Watson v Lancashire Teaching Hospitals NHS Foundation Trust [2022] EWHC 148 (QB)

Pickering v Cambridge University Hospitals NHS Foundation Trust [2022] EWHC 1171 (QB)

Both of these cases were stroke cases involving issues of causation. Both were decided by Mr Justice Andrew Ritchie.


As those with experience of stroke cases will know, patients with a high risk of stroke may be prescribed Aspirin or anticoagulation therapy to try to prevent further strokes. Aspirin may be prescribed to patients with previous heart disease or those who have had previous strokes but are in sinus rhythm. Heparin and Warfarin are both anticoagulants which reduce the tendency of the blot to clot. Anticoagulation is used where there is demonstrated cardiac arrhythmia and where the patient has hypercoagulability (sticky blood).  Heparin and Warfarin are used to treat deep vein thrombosis and pulmonary embolism, to prevent strokes in atrial fibrillation cases and in patients with mechanical heart valves.

Heparin (administered by injection) has a fast onset and fast offset and is effective within 1-3 hours.  Warfarin (a tablet) has a slow onset and a slow offset. Patients who need to come off their anticoagulation therapy prior to having surgery are usually advised to stop Warfarin five days before surgery. Patients requiring immediate anticoagulation tend to be started on Heparin and Warfarin; the Heparin is then stopped once Warfarin reaches therapeutic levels.


The Claimant, then aged 29, alleged that she had presented to A&E in March 2015 with symptoms caused by a transient ischaemic attack (TIA), a type of stroke. The Claimant’s case was that there had been a negligent failure to diagnose TIA and to refer her to a stroke clinic where she would have been started on Aspirin or Warfarin. The Claimant’s case was that the prescription of Aspirin or Warfarin would have prevented the serious stroke she suffered about 8 weeks later in May 2015.

The Defendant admitted that TIA should have been within the differential diagnosis in March 2015 and would have resulted in referral to a stroke clinic and advice to start Aspirin. However, the Defendant’s case was that the Claimant’s symptoms were in fact caused by migraine (not TIA) and so the diagnosis of TIA would not have been confirmed on imaging and Aspirin would have been discontinued. Accordingly causation was denied. It was further contended that Aspirin would not have been effective to prevent the Claimant’s later stroke which was caused by embolus from a cardiac clot and not atherosclerosis.

The Judge found against the Claimant on the basis that she had not suffered a TIA in March 2015. However he went on to consider the extent to which Aspirin reduced the risk of a recurrent stroke 8 weeks after an alleged previous stroke / transient ischaemic attack (TIA). The Judge heard evidence from neurology experts, Professor Brown and Dr Sare, with the evidence of Dr Sare being preferred. He regretted the absence of evidence from haematologists and cardiologists.

The Judge carefully considered the Rothwell et al (2016)paper. Whilst the Judge accepted that Aspirin may be effective in producing some reduction in recurrent stroke arising from all causes in the first 12 weeks after TIA, he identified the relevant question as being the extent of the reduction in risk in this case for this Claimant. He said that this required him to take into account not only her gender, but her lack of atherosclerosis and her lack of any proved heart condition in March 2015, her previous Ankylosing Spondylitis and her lack of any systemic or acute vascular disease and her age.

The Judge accepted that Aspirin would have been effective to prevent a recurrent stroke where the cause of the stroke was atherosclerosis. However, as C did not have atherosclerosis, that did not resolve the issue.

The Judge accepted that Rothwell shows that Aspirin is likely to be effective at preventing substantially more than 50% of recurrent serious or fatal strokes for those within the Rothwell study age group (84% were over the age of 52) with the organic characteristics that come with ageing in the first 12 weeks after TIA and so also at week 8. However he did not accept that it had been proven on balance that the same percentage applies to a 29 year old woman for multiple TIAs most probably generated by her heart in weeks 6-12 after TIA.

Ultimately the Judge concluded that the Claimant’s case on the effect of Aspirin in preventing secondary cardiac clots by over 50% in women aged 29-30 was not proven.


The Claimant was aged 52 at the time of the alleged negligence. She had a medical history of chronic atrial fibrillation but was not on anti-coagulation therapy to reduce the risk of blood clots. On 24 September 2015 she presented at A&E with what was subsequently diagnosed to be a blood clot in her left leg. The clot in her leg was an embolus from a thrombus which had formed in her left atrium. She was not advised immediately to start anti-coagulation therapy to reduce the risk of the left atrium thrombus embolising further.

Following the evidence of the A&E experts, Dr Jaffey and Mr Saab, breach of duty was conceded by the Defendant. The Defendant accepted that the Claimant should have been advised to start on Heparin and Warfarin by 0200 hours on 25 September 2015 and that the Claimant would have accepted such advice.

Therefore the only issue for the Judge to determine was whether Heparin would have prevented a further thromboembolic stroke 67 hours after anticoagulation should have been started.

The Judge heard evidence from haematology experts, Professor Mehta and Dr Patel, with the evidence of Professor Mehta being preferred.

Professor Mehta’s opinion was that the Heparin would have prevented propagation of the clot in the left atrium and thereby prevented the embolization which caused the stroke. Dr Patel’s opinion was that the thrombus would have dissolved after 3-4 weeks of Warfarin. However, relying on medical literature considering the efficacy of Heparin as bridging therapy in patients undergoing elective surgery who had to be taken off Warfarin, he opined that there was no evidence that Heparin decreased the thromboembolic risk generally and he therefore concluded that 67 hours of Heparin would not have prevented the stroke or materially reduced the risk of it.  

Both experts accepted that it would not be possible, for ethical reasons, to perform randomized trials on the efficacy of Heparin in the context of an arterial clot – hence there was never going to be literature which determined the issue in dispute in this case.

The Judge considered in detail the medical literature relied upon by the parties and the mechanisms by which Heparin might prevent embolization. He said that he did not find the literature on bridging therapy to be helpful in determining the issue in dispute in this case.

Professor Mehta relied on the established efficacy of Heparin in preventing pulmonary embolism (PE) in patients with deep vein thrombosis (DVT) and said that the Claimant’s situation was analogous. Whilst Dr Patel was not willing to accept the analogy, because he said that arterial clots are completely different to venous clots, it is apparent that the Judge considered this to be a useful analogy.

The Judge concluded that the same three processes by which Heparin works to prevent PE in patients with DVT would have operated in the Claimant’s situation. He concluded that the Heparin would have started working within 1-3 hours and would have prevented new clot formation, prevented propagation of the existing clot and would have enabled the Claimant’s body not only to reduce the size of the mother clot in the Claimant’s left atrium but also to make it less friable, more stable and more organised, so that on the balance of probability no embolus would have been fired off on 27 September 2015 and the Claimant would have avoided her stroke.


As ever, these judgments demonstrate the importance of experts being able to explain why they hold the opinions that they hold. They underline the need of the parties to have considered the type/mechanism of the stroke since the efficacy of preventative medication may depend upon it.

Pickering demonstrates the willingness of a judge to extrapolate, where appropriate, from one medical situation to another when it is necessary to determine causation in a novel situation. This well-reasoned judgment is likely to assist claimants with arterial clot who suffer a second stroke within days of the first.

I cannot help but wonder whether the judge in Watson would have reached the same findings about the effect of Aspirin if he had not already found against the Claimant by another route? The fact that his determination depended on the unusually young age of the Claimant means that another Aspirin case could have a different outcome.