The new MDS system is also helpful in that it distinguishes between two levels of certainty in diagnosis: clinically established PD, and probable PD. This has the virtue of acknowledging that, even in specialist centres, the initial diagnosis may need to be reconsidered on subsequent visits. For example, it is not uncommon for patients to be reclassified as having MSA.
For a patient to have clinically established PD, there must be no absolute exclusion criteria, at least two supportive criteria, and no red flags. For clinically probable PD, the absence of absolute exclusion criteria is again mandatory; there must be no more than two red flags present – and, in the presence of one or two such features, each has to be counterbalanced by the presence of supportive criteria.
Allowing for uncertainty recognises the significant implications for patients and carers of a definitive diagnosis of Parkinson’s Disease. And it may also aid research by allowing the search for causative factors -- including genetic variants – to be concentrated on people who clearly have the condition.
An aspect of diagnosis that continues to prove challenging is the presence of dementia. The UK Parkinson’s Disease Society Brain Bank criteria of the late 1990s, which continue to be widely used, regarded dementia as an exclusion criterion -- despite the fact that the existence of PD dementia had moved into the clinical foreground.
The new MDS criteria do not consider dementia as excluding PD, irrespective of the timing of its onset in relation to that of Parkinsonism. But the status of patients with a diagnosis of dementia with Lewy bodies (DLB) is likely to be a continuing source of controversy.
Acknowledging diagnostic uncertainty helps both clinical practice and research
The MDS Task Force accepts that future developments may see the incorporation into their diagnostic guidelines of biomarkers in tissue, CSF and blood. And imaging techniques may also become sufficiently well validated to allow them a role in diagnosis.
The Task Force was also concerned to make their criteria compatible with future developments in the expanding field of prodromal PD.
Being confident in our ability to identify “pre-PD” would give us the chance to research novel, potentially disease-modifying treatments, offering the prospect of early intervention to slow or even prevent the development of classical PD. But that requires the development of genuinely neuroprotective agents.
Both diagnostic and research criteria will need continuous updating as our understanding of PD and its antecedents evolves. Refinements in our diagnostic criteria offer the possibility of distinguishing between subtypes of the disease that would allow treatment to be tailored to the clinical circumstances of the individual patient.6, 7
The patient is “irresistibly impelled to take much quicker and shorter steps, and thereby to adopt unwillingly a running pace”
Although commonly supposed to have been a physician, James Parkinson’s role in early nineteenth century London was that of surgeon apothecary. He was something like a general practitioner of today, but also compounded and sold drugs from the family shop. And of course he was an astute observer of all that attracted his clinical interest.
Looking back to 1817, we see that Parkinson was perhaps prescient in several respects. First, he recognised that the chances of effectively relieving PD -- and perhaps even of cure -- depended on intervening “before the disease had been too long established”.
Secondly, he realised non-motor symptoms were involved. “Sleep,” he wrote, “Becomes much disturbed”. And the bowels, “which had been all along torpid, now, in most cases, demand stimulating medicines of very considerable power”. Although he could not explain how such a connection between brain and bowels might come about, Parkinson thought it possible that disorder of the gut could induce a “morbid action” and “derangement of structure” in the nervous system.8, 9
Thirdly, he realised that achieving a cure would be more difficult than developing a way of slowing the advance of the disease. Parkinson foresaw that “some remedial process may ... be discovered by which at least the progress of the disease may be stopped”.
As for when such a breakthrough might occur, he hoped that it would be “ere [before] long”. In fact, it would be almost 150 years before the introduction of dopamine replacement therapy, and that had of course to wait until we had solid post-mortem evidence for severe dopamine deficits in the substantia nigra.10
Parkinson had argued that anatomical examination was “the only sure foundation for pathological knowledge”. But, with little anatomical evidence, he was left largely with conjecture.
He speculated that the “proximate cause of the disease is in the superior part of the medulla spinalis”. The great mobility of the cervical vertebrae might cause injury to the region, Parkinson hypothesised. This perhaps led to “slow inflammation”, which then spreads to the medulla oblongata as the disease progresses from one affecting only the arms to one involving functions such as speech and swallowing.
But it was by application of reason – based on “the absence of any injury to the senses or intellect” -- that he concluded that “the morbid state does not extend to the encephalon”. With increased interest in the cognitive sequelae of PD, that is an idea we would now challenge. But we may judge James Parkinson to have done well with the clinical knowledge and conceptual framework then at his disposal.
