Apportionment in an Eye Claim: A Worked Hypothetical

Marc H. Shomer, MD, PhD, QME · Published August 3, 2026 · Last reviewed August 3, 2026

The Statutory Question, Stated Precisely

Two features of the statute shape everything that follows. First, apportionment is to the causation of the permanent disability, not of the injury: the question is not what caused the accident but what accounts for the impairment remaining once the condition is permanent and stationary. Second, the evaluator must address all causes — including pre-existing disease, prior injury, and ordinary progression of underlying pathology. A report reciting the statutory language without applying it to the record fails for the same reason a report that stops at impairment does. See apportionment of ophthalmic injury; this article is about how the reasoning gets built.

The Hypothetical Facts

A 58-year-old structural welder is struck in the left eye by a fragment of hot slag that penetrates the corneal epithelium and stroma. He is treated in an emergency department, referred to an ophthalmologist, and undergoes removal of the foreign body and a rust ring. The injury heals with a dense central corneal scar. Eighteen months later, with the scar stable and no further surgery contemplated, best-corrected acuity in the left eye is 20/80; the right eye is 20/20.

The prior records — obtained, not assumed — show that four years before the injury an optometrist documented a nuclear sclerotic cataract in the left eye with best-corrected acuity of 20/40, noting gradual progression at two subsequent annual visits, with 20/50 recorded fourteen months before the injury. There is no history of prior eye trauma, diabetes, or glaucoma.

Step One: Establish the Impairment Before Apportioning It

Apportionment operates on a number that must exist first. The evaluator rates permanent impairment under the AMA Guides to the Evaluation of Permanent Impairment, 5th Edition, using best-corrected visual acuity in each eye together with visual field data, expressed as whole person impairment. Reduced acuity in one eye with a normal fellow eye produces a modest whole person figure, because binocular function is substantially preserved — a point worth grasping before valuation assumptions harden, and developed under whole person impairment for vision. Rating also requires that the condition be permanent and stationary; in the hypothetical, eighteen months have elapsed, the scar is mature, refraction is stable, and no corneal procedure is planned. See permanent and stationary evaluations.

Step Two: Identify Every Candidate Cause

The evaluator lists the plausible contributors to the current 20/80 acuity and tests each against the record:

• The corneal scar from the industrial foreign body — visible on slit-lamp examination, central, dense, and absent from every pre-injury record.
• The nuclear cataract — documented before the injury, progressing on a recorded trajectory, and still present.
• Irregular astigmatism from the scarred corneal surface — measurable on corneal topography and distinguishable from lenticular opacity by its optical signature.
• Age-related change independent of both.
• Alternative pathology — macular disease, optic neuropathy, amblyopia — excluded here by normal optical coherence tomography (OCT, cross-sectional retinal imaging), normal pupillary responses, and a childhood vision history without patching.

Writing this list matters: it forces the evaluator to state what was considered and rejected — the difference between an opinion a judge can follow and an assertion a judge must take on faith.

Step Three: Assemble the Evidence for the Industrial Portion

The industrial contribution rests on citable facts rather than temporal sequence alone: the scar is anatomically consistent with a penetrating foreign body and sits centrally in the visual axis; no scar appears in any pre-injury examination; the emergency and treating records document mechanism and removal; and corneal topography shows irregular astigmatism corresponding to the scar, which refraction cannot fully neutralize.

A useful device is the counterfactual: what would this eye most likely see today had the injury not occurred? The pre-injury records answer with unusual precision. Acuity was 20/40 four years earlier and 20/50 fourteen months before the injury, progressing gradually, so absent the injury the cataract alone would be expected to produce acuity somewhat worse than 20/50 by the evaluation date. The gap between that projection and the measured 20/80 is the portion of the deficit the industrial event best explains.

Step Four: Assemble the Evidence for the Non-Industrial Portion

The non-industrial contribution is documented with the same rigor. The cataract predates the injury in dated records from an independent provider; its progression is recorded across multiple visits, establishing a rate rather than a snapshot; nuclear sclerosis is a recognized age-related process with an established natural history; and slit-lamp examination confirms it remains present and has advanced. The non-industrial portion is not inferred from the claimant's age or from a general statement that cataracts are common — it is anchored to this claimant's documented course.

Applying the counterfactual, the evaluator concludes on these illustrative facts that most of the current impairment is attributable to the industrial corneal scar and irregular astigmatism, with a smaller share to the pre-existing, independently progressing cataract. That split is the output of the reasoning, not its starting point: a different pre-injury trajectory would produce a different result, and a record with no pre-injury documentation would not support a confident split at all.

Why a Bare Percentage Fails

Consider the same conclusion written the way it too often appears: apportionment is stated as a bare split between industrial and non-industrial causes, with no further discussion. Nothing in that sentence permits review. It does not identify the non-industrial factor, cite the records establishing it, explain how the factor contributes to the disability, or disclose how the split was derived. Substantial medical evidence requires visible reasoning so the parties and the judge can test it, and a conclusion that cannot be traced to specific findings and records is functionally an unsupported estimate — which fails whichever party it happens to favor.

Two related errors recur. The first is apportioning to a risk factor rather than a cause: age, occupation, or family history is not itself a cause of this claimant's disability unless the causal pathway in this claimant is explained. The second is apportioning to a pre-existing condition that was asymptomatic and non-disabling without explaining how it contributes to the disability now being rated.

What a Defensible Apportionment Discussion Contains

• The impairment rating, stated first, with the measurements it derives from.
• An explicit statement that the condition is permanent and stationary, with reasons.
• Every candidate cause considered, including those excluded, with the evidence for each disposition.
• The specific pre-injury records relied on, identified by provider and date, and what they showed.
• The anatomic mechanism by which each retained factor contributes to the current disability.
• The reasoning connecting that analysis to the shares assigned — ideally the counterfactual, made explicit.
• What would change the opinion: missing records, alternative interpretations, unverifiable findings.
• Where a prior award exists, the separate section 4664 analysis, a legal as well as medical question that counsel must frame.

The Limits of an Illustration

The hypothetical was constructed to be tractable: clean pre-injury documentation, an anatomically obvious industrial lesion, one non-industrial factor with a recorded trajectory, no competing pathology. Real files rarely cooperate. Pre-injury records are often absent, in which case the honest report says the evidence does not permit a confident apportionment and explains why. Conditions may interact, so the whole is not the sum of the parts. Aggravation — where an industrial event accelerates a process already underway — requires reasoning about rate rather than presence. And apportionment interacts with legal doctrines a physician does not resolve: the evaluator supplies the medical analysis, and counsel applies the law.

The transferable lesson is narrow and durable. The percentages are the least interesting part of an apportionment opinion; the analysis that produced them is what survives cross-examination, and it is what attorneys on both sides should read for. See also How Ophthalmologists Analyze Vision Loss Causation and workers' compensation eye injuries.

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