Perception Threshold in SCS: Not One Size Fits All

CLC readers… thank you for your responses and the positive feedback on the first few instalments. I asked if there were any particular topics that you would like me to discuss and one recurrent theme to emerge was around threshold. This is probably going to take a few articles to cover because it’s quite complex. It’s also critically important to understanding how neurostimulation and neuromodulation works and how patients can get the most benefit.

We are all doing neuromodulation all the time. Our nervous system adapts and changes depending on the sensory inputs and our life experiences. I forgot my wife’s birthday twice in the 34 years we have been married. The first time was in the 90’s, I was preoccupied with work (all right, don’t judge me). A neuroscientist would call this phenomena masking, one sensory input (in this case work) drowning out another. Halfway through that morning I had to sign and date something, and as soon as I wrote the date down the blood drained from my head and my PA saw the panic and horror in my face. Here, my sympathetic nervous system fired up the autonomic involuntary body functions such as breathing, blood pressure, heartbeat. I told Danelle what I had done and like any great PA she said, “Don’t worry I’ll fix it.”

The second time was recently, in a COVID lock down. After 7 weeks of every day feeling like the same day over and over (shops closed, can’t go out for dinner, not really sure what day it is, etc.), I was in my study on video calls and the doorbell rang (to which I thought, “What the hell, who is ringing my doorbell? Don’t they know there is a global pandemic?”). It was the delivery of a bunch of flowers for my wife, Kathryn, for her birthday. Oh dear, but no fight or flight response this time; I didn’t even bother to pretend that I had ordered the flowers. “Sorry I forgot.” Kathryn said, “That’s ok, not like we can celebrate.” In a neurological sense, two things were going on here, one the environment was affecting Kathryn’s perception threshold of what I had done (in this case failed to do) and the second is called long-term potentiation.

In SCS for pain, we are used to the idea of sensation threshold for stimulation – the point at which the stimulation is only just felt by the individual and that there are some devices which are below threshold, commonly referred to as “paraesthesia free,” implying that the sensation experienced above threshold is a paraesthesia. Absolute threshold is defined as the point where 50% of the time a presented stimulus is felt by the individual. When threshold is determined in a programming session it usually is just the first point that a stimulus is perceived in an amplitude sweep. This is always larger than the absolute perception threshold. Up until recently, the only psychophysical parameter that can be measured in neuromodulation devices is the perception threshold, and so devices are classified by those that operate above threshold and those that don’t. But perception threshold depends on many factors, and over the next few articles we will go through a few of them. Some will surprise you.

Almost all the evidence for mechanism of action for neurostimulation has been obtained from studying animal models. The animal of choice has been the rat. One of the problems with rats is that they can’t tell you if they feel the sensation (i.e., perception threshold). This problem is solved by winding up the stimulation level until a motor threshold is reached and the animal twitches. In SCS for humans, reaching the motor threshold is undesirable but occurs often. With the leads placed for lower back pain (between T7 and T9) this is felt at first as rib stimulation. The Ia afferent from the dorsal root next to the lead is recruited by the stimulation and the rib muscles contract via the H-reflex. The motor thresholds and perception thresholds are different between different patients due to variation in their anatomy.

So how do we know how the stimulation levels used for research in animals relate to those used in humans? The settings are chosen based on a percentage of the observed motor threshold, 40%, 60%, etc. Recently, compound action potential measurements have been reported for rats (Obara et. al.) and the difference between ECAP threshold and motor threshold was determined. Now we can decide at which stimulation level the recruitment of dorsal column fibres begins and compare directly with the motor response.

Bottom line is that recruitment begins at 33% of the motor response in the rat. Most humans will use an SCS device at around 1.1x their ECAP threshold, which corresponds to 36% of the motor threshold so experiments conducted at 40% of motor threshold in the rat correspond to normal usage levels for SCS devices that generate sensations (paraesthesia). These stimulation levels are not comparable with stimulation settings used in humans with paraesthesia-free devices. All animal tests that I have seen have been conducted at stimulation levels that produce paraesthesia and so the evidence collected is relevant for dorsal column stimulation. Stimulation corresponding to 60% of motor threshold – which has been used in a number of studies – corresponds to stimulation levels which are never reached in humans and the relevance of this data collected at these levels to therapeutic human stimulation needs further investigation armed with our new knowledge from Obara.

The first time I forgot my wife’s birthday, Danelle did “fix it” and in the process decided she would teach me a lesson. At 5:00pm she came to my office with a list and instructions.

"You’re heading home early tonight, flowers have been delivered, etc…”

 That evening my hip pocket nerve was stimulated well above 60% of motor threshold, lesson learned.

3 months later, from out of nowhere, Kathryn says, “I do hope you thanked Danelle properly for organising my birthday.”

"What makes you think Danelle …” and before I could finish the sentence Kathryn interjects with “C’mon, like you could organise that!”