduced acuity expectations with contact lenses. Multifocal CLs may be unsuitable; distance single-vision CLs plus reading glasses over them are often better.
• Retinal detachment / retinoschisis — surgical repair (scleral buckle, vitrectomy) changes axial length and astigmatism; refit contact lenses postoperatively after refractive stability (~3 months).
• Retinitis pigmentosa — progressive rod dysfunction; constricted visual fields; photophobia. High-water-content lenses may improve comfort from reduced tear production.

Choroid

The choroid is the highly vascular layer between the retina and the sclera. It supplies oxygen and nutrients to the outer third of the retina (photoreceptors), which the retinal circulation does not reach.

• Choriocapillaris — single layer of capillaries; densest near the fovea
• Sattler's layer — medium vessels
• Haller's layer — large outer vessels
• Bruch's membrane — acellular layer between choroid and retinal pigment epithelium (RPE)

Choroidal pathology (choroidal neovascular membrane, central serous chorioretinopathy) reduces visual potential and therefore limits what contact lens correction can achieve. Choroidal melanoma is the most common primary intraocular malignancy in adults and is unrelated to contact lens wear but may be discovered on dilated exam prior to fitting.

Posterior Chamber

The posterior chamber is the small, aqueous-filled space between the iris and the lens zonules (anatomic posterior chamber, distinct from the vitreous-containing posterior segment). Aqueous humor produced by the ciliary body epithelium flows from here through the pupil into the anterior chamber.

Diabetes: The Cross-Segment Condition

Diabetes deserves special emphasis because it affects both segments:

• Cornea — reduced sensitivity, delayed epithelial healing, punctate keratopathy
• Retina — diabetic retinopathy, macular edema, neovascularization
• Lens — osmotic cataract ("snowflake" cataract) from sorbitol accumulation during hyperglycemia

Fitting diabetics requires:

1. High-Dk/t silicone hydrogel or scleral lenses
2. Daily wear or daily disposable modalities when possible
3. Close monitoring for epithelial defects and infiltrates
4. Co-management with the patient's retinal specialist
5. Refraction when glucose is stable (refractive fluctuation can be ±0.50 D or more with glucose swings)

Key Takeaways for the CLRE

• The crystalline lens contributes ~+19.00 D; aphakia requires ~+10 to +14 D of plus power—contact lenses minimize aniseikonia versus spectacles.
• The fovea contains cones only; macular disease limits contact lens acuity expectations.
• The choroid supplies the outer retina; choroidal disease reduces visual potential.
• Diabetic patients need higher-Dk/t lenses, shorter wearing schedules, and co-management.
• Pseudophakic anisometropia is a classic contact lens indication to avoid spectacle-induced aniseikonia.

Additional CL-Related Pathology High-Yield Items

Several Domain I pathology items span anterior and posterior findings and appear on both anatomy and follow-up questions:

• Bullous keratopathy — endothelial failure with epithelial bullae and pain. Soft bandage lenses may be used therapeutically for comfort under medical direction; elective cosmetic/EW soft wear is generally contraindicated until the cornea is managed.
• Media opacities — any opacity in the optical pathway (cornea, aqueous, lens/cataract, vitreous) that reduces best-corrected acuity. Contact lenses cannot improve acuity beyond the retinal/media limit; document acuity potential before promising CL outcomes.
• Motility problems — disorders of lid/eye position such as entropion, ectropion, and strabismus. Entropion/ectropion alter lid/lens interaction and staining risk; large-angle strabismus can complicate binocular CL goals and prism needs. Flag lid malpositions before fitting.