The pitfalls of material contribution in stroke negligence where arterial risk factors exist

The challenge of causation in stroke litigation

In neurology medico-legal practice, stroke negligence causation analysis frequently encounters the hurdle of pre-existing arterial risk factors. When a claimant presents with established hypertension, hyperlipidaemia, or atrial fibrillation, the task of isolating the specific impact of a delayed clinical intervention requires a granular evaluation of the neurological timeline. The core conflict in such cases often involves distinguishing whether the poor outcome was an inevitable consequence of the underlying pathology—despite competent care—or whether a quantifiable window of opportunity for improved outcome was lost due to a breach of duty.

The but-for test and material contribution

Under English law, the starting point for causation remains the ‘but-for’ test. The claimant must demonstrate that, on the balance of probabilities, the adverse neurological outcome would not have occurred but for the negligent act or omission. In the context of stroke, this often involves evidence regarding the time-critical nature of thrombolysis or mechanical thrombectomy. The expert witness must carefully evaluate the modified Rankin Scale (mRS) score that might have been achieved had the breach not occurred, compared with the actual neurological deficit sustained.

Difficulties arise when claimants seek to rely on ‘material contribution’ where the underlying pathology is progressive. The law is stringent; it requires evidence that the breach made a material contribution to the injury. Where vascular disease is widespread, the expert must assess whether the infarct was a direct result of the specific missed intervention or a progression of the underlying arteriopathy. This evidential burden requires careful analysis of serial imaging and clinical observations documented in the records, which necessitates expert consideration of how a thorough clinical impact assessment aligns with the established clinical risks of the claimant’s pre-existing condition.

Expert evidence and procedural compliance

Expert neurologists instructed in clinical negligence must adhere strictly to the overriding duty to the court as set out in CPR Part 35. The primary role is to provide the court with objective, independent evidence, rather than to advocate for either party. When instructing an expert, solicitors should ensure the letter of instruction clearly identifies the specific breach alleged, as the expert’s opinion on causation relies entirely on the premise that a breach of duty occurred.

Common pitfalls in instructing experts include requesting a definitive opinion on ‘apportionment’ before the breach has been clearly defined or understood through a process of document discovery. Experts must also avoid diagnostic overreach; for instance, in cases involving functional neurological disorders, the expert must apply standard diagnostic criteria rather than assuming a psychological overlay simply because imaging appears inconsistent with subjective reports.

Procedural distinctions in expert evidence

It is critical for legal professionals to distinguish between different expert procedures under the Civil Procedure Rules:

• Single Joint Experts (SJE): Under CPR 35.7, the court may direct that an expert be instructed jointly to provide a report. The procedure for instructing such an expert is governed by CPR 35.8. This expert owes a duty to the court to be independent and is not an agent of either solicitor.
• Expert Discussions: Under CPR 35.12, where the court has permitted separate experts, those experts may be directed by the court to discuss their opinions to reach a joint statement. This is a procedural exercise to clarify areas of agreement and disagreement for the court.
• Written Questions: Any party may use CPR 35.6 to put written questions to an expert, provided these are proportionate and limited to clarification of their report.

Practical guidance for solicitors

To ensure robust expert evidence, solicitors should prioritise the early collation of the relevant medical history. For stroke negligence cases, this includes imaging, ambulance records, and A&E triage notes. When instructing, the expert should be asked to address the ‘Bolitho’ component of the test for clinical negligence: whether the clinical decision-making stands up to logical analysis in the context of the available clinical data.

Reliability of analysis is the primary driver of resolution speed, as clear and objective expert opinion allows for a more informed assessment of the merits at the earliest possible stage in proceedings.