DBS Diary 04: Dr Stamford and Mr Hyde

When you stop to think about it, it’s hardly surprising that DBS is associated with some degree of behavioural change in those given the procedure. In part that’s the reason to do DBS – to change things.

That’s perhaps a little glib. When you “change things” in the brain you alter the activity of at least one neuronal pathway and, because such pathways are close proximity to others, most likely more than one. Depending on the placement of the electrodes one may get more or less stimulation of our desired pathway relative to the unwanted stimulation effects. Think of it like darts. If you aim for the treble 20 with your three darts you may get lucky (or be incredibly good at darts) and score 180 (60+60+60). If your aim is off by a couple of centimetres vertically, you may well score only 60 (20+20+20). A few centimetres horizontally and your score could be as low as 3 (1+1+1). And yes, we’re being ultra pedantic, you could miss the board altogether and score 0+0+0. But in that case you should probably just give up darts (or, by analogy, neurosurgery).

The point is that there is a ‘sweet spot’ within the subthalamic nucleus where one gets most benefit at the least cost. That seems to be in the superior lateral parts of the nucleus.

So in other words, perfectly placed electrodes can be thought of as a 180 score. But most of the time, and bearing in mind that this is the brain not a dartboard, the scores are lower. And that’s not placing any burden of responsibility upon the neurosurgeons; that’s just down to variability in our patient brains. None of us (well, few of us) have supermodel brains, perfect in every curve and tuck. No, most of us have rather frumpy brains, sometimes asymmetric but often – especially by the time we are considering DBS – misshapen or battered in some way or other. So, even if your neurosurgeon is the best darts player in the world, he will struggle with your tatty old darts board.

Okay, and I realise I’m testing your patience with a further analogy, but think of the subthalamic nucleus once more as a busy railway station. Lines (neuronal pathways) pass through the station. Some terminate, others carry on. Some stop briefly, others pass through without stopping. Think of DBS as the equivalent of a transient signal malfunction. Traffic through the station is disrupted for some lines more than for others. Some are even reversed. No, I know this doesn’t fit with the darts analogy. Forget the darts. We are on trains now.

So where do the railway lines go to and come from? As you can imagine, there are many. The subthalamic nucleus is Clapham Junction. Neurones come in from the cerebral cortex, and parts of the thalamus. Trains, sorry neurones, leave for the substantia nigra.and beyond. It has reciprocal connections with the internal and external globus pallidus and the pedunculopontine nucleus.

The point I’m (rather laboriously) making is that we cannot affect individual lines – yet. Whatever stimulation parameters we choose, it will always affect adjacent brain nuclei to some extent.

What does that mean?

In simple terms it can mean changes in behaviour, affect, emotion and action. Mostly trivial and perhaps even unnoticeable but occasionally more extreme. in some cases, changes of personality have been noted. Again mostly minor modifications but occasionally more profound changes.

If I’m honest, this worries me a little. I like to think, and perhaps I’m flattering myself, that I am a reasonably nice human being. I try to be friendly, to do the right thing and so on. I’m no saint but I hope there aren’t too many people out there I’ve offended or upset (unless intentionally obviously). I’m reasonably comfortable in my skin. I don’t want to find my personality changed out of all proportion. I don’t want to be apathetic, depressed, anxious or disinhibited.

Of course there is no way of knowing whether any of this will happen. It may be plain sailing. I may emerge psychologically indistinguishable from my pre-DBS form. That would be wonderful and, in many respects, is the most likely outcome. I’m probably fretting over nothing. But lurking at the back of my mind is the concern that inside my Dr Jekyll, there is Mr Hyde, just waiting for those electrodes to release him.