How Are Cataracts Graded and What Do the Grades Mean?

Cataracts are graded by comparing the clouding in your lens against a set of standard reference photographs, with the most widely used system scoring different parts of the lens on separate scales. The dominant framework, called LOCS III, assigns decimal grades for nuclear cloudiness, nuclear color change, cortical opacity, and posterior subcapsular opacity, each on its own scale. The grade your eye doctor assigns tells them how far along the cataract has progressed, but what that grade means for your daily vision depends heavily on which type of cataract you have and where in the lens it sits.

Why Cataracts Are Not Graded on a Single Scale

A cataract is not one disease with one progression path. The lens has distinct zones, and clouding in each zone behaves differently, causes different visual symptoms, and worsens at different rates. That is why every major grading system scores cataracts by type rather than lumping them together into a single number. The three main types are nuclear, cortical, and posterior subcapsular cataracts, and each gets its own grade.

Nuclear cataracts form in the center of the lens, where structural proteins gradually become oxidized and cross-linked over decades. Four hallmarks of this process are coloration, oxidation, insolubility, and covalent cross-linking of lens proteins, each worsening as the cataract advances.¹ In severe nuclear cataracts, more than 90% of certain amino acid residues in the lens center are oxidized.² The result is a lens that turns progressively yellow, then amber, then brown, scattering light and blurring your distance vision.

Cortical cataracts develop in the outer shell of the lens, where mechanical ruptures in the lens fiber create fluid-filled clefts. These wedge-shaped opacities start at the periphery and grow inward like spokes on a wheel. Posterior subcapsular cataracts form a plaque on the back surface of the lens, right in the path of light entering the eye. Because of their location, even a small posterior subcapsular opacity can cause disproportionate problems with glare and reading vision.

The LOCS III System

LOCS III, short for Lens Opacities Classification System III, is the standard grading framework used in both clinical practice and research worldwide. It was developed at the Center for Clinical Cataract Research in Boston and published in 1993. The system works by having a trained examiner compare what they see at the slit lamp against a set of standard photographs, then assign a decimal score for each feature of the cataract.³

LOCS III grades four features independently:

• Nuclear opalescence (NO): how cloudy the lens nucleus looks, scored against six slit-lamp reference images on a scale from 0.1 to 6.9.
• Nuclear color (NC): how yellow or brown the nucleus has become, also scored against six reference images on the same range.
• Cortical opacity (C): the area of the lens cortex covered by white wedge-shaped opacities, scored against five retroillumination images on a scale from 0.1 to 5.9.
• Posterior subcapsular opacity (P): the size of any plaque on the back surface of the lens, also scored against five retroillumination images from 0.1 to 5.9.

A healthy, clear lens scores NO0/NC0/C0/P0.⁴ The decimal scale is one of the system’s key innovations. Earlier versions of LOCS used whole-number grades with wide gaps between them, making it hard to detect small changes over time. By allowing graders to place a score anywhere on the decimal scale between reference images, LOCS III dramatically tightened the margin of disagreement between examiners. The 95% tolerance limits dropped to 0.7 for nuclear opalescence, 0.7 for nuclear color, 0.5 for cortical cataract, and 1.0 for posterior subcapsular cataract.³

In practical terms, if two experienced examiners grade the same eye, their scores for nuclear opalescence will typically fall within about 0.7 of each other. That level of agreement makes it feasible to track cataract progression over months or years and to compare results across different clinical trials.

Other Grading Systems Still in Use

LOCS III is the dominant system, but it is not the only one. The Oxford Clinical Cataract Classification and Grading System uses standard diagrams mounted next to the slit lamp rather than photographs. Features are classified by their shape and location, then graded in equal-interval steps. The system also includes a separate assessment of how much the cataract degrades the image of the retina, using a device that projects a resolution target through the lens.⁵ Head-to-head comparisons show that the relationship between Oxford and LOCS III grades is linear for nuclear and posterior subcapsular cataracts, with good agreement between observers using either system.⁶

For large epidemiological surveys in resource-limited settings, a simplified three-level grading system was developed by the WHO Cataract Grading Group. It uses just three standard photos for nuclear cataract and three severity levels for each cataract type, and field testing showed fair to very good agreement even among ophthalmology residents with minimal training.⁷ The tradeoff is obvious: you lose the fine granularity of LOCS III, but you gain a system that can be deployed by less specialized examiners across entire populations.

