Material contribution for expert witnesses

Published On: February 22nd, 2024
Author: Paul Sankey, Partner and Clinical Negligence Solicitor, Enable Law

Material contribution for expert witnesses

The law following CNZ v Royal Bath Hospitals NHS Foundation Trust [2023] 19 (KB) and Holmes v Poeton Holdings Ltd [2023] EWCA Civ 1377

There are broadly 3 key issues in most clinical negligence claims:

  1. Breach of duty;
  2. Causation – what harm the breach of duty caused; and
  3. Quantification – what loss flowed and how it should be quantified.

The claimant bears the burden of proof. Matters are proved ‘on the balance of probabilities. In other words, a matter is proved if it found to be more likely than not (more than 50% likely). This may seem a somewhat imperfect, rough and ready test, but it is what the courts require.

Causation – the ‘but for’ test

In most cases, the ‘but for’ test applies to causation. This is conceptually very simple. The question to ask is, ‘Would the claimant have suffered the injury but for the breach of duty?’

Although conceptually simple, it may be difficult to answer. Indeed, clinical negligence lawyers often spend more time trying to prove or disprove causation than breach of duty. However, there are cases whether a gap in our scientific knowledge makes it unanswerable.

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The scientific knowledge gap problem

  • Industrial disease cases blaze a trail

The issue first came to the fore in industrial disease cases. A claimant suffered pneumoconiosis after working with silica dust. Part of the exposure was from the work with pneumatic hammers but part from swing grinders. It was not possible to protect from dust from the hammers by the employer failed to comply with regulations to protect from dust from the grinders. Experts could not say to what extent each source – one ‘guilty’ and the other not – contributed. In another case, a claimant developed dermatitis from brick dust. Some exposure occurred through the normal course of work but some through the failure to provide showers. Medical science could not determine exactly how the dermatitis was caused and to what extent each source contributed.

In the first cases demonstrate circumstances where there was more than one cause, only one of which was ‘guilty’. In the second, there were cumulative causes, only part of which was ‘guilty’.

Should the claimant be deprived of a remedy in the face of a breach of duty because of the limits of our scientific knowledge? In these cases, the courts modified the causation test: where, on the balance of probabilities, a breach contributes to an injury, provided that contribution is more than ‘de minimis’ (negligible). This is the material contribution test. Note that the claimant still has to prove that the breach of duty contributed to the injury as opposed to just increasing the risk.

  • Clinical negligence cases follow

The material contribution test was then applied in the medical negligence context in a number of cases. Examples include:

  • A woman already weak from gallbladder surgery and then pancreatitis who was further weakened by post-operative care. She vomited but, because of the weakness, was unable to clear her airwaves. She aspirated, suffered a cardiac arrest and then a brain injury.
  • Delay in diagnosis of appendicitis contributing to sepsis.
  • Delay in managing a sub-dural haematoma leading to raised intra-cranial pressure and brain damage.
  • Ambulance delays after a patient developed a sub-arachnoid haemorrhage, leading to PTSD.
  • Delays contributing to hypoxic ischaemia.

Divisible and indivisible injuries

In these cases, the courts made a distinction between divisible and indivisible injuries.

  1. Divisible injuries: A divisible injury is dose-related. More exposure to harm worsens the outcome. It may therefore be possible to distinguish the harm which would have been caused in any event from the additional harm caused by the breach of duty.
  2. Indivisible injuries are not dose related. They either happen or they do not. More exposure to harm does not make them worse. Asbestosis from asbestos exposure is a divisible injury. The more the exposure, the worse the outcome. Mesothelioma from the same exposure is an indivisible injury. Greater exposure increases the risk but not the harm. One fibre is enough to trigger the condition. It either occurs or it does not.

Material contribution was applied to cases considered to be of indivisible injuries.

Unfortunately, the law became muddled when, in a number of cases, material contribution was held to apply to divisible but not indivisible injuries.

CNZ v Royal Bath Hospitals

This is the background for the case of CNZ v Royal Bath Hospitals Foundation NHS Trust where Mr Ritchie J sought to bring clarity to an area of confusion.

A mother had given birth to twins in 1996. In breach of duty, she had not been offered a C-section and, once it became necessary, there were delays. As a result, the second twin had suffered profound hypoxic ischaemia (PHI) and had quadriplegic cerebral palsy. The judge decided found that there had been 16 minutes of delay, of which the last 6 ½ minutes were due to a breach of duty. The first 10 minutes did not cause injury. After that point, every minute counted. However, but for the breach of duty, there would only have been 9 ½ minutes of delay and no injury. The claim therefore succeeded on the conventional but for test.

However, he went to consider what would have happened had there been only 2 or 5 minutes of negligent delay. Given that he had decided the case on the but for test, these further comments were not the reason for his decision. They are therefore not binding as a precedent but courts may take them into account as ‘obiter dicta’.

