The anterior segment encompasses all structures from the cornea to the lens -- the cornea, limbus, conjunctiva, sclera, anterior chamber, iris, ciliary body, and crystalline lens. Anterior segment examination with the slit lamp is one of the COA's core clinical skills, and knowledge of common anterior segment conditions is tested throughout the COA exam in the Assessments, Imaging, and Assisting with Procedures domains.
This guide covers the most commonly tested anterior segment conditions on the COA exam: corneal diseases (HSV keratitis, bacterial keratitis, keratoconus, ABK/PBK), conjunctival conditions (pterygium, pinguecula, conjunctivitis types), anterior chamber findings (flare, cell, hypopyon, hyphema), and important lens and iris conditions (pseudoexfoliation, phacodonesis, rubeosis iridis).
Mastery of the slit lamp is essential for all of this content. The COA uses the slit lamp daily to document findings, recognize abnormalities, and flag urgent findings for the physician. Understanding what you are seeing -- and why it matters -- separates a good COA from an exceptional one.
Corneal Conditions
HSV Keratitis (Herpes Simplex)
Classic Slit-Lamp Findings
- Dendritic ulcer: Branching (tree-shaped) epithelial defect with terminal bulbs at the ends -- pathognomonic for HSV epithelial keratitis
- Stains with fluorescein (base of ulcer) and rose bengal/lissamine green (edges -- devitalized cells)
- Reduced corneal sensation (hypoesthesia) -- key distinguishing feature
- Geographic ulcer: coalesced dendrites forming a large amoeboid ulcer (more advanced)
- Disciform keratitis: central stromal edema (endotheliitis) -- coin-shaped opacity
Clinical Importance
- Most common cause of infectious corneal blindness in developed countries
- Recurrences common; stress, UV, illness are triggers
- Treatment: topical antiviral (trifluridine, ganciclovir gel) or oral acyclovir/valacyclovir
- NEVER use steroids alone on HSV epithelial keratitis -- can cause severe worsening
- Corneal sensitivity testing (wisp of cotton) should be part of any corneal exam
Bacterial Keratitis (Corneal Ulcer)
Key Features
- Stromal infiltrate (white/gray opacity in stroma) with overlying epithelial defect
- Pain, photophobia, discharge, decreased vision
- Surrounding corneal edema and stromal infiltrate
- Anterior chamber reaction (cell and flare, possible hypopyon)
- Most common pathogens: Pseudomonas (contact lens), Staphylococcus, Streptococcus
COA Actions
- This is an ophthalmic urgent/emergency -- notify physician immediately
- Corneal scraping for culture (physician performs; COA prepares plates)
- Contact lens history critical (extended wear = Pseudomonas risk)
- Fortified antibiotic drops started promptly
- Document infiltrate size, location, depth, and AC reaction
Keratoconus
Classic Slit-Lamp Signs
- Vogt's striae: Fine vertical stress lines in posterior stroma/Descemet's; disappear with digital pressure
- Fleischer's ring: Iron deposit ring at cone base; best seen with cobalt blue light
- Munson's sign: V-shaped corneal indentation of lower lid on downgaze
- Corneal thinning and protrusion (ectasia), typically inferior
- Scissor reflex on retinoscopy
Additional Features
- Progressive irregular astigmatism beginning in teens/20s
- Corneal topography: inferior steepening ("cone"), asymmetric bowtie pattern
- Associated with atopy, eye rubbing, Down syndrome
- Acute hydrops: sudden Descemet's rupture → severe corneal edema, pain, blur
- Corneal cross-linking (CXL) to halt progression; severe cases may need keratoplasty
Aphakic and Pseudophakic Bullous Keratopathy (ABK/PBK)
Bullous keratopathy is irreversible corneal edema from endothelial cell failure. ABK occurs in eyes without a lens (aphakic), PBK in eyes with an IOL (pseudophakic). Endothelial cell loss from cataract surgery trauma, IOL-endothelial contact, or pre-existing endothelial disease (Fuchs' dystrophy) leads to progressive corneal thickening, stromal edema, and epithelial bullae (fluid blisters), causing severe pain and blurred vision. The slit lamp shows epithelial and stromal edema, bullae, and sometimes subepithelial fibrosis in chronic cases. Treatment: hypertonic saline drops temporarily, corneal transplant (DSAEK, DMEK) for definitive treatment.
Conjunctival Conditions
Pterygium vs. Pinguecula
Pterygium
- Fleshy, vascularized growth
- Extends ONTO the cornea (crosses limbus)
- Usually nasal; can cause astigmatism
- Stocker's line: iron deposit at advancing tip
- May threaten visual axis → surgical excision
- High recurrence rate without adjuvant therapy
Pinguecula
- Yellow-white raised deposit
- Does NOT cross limbus onto cornea
- Degenerated collagen/elastin
- Can become inflamed (pingueculitis)
- Treatment: lubricants; rarely excision
Conjunctivitis: Three Types
Bacterial
Purulent/mucopurulent discharge, papillary reaction, no preauricular node. Treat: topical antibiotics.
