Cataract surgery is the most commonly performed surgical procedure in ophthalmology, and the COA plays a central role in patient assessment, surgical preparation, and post-operative care. The COA exam tests your ability to classify cataract types, understand their clinical presentations, perform pre-operative measurements accurately, and describe the basic principles of IOL selection.
A cataract is any opacity of the crystalline lens that reduces visual clarity. The lens is composed of a central nucleus, surrounding cortex, and a thin capsular bag (anterior and posterior capsule). Cataracts are classified by their location within the lens, their morphologic appearance, and their etiology. On the COA exam, morphologic classification is the primary framework tested.
This guide covers the three main cataract types and their distinguishing features, the LOCS III grading system used for documentation, the visual symptoms each type produces, the pre-operative workup the COA performs, IOL types and their indications, and the COA's role at each stage of the surgical process.
The Three Main Cataract Types
On the slit lamp, cataracts are examined in multiple illumination modes: direct illumination (broad beam, optic section) reveals depth and density, while retroillumination (reflected light from the fundus or iris) shows opacity distribution and morphology most clearly. The COA should be able to describe cataract findings using proper terminology.
1. Nuclear Cataract (NC)
Characteristics
- Located in the central nucleus of the lens
- Progressive yellowing, browning (brunescence)
- Dense central opacity on optic section view
- LOCS III: N0 (clear) → N6 (brunescent)
- Most common type; increases with age
Symptoms
- Gradual distance blur
- Myopic shift (index myopia -- may improve near vision temporarily)
- Reduced contrast sensitivity
- Color shift toward yellow/brown
- Less dramatic glare than PSC
Etiology: Aging (most common), UV radiation exposure, smoking, systemic factors. The "second sight" phenomenon -- temporary improvement in near vision as nuclear sclerosis increases the refractive index of the lens -- is a classic exam teaching point.
2. Cortical Cataract (C)
Characteristics
- Located in the outer lens cortex
- Spoke-like (cuneiform) opacities radiating from periphery
- Best seen on retroillumination
- LOCS III: C0-C5 (% of cortex affected)
- Vacuoles and water clefts in early stages
Symptoms
- Glare and light scatter (especially when spokes are central)
- Monocular diplopia (double images in one eye)
- Variable visual acuity depending on spoke position
- Less myopic shift than nuclear
Etiology: Diabetes (most strongly associated with cortical cataracts), UV-B radiation, dehydration. Diabetic patients may develop cataracts decades earlier than non-diabetic patients. The combination of diabetes + cortical cataract is a high-yield COA exam pairing.
3. Posterior Subcapsular Cataract (PSC)
Characteristics
- Located at posterior pole, under posterior capsule
- Granular/plaque-like opacity on retroillumination
- Appears as a bright frosted area on retroillumination
- LOCS III: P0-P5
- Can cause severe symptoms even when small
Symptoms
- Severe glare and halos (especially at night / oncoming headlights)
- Disproportionate near vision difficulty
- Better vision in dim light (dilated pupil bypasses opacity)
- Rapid progression compared to nuclear
Etiology: Long-term systemic or topical corticosteroids (most important association), diabetes, radiation exposure, trauma. PSC is the type most likely to cause disproportionate symptoms -- a patient with 20/40 vision and PSC may be functionally more disabled than a patient with 20/80 from nuclear sclerosis.
LOCS III Grading System
The Lens Opacities Classification System III (LOCS III) is the standardized photographic grading scale used to document and compare cataract severity. It uses reference photographs for objective comparison, making it useful for tracking progression and for clinical research. The COA should understand the grading scales and the examination techniques used for each.
| Cataract Type | Scale | Grade Range | Exam Technique |
|---|---|---|---|
| Nuclear Opalescence | NO / NC | N0 (clear) → N6 (brunescent) | Slit-lamp optic section (2mm beam at 45°) |
| Cortical | C | C0 → C5 (% area involved) | Retroillumination against red fundus reflex |
| Posterior Subcapsular | P | P0 → P5 | Retroillumination (most sensitive) |
Exam Pearl: Grading vs. Surgical Decision
Cataract surgery is indicated when the patient's visual function is impaired enough to affect their quality of life -- not based on cataract grade alone. A patient with N3 nuclear sclerosis who drives at night and notices halos may qualify for surgery before a patient with N4 who is happy reading at home. The COA documents symptoms and BCVA; the physician makes the surgical decision.
Pre-Operative Workup: The COA's Core Role
Pre-operative cataract measurements are among the most high-stakes tasks a COA performs. Errors in biometry or keratometry translate directly into a patient walking out of surgery with the wrong refractive outcome. Every measurement has specific technique requirements, quality indicators, and potential sources of error the COA must understand.
Practice Cataract Questions for the COA Exam
Opterio includes surgical prep, lens classification, and pre-op measurement questions with full AI explanations.