Had there been 2 or 5 minutes of negligent delay, some damage would then have occurred anyway but the negligence caused additional harm.

He first distinguished indivisible for divisible injuries as explained above. Indivisible injuries have a ‘trigger’. Once the trigger is pulled the injury happens. Its course is unaffected by the dose. The example he gave was malaria from a mosquito bites. Just as it takes one fibre to cause mesothelioma, it only takes one mosquito bite to cause malaria. More bites increase the risk but they do not increase the severity of the condition. Divisible injuries, in contrast, are dose-related.

If there is a scientific gap as to contribution of different causes (multiple or cumulative) to an indivisible injury, the but for test is modified. The test is then whether the breach of duty made a material contribution (one that is more than negligible) to the harm.

He categorised brain damage from PHI, the injury in this case, as a divisible injury. The longer the ischaemia, the worse the outcome. Every minute counts. But for the breach of duty, at least some of the harm would have been avoided.

However, the next issue is whether, in quantifying the loss, there should be an apportionment. Ritchie J thought in principle there should be, so long as there was a good evidential basis to do so. If apportionment were possible, a defendant should only have to pay for the damage it caused and not for damage the claimant would have suffered anyway. In previous industrial disease divisible injury cases, the court has made an apportionment using a ‘broad brush’ approach where damage was non-linear and precise quantification was impossible.

The expert evidence in this case was that although ‘every minute counted’ but the damage was non-linear. Organs ‘fell off a cliff’ at different points but those points were unknowable. It was impossible to attribute particular causes to particular loss. But unlike those industrial disease cases, the potential damages for CNZ’s injury were very large and a broad brush approach was not relevant. In those circumstances, although the injury (PHI) was divisible, the functional outcome was treated as indivisible. The claimant was entitled to recover in full.

The implications of this case are therefore that:

  1. Material contribution applies to indivisible injuries;
  2. It also applies where, although the injury is divisible, damages are likely to be substantial, so a broad brush approach is not appropriate and there is no evidential basis for apportionment. In other words, the functional outcome is treated as an indivisible injury.

Is it fair?

Is this unfair to defendants? Ritchie J clearly would have apportioned damages if there had been an evidential basis to do so. Not to do so, meant a degree of unfairness to defendants. However, the alternative was unfairness to the claimant. Given that the defendant is the tortfeasor – it has breached it duty to the claimant and caused injury to an innocent claimant – it is the defendant rather than the claimant who should bear any unfairness.

Holmes v Poeton Holdings Ltd

On 22 November 2023, the Court of Appeal give its judgment in Holmes v Poeton Holdings Ltd. This was an industrial disease case (and not a clinical negligence case) brought by a man who have been exposed by his employer to unsafe levels of Trichloroethylene, which, he alleged, had materially contributed to his developing Parkinson’s disease, an indivisible condition. The claim succeeded at first instance both on breach of duty and causation. The Court of Appeal allowed an appeal on the grounds that the judge was not entitled to find that the breach of duty had actually contributed to the condition, only that it might have done.

It defined divisible injuries as dose-dependant: ‘their severity will be influenced by the total amount of the agent that has caused the disease’. By contrast, indivisible injuries are those whose ‘severity will not be influenced by the amount of the agent that caused it’.

It endorsed the proposition that material contribution applies to indivisible injuries.

In relation to divisible injuries, it endorsed the approach taken in 2 industrial disease cases, one of asbestosis and the other of noise-induced hearing loss under which the claimant had to prove that the extent of the injury for which the defendant was liable. The fact that precise apportionment was not possible did not mean no apportionment should be made.

No consideration was given to Ritchie J’s view in CNZ that it applies to divisible injuries where, in relation to the functional outcome, damages are very large and there is no evidential basis to apportion.

Questions for experts to address

In cases where an injury has multiple causes or one cumulative cause, I suggest that the first step is to consider whether the injury is divisible or indivisible. The questions to address are then:

(a) Divisible injuries

  1. Would the claimant have suffered the entire injury but for the negligence?
  • Yes: claim fails
  • No: claim succeeds – go to question 2
  1. Would the claimant have suffered none or only some of the injury but for negligence?
  • None: claim succeeds in full
  • Some: go to question 3
  1. Is it possible (ie is there an evidential basis) to apportion?
  • Yes: apportion
  • No: recover in full

(b) Indivisible injuries:

  1. Would the claimant have suffered the injury but for the negligence?
  • Yes: claim fails
  • No: claim succeeds
  • Don’t know (scientific knowledge gap) – go to question 2
  1. Did the negligence make a material contribution to the injury?
  • Yes: go to question 3
  • No: claim fails

Paul Sankey, Partner and Clinical Negligence Solicitor Enable Law
[email protected]

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