Viral (Adenoviral)
Watery discharge, follicular reaction, preauricular lymphadenopathy. Highly contagious. Supportive care only.
Allergic
Bilateral itching (hallmark), ropy discharge, giant papillary reaction (superior palpebral). Treat: antihistamines, mast cell stabilizers.
Subconjunctival Hemorrhage
A bright red patch of blood beneath the conjunctival epithelium, sharply demarcated from surrounding white sclera. Caused by rupture of a small conjunctival blood vessel -- typically from Valsalva (coughing, straining), trauma, or hypertension. Alarming in appearance but almost always benign. Vision and IOP are unaffected. Resolves spontaneously in 2-4 weeks. The COA reassures the patient and checks blood pressure (recurrent SCH without obvious cause warrants BP evaluation). No treatment needed; artificial tears for lubrication if irritation occurs.
Anterior Chamber Assessment
Assessment of the anterior chamber on the slit lamp requires a darkened room and a narrow, high-magnification beam. Flare and cell indicate inflammation (uveitis, trauma), while hypopyon and hyphema indicate more severe pathology requiring urgent attention.
Practice Anterior Segment Questions
Opterio includes COA questions on corneal diseases, conjunctivitis types, anterior chamber grading, and slit-lamp findings with AI explanations.
| Grade | Cells (per 1mm x 1mm field) | Flare (protein, Tyndall effect) |
|---|---|---|
| 0 | <1 cell -- no cells | No flare visible |
| 0.5+ | 1-5 cells | — |
| 1+ | 6-15 cells | Faint -- just perceptible |
| 2+ | 16-25 cells | Moderate -- iris/lens details still clear |
| 3+ | 26-50 cells | Marked -- iris/lens details hazy |
| 4+ | >50 cells (too many to count) | Intense -- fibrinous / "plastic aqueous" |
Hypopyon
A visible layering of white blood cells (pus) in the inferior angle of the anterior chamber. Appears as a white horizontal meniscus level at the bottom of the AC. Indicates severe intraocular inflammation or infection. Causes: bacterial keratitis (with AC involvement), endophthalmitis, severe uveitis, HLA-B27 associated uveitis. Hypopyon is an urgent finding requiring same-day physician evaluation.
Hyphema
Blood in the anterior chamber. Unlike hypopyon, hyphema appears red and often layered at the inferior AC. Graded by percentage of AC filled (Grade 1: <1/3; Grade 2: 1/3-1/2; Grade 3: 1/2-near total; Grade 4: total/blackball hyphema). Causes: trauma (most common), microhyphema from neovascularization. Risk of rebleed (days 2-5), corneal blood staining (with high IOP), and IOP spike. Urgent physician management.
Iris and Lens Conditions
Pseudoexfoliation (PXF) -- Lens and Iris
White/gray fibrillar material deposited on the anterior lens capsule in a classic target pattern: central disc, clear midzone, peripheral granular ring. Material also deposits on the pupil margin, zonules, and trabecular meshwork. Clinically important for two reasons: (1) PXF glaucoma -- the most common identified secondary open-angle glaucoma worldwide; (2) Surgical risk -- PXF weakens zonules (the fibers that hold the lens), increasing risk of zonular dialysis and lens subluxation during phacoemulsification. The COA notes any pupil dilation difficulty (PXF causes poor dilation due to iris sphincter deposits) and flags PXF findings for pre-operative planning.
Phacodonesis (Lens Trembling)
Visible trembling or wobbling of the crystalline lens with eye movements, seen on slit-lamp exam. Indicates zonular weakness or dehiscence (zonular fibers are broken or attenuated). Causes: trauma, pseudoexfoliation, Marfan syndrome, homocystinuria. A lens with phacodonesis is at high risk of subluxation or dislocation. This is a significant surgical risk factor -- the COA should document phacodonesis and ensure the surgeon is aware. In severe cases, the lens may subluxate into the vitreous or anterior chamber, causing acute vision loss and IOP elevation.
Rubeosis Iridis (Iris Neovascularization, NVI)
Abnormal new blood vessels growing on the iris surface and into the anterior chamber angle. Caused by retinal ischemia releasing VEGF (most common causes: proliferative diabetic retinopathy, central retinal vein occlusion, central retinal artery occlusion). Rubeosis can obstruct the trabecular meshwork, causing severe neovascular glaucoma with markedly elevated IOP and rapid vision loss. On slit lamp: fine red vessels on iris surface, especially at pupil margin and angle (best seen with gonioscopy). Urgent treatment needed: anti-VEGF injections + panretinal photocoagulation.