LOCS Cataract Grading System
Overview and Purpose
The Lens Opacities Classification System (LOCS) is a standardized photographic reference system that grades cataracts by type and severity using slit-lamp and retroillumination images, with LOCS III being the most widely used version employing decimal scoring for nuclear opalescence, nuclear color, cortical, and posterior subcapsular opacities. 1, 2
The system serves multiple clinical functions beyond simple documentation:
- Tracks disease progression over time, with posterior subcapsular cataracts typically progressing more rapidly than nuclear and cortical types 2
- Predicts surgical complexity and phacoemulsification energy requirements, with exponentially greater ultrasonic energy needed as nuclear color and opalescence scores increase 3
- Facilitates research standardization by allowing comparison across studies and meta-analyses through linear calibration between different grading systems 4
Classification of Cataract Types
Nuclear Cataracts
Nuclear cataracts involve central lens opacification or discoloration, classified as brunescent (brown), opalescent (white), or both 1, 2. These cataracts progress slowly and predominantly affect distance vision rather than near vision 2, 5. LOCS grades nuclear cataracts on two scales: nuclear opalescence (NO) and nuclear color (NC), with annual progression rates of approximately 12.4% 6, 7.
Cortical Cataracts
Cortical opacities appear as spoke-like or oil droplet patterns, either centrally or peripherally located 1, 2. Patients characteristically complain of glare symptoms 2. When the entire cortex becomes white and opaque, it is termed a mature cortical cataract 1. These progress at approximately 17.9% annually 6.
Posterior Subcapsular Cataracts (PSC)
PSC cataracts are located just inside the posterior lens capsule and cause substantial visual impairment when involving the axial region 2, 5. These cataracts produce glare and poor vision in bright light, with near vision typically more affected than distance vision due to miosis during near accommodation 2, 5. PSC cataracts are more common in younger patients and progress more rapidly than other types, with annual progression of 6.5% 2, 6.
Less Common Types
Anterior subcapsular, anterior polar, and posterior polar cataracts occur less frequently 1, 2. Anterior polar cataracts are localized opacities at the anterior lens pole causing mild to moderate amblyopia, while posterior polar cataracts are localized posterior opacities with similar visual impact 5.
LOCS Grading Methodology
LOCS II System
LOCS II uses colored slit-lamp and retroillumination transparencies with four nuclear standards (grading opalescence and color), five cortical standards, and four subcapsular standards 7. The system demonstrates very good interobserver reproducibility at the slit lamp and excellent intraobserver reproducibility 7.
A critical caveat: reliability varies by opacity severity and coexisting cataracts 8. Clinical grading of posterior subcapsular opacities becomes more reliable as severity increases, while more advanced coexisting opacities decrease agreement in diagnosing nuclear (but not cortical or PSC) opacities 8.
LOCS III System
LOCS III employs photographic transparencies as standards and assigns decimalized scores for four characteristics: nuclear opalescence (NO), nuclear color (NC), cortical cataract (C), and posterior subcapsular cataract (P) 3, 4. The relationship between LOCS III and other systems like the Oxford Clinical Cataract Classification and Grading System is linear for nuclear and PSC cataracts, but requires squaring LOCS III scores for cortical cataracts 4.
Clinical Application for Treatment Decisions
Surgical Planning
Preoperative LOCS III grading directly predicts phacoemulsification parameters 3. There are exponential relationships between machine-measured phacoemulsification time and nuclear color (R² = 0.48) and nuclear opalescence (R² = 0.40) 3. This allows surgeons to create customized operative plans with anticipated ultrasonic energy expenditure and balanced salt solution volume requirements 3.
Functional Assessment Beyond Grading
LOCS grading alone does not determine surgical timing—the decision to operate depends on functional impairment and patient preference 1. The American Academy of Ophthalmology emphasizes that surgery should be performed when it will enhance patient function and when the informed patient elects this option, with timing mutually agreeable between patient and surgeon 1.
Additional functional assessments complement LOCS grading:
- Glare testing is particularly valuable for cortical and PSC cataracts, which significantly impact vision in bright light 2
- Contrast sensitivity testing may reveal functional vision loss not detected by standard visual acuity alone 2
- Wavefront imaging can detect visual aberrations from even mild cataracts 2
Indications Beyond Visual Symptoms
Surgery may be indicated even with less severe LOCS grades when needed for improved visualization and management of coexisting ocular disease such as glaucoma, macular degeneration, or diabetic retinopathy 1.
Common Pitfalls
Do not rely solely on LOCS grading to determine surgical candidacy—the system quantifies opacity severity but does not directly measure functional impairment or quality of life impact 1, 2. A patient with moderate LOCS grades but significant functional limitation may benefit from surgery more than a patient with higher grades but minimal symptoms.
Recognize that grading reliability decreases with mild PSC opacities and in eyes with severe coexisting nuclear or cortical opacities 8. In these cases, functional testing becomes even more critical for decision-making.
Several classification systems exist, making direct comparison between studies difficult 1. When reviewing literature or comparing progression rates, verify which system was used and apply appropriate conversion factors if comparing LOCS III to other systems 4.