How Different Grades Affect Your Vision

A higher grade on any of the LOCS III scales means more light scatter and more visual impairment, but the specific symptoms depend on which type of cataract is progressing. Among patients with different cataract types, nuclear sclerosis tends to cause the worst uncorrected distance vision, while posterior subcapsular cataracts hit best-corrected visual acuity hardest.⁸ Both visual acuity and contrast sensitivity drop as cataract severity increases across all types, and the contrast sensitivity loss shows up at every spatial frequency tested.⁹

An interesting wrinkle is that standard eye-chart acuity does not always capture early cataract damage. Research has shown that testing with small, low-contrast letters can reveal cataract-related impairment that a normal eye chart misses entirely.¹⁰ This explains why some people with early cataracts complain of difficulty driving at night or reading in dim light even when their eye-chart score looks fine. Glare sensitivity also worsens as cataract grade climbs, and it is most common with posterior subcapsular cataracts, where more than nine out of ten patients report it.⁸

Cortical cataracts produce a somewhat different pattern. Because the spoke-like opacities are at the periphery of the lens, the center may remain relatively clear for a long time. Contrast sensitivity tends to be better preserved in cortical cataracts compared to the other two types, but glare from oncoming headlights can be severe because light scatters off the cortical wedges.

The Myopic Shift and “Second Sight”

One of the more surprising effects of rising nuclear cataract grade is a shift toward nearsightedness. As the nucleus becomes denser and more yellow, it changes the way light bends through the lens. People in their 60s and 70s who had been wearing reading glasses for years sometimes find they can suddenly read again without them. This phenomenon, sometimes called “second sight,” is not a sign of improvement. It is a sign that the nuclear cataract is advancing.

Research quantifies this clearly. In one study, people without nuclear cataracts showed a slight yearly drift toward farsightedness of about 0.05 diopters. In those with nuclear cataracts, that drift vanished, and a meaningful myopic shift appeared only in people whose nuclear opacity reached level 4 or higher on the grading scale.¹¹ Another study found that the nuclear cataract group had a myopic shift averaging about a third of a diopter compared to control eyes, with over half of the nuclear cataract eyes exceeding the normal range.¹²

This grade-dependent refractive change matters because it means your eyeglass prescription may need updating more frequently as a nuclear cataract progresses, and each update is a temporary fix. Cortical cataracts, by contrast, tend not to produce the same myopic shift. They more commonly cause changes in astigmatism, where the amount and orientation of astigmatic error shifts as the cortical spokes grow.¹³

Objective Tools That Bypass Human Judgment

LOCS III depends on a trained human comparing what they see to a set of photos, and that introduces some subjectivity. Over the past two decades, several objective technologies have emerged that can measure lens density with a number rather than a judgment call.

Scheimpflug imaging uses a camera with a tilted lens to photograph a cross-section of the eye’s own lens. Software then measures the density of light scatter in the nucleus and converts it to a numerical value. Studies show a strong positive correlation between Scheimpflug-measured nuclear lens density and LOCS III grades for both nuclear opalescence and nuclear color.¹⁴ The technology is built into devices already common in eye clinics, making it practical to add an objective density measurement to the routine exam. That said, when automated cataract-staging software built into these devices tries to assign its own grade, the match with LOCS III becomes less reliable.¹⁵

Anterior segment optical coherence tomography (OCT) offers another objective approach. By analyzing how the lens scatters a beam of near-infrared light, OCT can generate pixel-intensity maps of lens density. Nuclear density measured this way shows a positive linear correlation with LOCS III grades, and the nuclear region of the lens correlates more strongly than whole-lens density does.¹⁶ Neither Scheimpflug nor OCT has replaced LOCS III as the clinical standard, but both add a reproducible number to the conversation, which is especially valuable in research trials tracking whether a drug slows cataract progression.

Another device, the double-pass optical quality analyzer, measures something called the objective scatter index, which quantifies how much light the cataract scatters before it reaches the retina. Among various objective metrics studied, the scatter index showed the strongest correlation with patients’ self-reported visual function scores.¹⁷ That finding hints that scatter-based measurement may come closer to capturing what the patient actually experiences than a density number alone.

What Grade Means for Surgery

Cataract surgery decisions are not made on grade alone, but the grade matters in ways that go beyond just “is the cataract bad enough.” A higher nuclear sclerosis grade means the lens nucleus is physically harder, and that translates directly into how much ultrasound energy the surgeon needs to break it apart during the standard procedure. Each step up in nuclear sclerosis grade is associated with a measurable increase in cumulative dissipated energy, the total ultrasound power delivered during the operation.¹⁸ More energy means more heat, more turbulence inside the eye, and more stress on the delicate corneal endothelial cells that keep the cornea clear.

