Mugweni v NHS London

In a cumulative-cause medical negligence claim, a claimant succeeds where negligence is proved to have made more than a negligible contribution to injury, even if medical science cannot establish conventional but-for causation. The court must assess the evidence of both the duration and severity of each alleged causal episode. A further period of hypoperfusion cannot be treated as causative merely because it followed an earlier injury or because additional damage appears logically possible. On appeal, factual findings based on expert evidence will stand where they were reasonably open to the trial judge.

Grace Mugweni suffered severe neurological disabilities after cardiac surgery in 1983. Her claim alleged that an anaesthetist negligently failed to detect and treat a developing tension pneumothorax before cardiac arrest. The respondent argued that the brain injury resulted from the non-negligent consequences of cardiopulmonary bypass and Grace’s pre-existing condition.

Langstaff J found a breach of duty but held that it had not caused or materially contributed to the brain injury, and dismissed the claim in [2011] EWHC 334(QB). The appellant appealed on causation and the respondent cross-appealed on breach. The central issues were whether the alleged negligent period before and after cardiac arrest materially contributed to the watershed brain injury, and whether the trial judge was entitled to find that the relevant signs should have been detected.

Lord Justice Davis delivered the substantive judgment. Lord Justices Hughes and Ward agreed. The appeal and the cross-appeal were both dismissed.

1. Cumulative causation. The court accepted the formulation in Bailey v Ministry of Defence & Anor, [2008] EWCA Civ 383. If the evidence shows that non-tortious causes would have caused the injury in any event, the claim fails. If the injury probably would not have occurred without the tortious contribution, the claimant succeeds. Where medical science cannot establish conventional but-for causation but can establish that the negligent contribution was more than negligible, the but-for test is modified.
2. Evidence of a second causal episode. The question whether the alleged negligent period added to the injury was evidential, not logical. Both the duration and the depth of hypoperfusion mattered. Additional watershed damage required evidence that the later episode was sufficiently prolonged and more severe than the earlier bypass-related episode. The contemporaneous surgical note supported the finding that there had been no significant period of circulatory arrest and that circulation was rapidly restored. The accepted expert evidence did not establish a sufficiently prolonged or severe second episode.
3. Breach. The rarity of tension pneumothorax did not determine the breach issue. Tamponade was an ever-present operative risk with similar signs. The trial judge was entitled to find that changes in arterial and central venous pressure should have prompted urgent investigation and should have been detected by the responsible anaesthetist within the available two-to-three-minute period.
4. Appellate restraint. The judge’s conclusions on expert evidence, timing and causation were findings open to him. The Court of Appeal found no proper basis for substituting its own assessment. Causation therefore remained unproved despite the finding of breach.

• Court of Appeal (Civil Division) — Dismissed the appellant’s appeal on causation and the respondent’s cross-appeal on breach.
• High Court of Justice, Queen’s Bench Division — Langstaff J found a breach of duty but held that it had not caused the relevant brain injury, and dismissed the claim in [2011] EWHC 334(QB).