When variation falls silent
On healthy mobility, and what its loss can tell us — a look at variability as a diagnostic signal.
A heart that always beats the same is seldom a healthy heart. Cardiologists have understood this since the 1980s: a richly varied heart-rate variability is a sign of a fit autonomic nervous system, while an impoverishment of that variability usually points to disease, to fatigue, to old age. The heart tells its story not only in its frequency, but as much — and sometimes more — in its fluctuations.
Comparable observations now come from elsewhere. Sleep that unfolds night after night in the same pattern often proves an indication of underlying dysregulation. Hormonal curves that lose their usual daily mobility suggest there is something to be repaired. And in the psyche, so research by Marieke Wichers, Laura Bringmann and their Groningen colleagues seems to suggest, a comparable story unfolds. A depression often announces itself as a silencing of the healthy daily variation. The peaks flatten, the troughs grow slower, the system ceases to breathe in its own rhythm.
Two patients, the same average
It is a remarkable thought, and it sits somewhat at odds with an assumption deeply embedded in our diagnostics. The usual questionnaire measures the average state of the past two weeks, and assumes that this average is representative of what took place in that interval. But two patients with identical averages may operate, in an underlying dynamics, in entirely different ways. The one flexible, mobile, capable of bending with what the week brings. The other monotone, predictable, riveted to one place in the state space.
What might it mean were this distinction to become manageable in practice? First of all, that 'recovery' would be something other than a lower number on a scale. It would also — perhaps above all — mean a return of lively variation. A spectrum of feeling that again dares to stir. A day that need no longer resemble the previous in order to be bearable.
Many names for one phenomenon
In complexity science this phenomenon has several names. It is sometimes called entropy, sometimes dimensionality, sometimes simply degrees of freedom. Hermann Haken preferred to speak of order parameters: higher-level variables that describe the behaviour of a complex system. Whatever name is chosen — a healthy human system seems to be recognised by its richness, not by its rest.
"Not the absence of complaints, but the presence of flexibility seems the best mark of recovery." — freely after Schiepek & Wichers
Consequences for the consulting room
For diagnostics this thought has consequences. One who asks a patient to fill in a short questionnaire twice a day for several weeks soon has a dataset in which the average state is only one of many readable aspects. The shape of the fluctuations, the coherence between variables, the degree to which the system returns to equilibrium after a setback — these are equally telling quantities, if not more telling. The work of Borsboom and Fried in the field of network analysis, and of Olthof and Lichtwarck-Aschoff in the field of dynamic measurement, is bringing this way of looking ever closer to the Dutch consulting room.
All this does not call for a radical revision of what we do. It calls rather for a complement to what we measure. And it calls for a certain readiness to accept that 'health' does not always mean the same as 'rest'. Sometimes a patient who swings more than a week ago is not more ill, but more mobile — a distinction that, without the clinical eye, the reading of a small graph not seldom misses.