For extremely dense, “rock hard” cataracts, specialized surgical techniques may be required to protect the cornea. Even with protective maneuvers, corneal endothelial cell loss of about 7% at three months has been documented in these advanced cases.¹⁹ This is one reason surgeons sometimes recommend operating before a cataract reaches the most advanced grades: waiting too long can make the procedure technically harder and increase the risk of corneal complications.

A subtlety that sometimes frustrates patients is the gap between the clinical grade and the functional improvement they experience after surgery. One study found that while patients with more advanced cataracts showed significantly greater improvements in visual acuity measurements after surgery, their functional improvement in daily activities was not significantly different from those with less advanced cataracts.²⁰ In other words, a person with a moderate cataract who struggles with driving at dusk may experience just as much real-world benefit from surgery as someone whose eye chart says they are much worse off.

When Cataracts Are Left Too Long

Grading systems assume a cataract that is being monitored and eventually treated. But when cataracts go unaddressed for years, they can progress beyond what the standard scales were designed to measure. A hypermature cataract is one in which the cortical lens fibers have broken down extensively, sometimes to the point where the lens contents liquefy. In one pattern called a Morgagnian cataract, the hard brown nucleus sinks to the bottom of the liquefied cortex.

The concern with hypermature cataracts goes beyond just poor vision. As the degraded lens proteins leak through the capsule into the front chamber of the eye, they can trigger an inflammatory response. Immune cells rush in to clean up the leaked proteins, and the combination of protein debris and inflammatory cells clogs the eye’s drainage system, causing a rapid, painful spike in eye pressure known as phacolytic glaucoma.²¹ This is a surgical emergency, and it underscores why regular grading and monitoring matter: catching a cataract in the moderate-to-advanced range and operating electively is far safer than dealing with a hypermature lens and its secondary complications.

AI-Assisted Grading

Deep learning systems trained on thousands of slit-lamp and retroillumination photographs can now grade cataracts with accuracy that approaches human experts. One large validation study reported that an AI platform could detect the presence of nuclear cataract with accuracy above 98% and predict LOCS III severity grades for nuclear opalescence and nuclear color with accuracy above 90%. For cortical and posterior subcapsular opacities, detection accuracy was above 92%, and grading accuracy ranged from roughly 88% to 91%.²²

The appeal is reproducibility. A human grader’s assessment can shift depending on the lighting conditions in the exam room, their level of fatigue, and how recently they recalibrated their mental reference images. An AI system gives the same answer every time for the same photograph. As these tools move from research prototypes into clinical software, they could standardize grading across clinics and make it easier to track small changes over time, particularly in trials testing whether supplements, eye drops, or other interventions slow cataract progression.

Grading Cataracts in Children

The systems described above were developed for age-related cataracts in adults. Congenital cataracts in children are a different challenge. They vary enormously in shape, size, location, and associated eye abnormalities, and no single adult grading scale captures that variety well.

Recent research has tried a different approach: clustering children’s cataracts into groups based on multiple features at once, including the length of the eye, the presence of retinal or macular abnormalities, and whether the opacity involves the back of the lens. One study of 164 children divided their eyes into two clusters. The cluster with shorter eye length, almost no macular abnormalities, and no posterior cataracts had much better visual outcomes, with eyes in that group being roughly twice as likely to achieve good corrected vision compared to the other cluster.²³ This multidimensional approach acknowledges that in children, the cataract itself is only part of the picture. The health of the retina and the development of the eye as a whole matter at least as much for predicting how well the child will see after treatment.

Lens Clouding Across Species

Cataract grading is not limited to human eyes. Veterinary ophthalmologists face a similar need to track lens opacity in animals, whether for companion animal care or for studying aging and eye disease in model organisms. One approach used in comparative studies is to determine the age at which half the individuals in a population show lens opacity. In a survey of 2,000 companion dogs, this milestone fell at a mean age of about 9.4 years.²⁴ Comparing these values across species gives researchers insight into how lens biology and lifespan interact, and whether protective mechanisms that work in some animals could eventually inform human cataract prevention. The grading challenge in veterinary settings is complicated by the fact that animal lenses differ in size, shape, and baseline transparency, so human grading scales cannot simply be borrowed and applied directly.