It would be a mistake to view the judgment in MacLennan v Hartford Europe  EWHC 346 (QB) handed down on 24 February as an unequivocal finding that Chronic Fatigue Syndrome (CFS) can never be caused by stress at work.
It is fair to say the Claimant was running a ‘courageous’ case. CFS remains a controversial illness with ongoing debates about its nature (and especially whether it is predominantly a physiological or psychological condition), its diagnosis and even its name.
Without any certainty about the causative mechanism for CFS, it is hardly surprising that there is no agreement that stress may cause CFS at all and so the Claimant was facing an uphill task even before looking at the evidence itself.
In principle, the theory advanced was logically sound. CFS is often called ‘post viral’ CFS as one of the commonly observed histories is that the symptoms of CFS follow a seemingly unrelated viral infection. Here, the Claimant’s CFS had followed a serious chickenpox infection which was said to have been the result of stress at work causing a lowering of her immune system.
However, it was on the evidence that her claim failed. As the judgment pointedly summarised:
In respect of the undermining of her immune system … Dr Weir … said that, if Mrs MacLennan had suffered consistent illness throughout 2005 … then that would be clear evidence of an undermined immune system as a result of stress at work … However, he conceded that, if she had not had any relevant illness from mid-May to the contraction of chickenpox in September, then there would be insufficient evidence of a deficiency in her immune system to support his hypothesis; and he would conclude, on the balance of probabilities, that she would have contracted CFS after her chickenpox in any event. There is no evidence of a relevant illness in that period … and, hence, on Dr Weir’s evidence, Mrs MacLennan’s case would fail on medical causation.
In short then, the judgment makes no finding on the underlying principle because it did not need to. The Claimant was unable to establish the facts on which her case on causation relied even before asking the Court to favour her medical explanation (which, it should be said, was not agreed by the Defendant’s expert).
Clearly there is a case to be made that CFS may be caused by stress at work but to rely on this is arguably a mistake. It seems to me it falls into the trap of relying on the diagnostic label at the expense of viewing the symptoms themselves.
The nature of CFS is such that it is diagnosed by reference to a wide range of criteria (these are not without controversy but the most commonly used are the CDC 1994 aka Fukuda criteria). In practice, however the constellation of symptoms which may satisfy the diagnosis of CFS will likely also meet a diagnosis of some other illness such as a depressive illness or an adjustment or anxiety disorder.
Given there is often little practical distinction between both the symptomatology and treatment of many of these illnesses, becoming fixated upon one precise diagnosis, and especially one which has inherent causation difficulties, may well be unwise; at least in the context of building a claim.