When variation falls silent
On healthy mobility, what its loss can tell us, and variability as a diagnostic signal.
A heart that always beats in exactly the same way is seldom a healthy heart. In cardiology this has been known since the 1980s: a richly varied heart rate variability is regarded as a sign of a well-functioning autonomic nervous system, whereas an impoverishment of that variability often goes together with illness, fatigue or old age. The heart thus tells its story not only in its rate, but also, and sometimes above all, in its fluctuations.
Similar observations come from other quarters. Sleep that follows exactly the same pattern night after night may point to an underlying dysregulation. Hormonal curves that lose their usual daily mobility suggest that something is out of balance. Research by Marieke Wichers, Laura Bringmann and their Groningen colleagues, among others, seems to suggest that something comparable occurs in the psyche. A depression sometimes announces itself as a silencing of healthy daily variation: the peaks flatten, the troughs become slower, and the system seems to lose its own rhythm of breathing.
Two patients, the same average
It is a remarkable thought, and one that sits somewhat uneasily with an assumption deeply anchored in our diagnostics. The usual questionnaire measures the average state over the past two weeks and assumes that this average is representative of what took place during that period. Two patients with identical averages may, however, have very different underlying dynamics. One is flexible and mobile, able to bend with whatever the week brings. The other is monotonous and predictable, as if fixed in one place in the state space.
What would it mean if this distinction became workable in practice? In the first place, that recovery would be more than a lower number on a scale. It could also, and perhaps above all, involve the return of lively variation: an emotional life that dares to move again, a day that need not resemble the previous one in order to be bearable.
Many names for one phenomenon
In complexity science this phenomenon goes by several names. Sometimes it is 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. Whichever term one chooses, a healthy human system seems to be recognisable by its richness rather than by its calm.
"Not the absence of complaints, but the presence of flexibility seems to be the best mark of recovery." (freely after Schiepek & Wichers)
Meaning for the consulting room
This idea has consequences for diagnostics. Asking a patient to complete a short questionnaire twice a day for a few weeks soon yields a data series in which the average state is only one of many aspects. The shape of the fluctuations, the coherence between variables and the degree to which the system returns to its equilibrium after a setback can be at least as 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.
None of this calls for a sweeping revision of what we do, but rather for an addition to what we measure. It also calls for a willingness to accept that health does not always coincide with calm. A patient who fluctuates more than a week ago has sometimes not become more ill, but more mobile. Without a clinical eye, that distinction cannot always be seen in a small graph.