Cataract Grading — LOCS III Classification System

About

The LOCS III (Lens Opacities Classification System III) was developed by Chylack et al. (1993) at Harvard Medical School and published in Archives of Ophthalmology. It succeeded earlier LOCS I and LOCS II systems, providing improved inter-grader reliability and finer gradations. LOCS III uses standardized color slit-lamp and retroillumination photographs as reference standards for grading. The system grades three types of cataract independently: Nuclear sclerosis (opacification of the central lens nucleus) — graded from 0-5 based on increased nuclear yellowing/brunescence and opalescence on slit-lamp examination. Cortical cataract (opacification of the lens cortex) — graded from 0-5 based on the circumferential extent of cortical spokes/opacities on retroillumination. Posterior subcapsular cataract (PSC — opacification at the posterior lens capsule) — graded from 0-5 based on the area of involvement on retroillumination. The LOCS III is the most widely used cataract grading system in clinical research and has been validated against other measures of visual function including contrast sensitivity and glare disability, not only visual acuity.

Formula

Nuclear (0-5), Cortical (0-5), Posterior Subcapsular (0-5)

Each cataract type is graded independently on a 0-5 scale. Nuclear grade: 0 = clear lens, 1 = mild nuclear opalescence, 2 = moderate nuclear yellowing/opalescence, 3 = obvious nuclear brunescence, 4 = advanced nuclear brunescence, 5 = very advanced nuclear opacity/cataract. Cortical grade: 0 = no cortical opacities, 1 = minimal cortical spokes (<5%), 2 = moderate cortical involvement (5-25%), 3 = significant involvement (25-50%), 4 = extensive involvement (50-75%), 5 = very extensive involvement (>75%). PSC grade: 0 = no posterior subcapsular opacity, 1 = minimal PSC (<5% of posterior capsule), 2 = moderate PSC area, 3 = significant PSC, 4 = extensive PSC, 5 = very extensive PSC. The overall cataract severity is usually determined by the highest grade among the three types. Surgical decision-making also incorporates visual acuity, visual function questionnaire, glare testing, and patient-reported visual disability.

Score Interpretation

Cataract is the leading cause of reversible blindness worldwide, affecting over 95 million people globally. The LOCS III grading system is the standard for: (1) Clinical documentation — standardized terminology for describing cataract type and severity in medical records. (2) Surgical decision-making — grades ≥3 are typically associated with visual function impairment warranting surgical consideration, though patient-reported symptoms and visual needs are equally important. (3) Surgical planning — nuclear grade affects phacoemulsification energy requirements and may influence IOL power calculation formulas. (4) Research — LOCS III is the standard classification for clinical trials on cataract prevention (e.g., AREDS, vitamin studies). (5) Medico-legal — provides objective documentation of pre-operative cataract severity. Correlation with visual acuity is moderate — some patients with grade 3-4 cataracts maintain 20/40 vision while others with grade 2 have significant glare disability, underscoring the need for functional assessment alongside morphological grading.

Reference Ranges