Pre-Existing Conditions in Workers' Comp: Apportioning to Degenerative Disc Disease Nobody Knew Was There
Short answer: a pre-existing condition does not bar a workers' comp claim in California. The employer takes the worker as it finds them, and an injury that aggravates a degenerative spine is still an industrial injury. What the pre-existing condition affects is the permanent disability award, through apportionment under Labor Code sections 4663 and 4664. The evaluator may apportion part of the permanent disability to the degenerative process, even when it was asymptomatic before the injury, but only by explaining how that process is contributing to the disability now. The imaging literature makes the first half easy and the second half unavoidable.
The Friday morning musculoskeletal breakout at the 2026 CSIMS MedLaw Conference, "Apportionment & Impairment: Complexities in Musculoskeletal," was built around this problem (conference agenda). Almost every spine claim in a worker past thirty-five arrives with degenerative findings on imaging, and almost every one of those findings predates the injury. What the evaluator does with that fact decides the case.
Does a pre-existing condition disqualify a workers' comp claim?
No. California applies the rule that an employer is liable for an industrial injury that aggravates, accelerates, or lights up a pre-existing condition. A worker with lumbar degeneration who lifts a box and herniates a disc has a compensable injury. The degeneration does not defeat causation of the injury.
Where the pre-existing condition matters is later, once the worker is permanent and stationary and there is a permanent impairment to divide. Labor Code section 4663 requires the physician to state what approximate percentage of the permanent disability was directly caused by the industrial injury and what approximate percentage was caused by other factors, before or after. Section 4664 limits the employer's liability to the percentage directly caused by the injury. The apportionment post covers the statutory framework; this one is about the musculoskeletal case where the "other factor" is degeneration the worker never knew about. Where that factor is a disease with its own chart, the internal medicine analysis runs differently.
Can a QME apportion to an asymptomatic degenerative condition?
Yes. This was settled in E.L. Yeager Construction v. WCAB (Gatten) (2006) 145 Cal.App.4th 922, where the Court of Appeal held that section 4663 permits apportionment to a pre-existing degenerative condition even where it was asymptomatic before the injury. The older rule, under which only a previously disabling or symptomatic condition could be apportioned, did not survive the 2004 reforms.
That answers whether. It does not answer how much, and it does not relieve the physician of the reasoning requirement. The apportionment still has to meet Escobedo v. Marshalls (2005) 70 Cal.Comp.Cases 604 (en banc): the physician has to explain the nature of the pre-existing condition, how and why it is contributing to the current permanent disability, and the basis for the approximate percentage. "Degenerative changes on MRI, 40 percent non-industrial" is a number with no reasoning. Escobedo itself involved pre-existing degenerative arthritis of the knees, and the opinion was upheld because the physician explained the contribution rather than assumed it.
How common are degenerative findings in people with no symptoms?
Common enough that their mere presence proves almost nothing. This is the finding the evaluator has to hold in mind while writing.
Brinjikji and colleagues pooled thirty-three imaging studies of asymptomatic people and reported the age-specific prevalence of degenerative features in the spine (AJNR Am J Neuroradiol, 2015;36(4):811-816, PMID 25430861). In people with no back pain:
| Finding | Age 20 | Age 40 | Age 60 | Age 80 |
|---|---|---|---|---|
| Disc degeneration | 37% | 68% | 88% | 96% |
| Disc bulge | 30% | 50% | 69% | 84% |
| Disc protrusion | 29% | 33% | 38% | 43% |
| Annular fissure | 19% | 22% | 25% | 29% |
| Facet degeneration | 4% | 18% | 50% | 83% |
The older study everyone cites reached the same place with a smaller sample. Boden and colleagues imaged sixty-seven people who had never had low back pain and found a herniated disc in about one in five of those under sixty, and abnormal scans in more than half of those sixty and over, with a third showing a herniation and a fifth showing stenosis (J Bone Joint Surg Am, 1990;72(3):403-408, PMID 2312537).
The consequence for apportionment is direct. A degenerative finding at age fifty is close to the population norm for a fifty-year-old with no symptoms. Its presence on the post-injury MRI is not, by itself, evidence that it is causing any part of the current disability. The physician has to say what about this degeneration, in this examinee, is producing disability, and why the injury alone would not have produced the same disability.
