Evidence of loss of chance in vascular surgery: application of Gregg v Scott to the expert report

In clinical negligence litigation involving vascular surgery, the claimant often faces an outcome that is less favourable than it would have been had the alleged breach not occurred. Where a clinician fails to diagnose or treat a condition, such as peripheral arterial disease or an impending aneurysm rupture, the legal question often shifts from an all-or-nothing causation analysis to the quantification of a lost chance of a better outcome. For the instructing solicitor and the expert witness, understanding the threshold established in Gregg v Scott [2005] is central to the viability of a claim where the clinical prospect of success was less than 50% even with appropriate care.

The threshold for loss of chance in vascular surgery

In English civil law, the general rule remains that a claimant must prove on the balance of probabilities that the breach of duty caused the damage. Under the Gregg v Scott principle, a loss of chance claim is typically unsuccessful where the claim is for the loss of a prospect of a better medical outcome that was, at the time of the alleged breach, less than 50%. The House of Lords held that the claimant must generally demonstrate that the negligence caused the adverse outcome itself, rather than merely the loss of a chance to avoid that outcome, if the probability of a better result was not already more likely than not.

When instructing an expert in vascular surgery, the expert must clarify whether they are opining on the probability of a successful outcome based on the available clinical evidence. If the clinical evidence suggests that even with timely intervention, the probability of limb salvage or prevention of rupture was 40%, a claim based on the loss of that 40% chance will typically fail under the current law of England and Wales. The expert report must address whether, on the balance of probabilities, the treatment would have resulted in a different clinical outcome.

Evidential base for causation analysis

The expert opinion relies on the synthesis of primary source material, including:

• Initial presentation records and triage notes.
• Imaging evidence (CT angiography, duplex ultrasound, or invasive angiography) at the time of the index event.
• Operative notes and subsequent interventional radiology reports.
• Histopathology reports where applicable to the progression of vascular disease.
• Documentation of co-morbidities that may have influenced surgical candidacy or prognosis.

Where key imaging or contemporaneous notes are absent, the expert must qualify their opinion. A retrospective reconstruction of what might have occurred had intervention been prompt at a specific time point requires a precise evidential foundation. If the claimant’s vascular status was deteriorating, the expert must distinguish between the progression of the underlying disease and the specific exacerbation caused by the alleged delay or failure in treatment.

Discipline boundaries in vascular causation

Causation in vascular surgery frequently requires an understanding of both the surgical and the radiological aspects of care. Where a patient undergoes endovascular intervention that is alleged to be negligent, the instruction of both a vascular surgeon and an interventional radiologist may be required to delineate the standard of care and the causative effect of the intervention. A vascular surgeon may opine on the indication for the procedure, while the interventional radiologist is better placed to report on the technical adequacy of catheterisation, stent deployment, or embolisation technique.

These roles are distinct. A surgeon should not be asked to opine on the technical intricacies of endovascular imaging interpretation unless they possess appropriate sub-specialist training. Conversely, an interventional radiologist may not be the appropriate expert to opine on the systemic vascular management of the patient’s wider arterial disease. Instructing parties should ensure that the expert’s remit is bounded by their specific area of clinical practice to ensure the evidence provided satisfies the requirements of CPR Part 35.

Procedural requirements for expert evidence

The expert’s duty to the court, as set out in CPR Part 35, necessitates that the report remains independent. When causation is contested, the process of written questions under CPR Part 35.6 allows for the clarification of the expert’s position on the probability of a successful outcome. Where multiple experts are instructed, CPR Part 35.12 facilitates discussions to narrow the issues, potentially leading to a joint statement.

In cases where the prognosis is uncertain, the joint statement process is an opportunity to clarify whether the experts agree on the probability of success at the material time. If the experts disagree on the statistical likelihood of success, this should be explicitly set out in the joint statement. This enables the court to resolve the factual dispute regarding the clinical prospects. Where the court directs the instruction of a single joint expert, that expert is instructed under CPR Part 35.8.

The application of these principles is specific to England and Wales under the Civil Procedure Rules. In Scotland, the instruction of a skilled witness under the principles established in Kennedy v Cordia (Services) LLP [2016] requires the expert to provide a technical opinion that assists the court, though the procedural mechanisms for written questions and joint statements differ from those in the England and Wales regime.