LOCS Systems

Cataract is the leading cause of blindness worldwide. Approximately 20 million Americans, age 40 and over, have cataracts. Cataracts cloud the ocular lens and impair vision. Less severe clouding of the lens is called lens opacification, and this may be present without any visual symptoms.

    The severity of the visual impairment can vary from no symptoms at all, to slight blur and glare, to complete blindness. There are many different factors identified as potential causes of lens opacification. These include: age, gender (females more than males), smoking, ultra-violet radiation, ionizing radiation, and several diseases (e.g. diabetes mellitus) and some drugs (e.g. corticosteroids). The LOCS II and III systems of classifying lens opacities were developed for use in epidemiological studies of the risk factors of age-related cataract. It was anticipated that these systems would be used to assess the efficacy of anti-cataract medications as they were developed. Alas, no such medications have been developed. It is, perhaps, ironic that the LOCS systems have found their widest application in assessments of actual or potential lens toxicity of new systemic drugs. 
    Leo T. Chylack, Jr., M.D. first-authored publications describing the Lens Opacities Classification System (LOCS I, II, and III). These systems constituted a new method of classifying age-related lens opacification and browning.

    The LOCS systems have been through three iterations (I-III), each one containing improvements. The LOCS system allows standardized, validated, classification of the type and severity of age-related cataractous change viewable by slit lamp. It does not require lens photographs, although it has been widely used to grade standardized lens images. Since the late 1980s, LOCS II and III have become widely accepted clinical standards for assessing cataract severity and type. LOCS II, and to a greater degree LOCS III, have become important tools for pharmaceutical companies needing to evaluate the potential for lenticular toxicity of newly discovered systemic drugs in Phase I, II, and III clinical studies. For the past several years, studies in which LOCS methods have been used have helped the pharmaceutical sponsors and the FDA to accurately assess the cataractogenic risks, if any, of new therapeutic products.

Both LOCS II and LOCS III are systems for classifying the type and severity of age-related lens opacification at the slitlamp during an ocular examination with the pupils maximally dilated. The eye specialist or scientist examines the patient’s ocular lenses under standardized viewing conditions. The appearance of the lens is compared to standard images in the LOCS II or III transparency and the severity of four features of the lens:
  1. nuclear opalescence (NO),
  2. nuclear color (NC),
  3. the aggregate extent of cortical (C) opacification, and
  4. posterior subcapsular (P) opacification
NO is a proxy for the severity of nuclear opacification; NC is a proxy for the intensity of nuclear brunescence, and C and P are proxies for the aggregate extent of opacification of the cortex and posterior subcapsular areas, respectively. There is no need for specialized lens photographs in LOCS III grading; rather, the eye specialist or scientist may assess the lens consistently and accurately during a routine eye exam.

LOCS II: The LOCS II standard transparency contains fewer standard images than that of LOCS III. As reference standards, it uses slit images of various stages of nuclear cataract and nuclear color, and retro-illumination images of various cortical and posterior subcapsular cataracts. LOCS II uses an integer scale (e.g. I, II, III, IV, etc.) and intermediate grades (e.g. between I and II, II and III, III and IV, etc) are not possible.

    LOCS II was widely used in epidemiological studies of risk factors for the three classes of age-related cataract (see bibliography), but when users expressed interest in assessing the effect of various factors on the rates of cataract progression, a more robust system of grading longitudinal changes (rates) was needed. This led to the introduction of LOCS III.

LOCS III: The LOCS III transparency contains an expanded set of standard images on all four scales, and the grading scales are decimalized with each standard illustrating severity representing 1.0, 2.0, 3.0 etc. Intermediate grades (e.g.2.6) are possible with LOCS III.

   The system is designed to be used at a slit lamp configured for LOCS III grading. The configuration is simple and does not interfere with the other uses of the slit lamp. LOCS III has proved to be an effective means of grading longitudinal changes in each of the four features (NO, NC, C and P) of the lens, and it has been used in a wide variety of basic science applications, pharmaceutical trials, cross-sectional and longitudinal epidemiological studies, surgical studies, and in the testing and development of surgical instruments and methods (see the bibliography section in the LOCS III Instructions). If you are interested in reading a detailed comparison of LOCS II and III, please see the original publication describing LOCS III.

​​​​​​​LOCS III is the system currently recommended by Chylack Incorporated. CI offers the LOCS III Standard Transparency Set with comprehensive grading instructions and the original publication describing LOCS III for sale via this website. 

Which one is right for my project?
Since LOCS III was developed to obviate the limitations of LOCS II, it is correct to say that LOCS III can do all that LOCS II can do, but better. LOCS II continues to find application in certain population-based studies aimed at describing the types and severity of lens opacification in various regions or countries. In such a setting precise measures of severity are usually not necessary. Where precise grading is needed or accurate measures of the rates of lens opacification are required,  LOCS III is the better system. If there is doubt as to which system might be best for your application, please contact Dr. Chylack.