A-Scan Biometry (Axial Length)
Measures the axial length of the eye (cornea to retina) using ultrasound (contact/immersion A-scan) or optical coherence (IOL Master, Lenstar). Axial length is the single most important variable in IOL power calculation. A 1mm error in axial length produces approximately 2.5D of IOL power error. The IOL Master (optical biometry) is preferred as it is non-contact, faster, and more reproducible. Contact A-scan can compress the cornea, artificially shortening axial length and leading to a hyperopic outcome.
Normal axial length: ~24mm. Short eyes (<22mm, hyperopia) and long eyes (>26mm, high myopia) require special IOL formulas (Barrett, Kane) for accuracy.
Keratometry (K Readings)
Measures corneal curvature in two principal meridians. Average K readings (approximately 43.5D) are used in IOL power formulas. For toric IOL calculation, accurate K readings and corneal astigmatism axis are critical -- an error of just a few degrees in axis marking causes significant residual astigmatism. Manual keratometry (Javal-Schiötz or auto-keratometer) measures the central 3mm; corneal topography (Placido disc) maps the entire corneal surface and detects irregular astigmatism.
COA tip: Repeat keratometry on any day contact lens wearers were wearing lenses -- lenses temporarily distort the corneal surface, giving inaccurate K readings. Patients should be instructed to discontinue soft lenses 1-2 weeks before measurement.
Specular Microscopy (Endothelial Cell Count)
Counts and images the corneal endothelial cells (the pump cells that maintain corneal clarity). Endothelial cells do not regenerate. Phacoemulsification (cataract surgery using ultrasound) temporarily stresses and can reduce endothelial cell count. Patients with low cell counts (<1,500 cells/mm²) are at higher risk for corneal decompensation post-operatively (bullous keratopathy).
Normal ECC: ~2,500-3,000 cells/mm² in adults. Risk increases significantly below 1,000 cells/mm². COA documents the cell count, coefficient of variation (CV), and hexagonality. High CV and low hexagonality indicate stressed endothelium.
Pachymetry (Corneal Thickness)
Measures central corneal thickness (CCT) using ultrasound or optical methods. CCT averages ~550 microns. Relevant for cataract surgery in the context of combined procedures (cataract + corneal transplant planning) and for assessing risk in patients with compromised endothelium. Also affects IOP measurement accuracy (thick corneas read artificially high; thin corneas read artificially low).
Intraocular Lens (IOL) Types
After the cataractous natural lens is removed via phacoemulsification, an IOL is implanted in the capsular bag. IOL selection depends on the patient's refraction goals, corneal status, lifestyle needs, and financial capacity. The COA should understand the categories well enough to assist in patient counseling and pre-operative documentation.
| IOL Type | Focal Point(s) | Best For | Trade-Offs |
|---|---|---|---|
| Monofocal | One distance (usually distance) | Most patients; insurance-covered | Reading glasses needed for near |
| Toric | One distance (distance) + astigmatism correction | Patients with ≥0.75D corneal astigmatism | Must align to correct axis; rotation causes refractive surprise |
| Multifocal | Distance + near (2-3 focal zones) | Motivated patients wanting spectacle independence | Halos, glare, reduced contrast; premium cost |
| EDOF (Extended Depth of Focus) | Distance + intermediate (elongated focal range) | Computer users; fewer dysphotopsias than multifocal | Near vision may require reading glasses; premium cost |
| Accommodating | Shifts focal point with ciliary muscle action | Limited range of accommodation; some near improvement | Variable results; may be less predictable |
Phacoemulsification: What the COA Needs to Know
Phacoemulsification (phaco) is the standard surgical technique for cataract removal. A small-incision approach uses ultrasound energy to break up the cataractous lens (emulsification) and aspirate it from the capsular bag, leaving the posterior capsule intact to support the IOL.
COA Pre-Op Role
- Biometry and keratometry measurements
- Dilate pupils (ordered mydriatics)
- Instill topical antibiotics and NSAIDs
- Verify informed consent documentation
- Toric axis marking (if applicable)
- Vital signs and allergy confirmation
COA Intra-Op Role
- Maintain sterile field
- Prepare and pass instruments
- BSS (balanced salt solution) preparation
- OVD (viscoelastic) preparation
- IOL cartridge loading assistance
- Monitor patient positioning
COA Post-Op Role
- Check IOP at 1-hour post-op
- Visual acuity measurement
- Slit-lamp check (corneal edema, wound)
- Patient education (drop schedule)
- Shield/patch application
- Activity restriction counseling
Posterior Capsule Opacification (PCO)
PCO ("secondary cataract") is the most common post-operative complication of cataract surgery, occurring in 20-40% of patients within 2-5 years. Residual lens epithelial cells proliferate on the posterior capsule, causing it to become hazy and reducing visual acuity. Treatment is Nd:YAG laser posterior capsulotomy -- a quick in-office procedure. The COA may perform pre-YAG visual acuity assessment and post-YAG IOP checks (YAG capsulotomy can temporarily spike IOP).