What does a defensible degenerative-disease apportionment look like?
It answers four questions in order.
What was there before the injury, and how do we know? Prior imaging is the best evidence. A 2019 MRI in the production showing moderate L4-L5 degeneration establishes the condition and its severity two years before the 2021 injury. Absent prior imaging, the physician is inferring from post-injury imaging and age, and should say so.
Was it producing disability before the injury? Symptoms and treatment in the records answer this: prior episodes of back pain, chiropractic or physical therapy courses, work restrictions, prescriptions. Under Gatten, a prior asymptomatic condition can still be apportioned, but a condition with a treatment history supports a larger share and a condition with none supports a smaller one, and the report has to say which it found.
How is the degeneration contributing to the disability now? This is the sentence that separates substantial evidence from a percentage. The mechanism has to be stated in medical terms: the degenerative disc has reduced the segment's tolerance for load, so the residual mechanical pain reflects both the injury and the baseline; or the facet arthropathy is producing a component of the pain independent of the disc injury; or the multilevel disease explains why the examinee has not recovered as a person with a single-level injury would.
Why this percentage? Approximate is the statutory word, and no judge expects precision. What the judge expects is a reason the number is where it is: the severity of the prior imaging, the prior treatment history, the proportion of the current findings attributable to the injury rather than the baseline.
One boundary applies to all of this. Hikida v. WCAB (2017) 12 Cal.App.5th 1249 holds that disability resulting from the medical treatment of the industrial injury is not apportionable to the pre-existing condition. If a fusion for the industrial herniation produces adjacent-segment disease, that new disability belongs to the injury.
Where do these apportionments fail in practice?
In the records. The prior imaging report is on page 412 of a 2,000-page production, the 2018 chiropractic course is on page 340, and the physician who did not find them wrote an apportionment with no history of prior symptoms and no prior imaging, which the defense then rebuts with both. Or the physician found the MRI and apportioned to it without checking whether the worker had ever been symptomatic, and the applicant's attorney asks at deposition why a finding present in two-thirds of asymptomatic forty-year-olds is worth 40 percent of this worker's disability.
Lexamed's role in a spine case is finding page 340 and page 412 before the deposition does. It reads the full production, dates every imaging report and treatment encounter, cites the page for each, and surfaces prior complaints to the same body part as leads. The apportionment reasoning stays with the physician; the tool makes sure the reasoning has the record in front of it.
Frequently asked questions
Can you get workers' comp for a pre-existing condition in California? Yes, if a work injury aggravated or accelerated it. The pre-existing condition does not defeat the claim. It can reduce the permanent disability award through apportionment under Labor Code 4663 and 4664.
What is degenerative disc disease apportionment? The portion of a permanent spinal disability the evaluator attributes to pre-existing degeneration rather than the industrial injury. Under Gatten it can apply even if the degeneration was asymptomatic before the injury, provided the physician explains how it contributes to the disability.
How common is degenerative disc disease in people without symptoms? Very. In pooled imaging studies of asymptomatic people, disc degeneration was present in 37 percent at age twenty, 68 percent at forty, 88 percent at sixty, and 96 percent at eighty (Brinjikji et al., AJNR 2015).
Does an MRI finding alone justify apportionment? No. The finding must be shown to be contributing to the current disability. Because degenerative findings are near-universal with age, their presence on imaging is not by itself evidence of contribution.
Can apportionment be made to a condition that never caused symptoms? Yes, under E.L. Yeager Construction v. WCAB (Gatten) (2006). The physician must still explain the contribution under Escobedo; the lack of prior symptoms generally supports a smaller percentage.
What about disability caused by surgery for the work injury? Under Hikida v. WCAB (2017), disability resulting from the treatment of the industrial injury is attributed to the injury and is not apportioned to the pre-existing condition.
What records matter most for degenerative apportionment? Prior imaging, prior treatment for the same body part, and prior work restrictions. Their presence or absence in the production determines both the percentage and whether the opinion survives cross-examination.