Wednesday, 14 March 2012

An Introduction: What is NMES?


Welcome to NMES for CP.com! The aim of this blog is to provide physiotherapists and students an overview of the use of NMES in children with cerebral palsy. Over the next few weeks, we will be posting information regarding clinically important treatment parameters, precautions and contraindications, as well as research and support for it’s use. But to start off with, here’s some information about what NMES is, what it’s used for and how to differentiate it from other electrical stimulation modalities.

Figure 1. Photograph taken the first time NMES was used 
on the forearm of a child for wrist extension and finger
flexion, 21 months of age [3].
Neuromuscular Electrical Stimulation (NMES) describes the application of electrical stimulation to weakened, but otherwise normally innervated muscles. Treatments are performed transcutaneously, using surface electrodes over motor points within a target muscle/group of muscles. Percutaneous electrodes are available but are generally reserved for experimental conditions. Most researchers use biphasic rectangular pulses that are symmetrical to avoid accumulation of charge within the tissue, thereby decreasing risk of injury to the patient [1].

Stimulation can be applied while performing functional tasks (for example walking or reaching), and this is termed Functional Electrical Stimulation (FES) [1].

The two main types of electrical stimulation used in children with cerebral palsy (CP) are:

1) Neuromuscular electrical stimulation (NMES) – this is a high intensity, short duration stimulation which elicits a muscle contraction. NMES is predominantly based on the principle of overload, as well as its’ selective recruitment of Type II muscle fibres.

2) Threshold electrical stimulation (TES) – a low level sub-contraction stimulation generally applied during sleep. TES is thought to increase blood flow to the target musculature causing heightened trophic hormone secretion, thereby increasing the bulk of the muscle [2].

Kerr, McDowell [2] compared the effects of NMES and TES in children with CP. The following parameters were used:
Table 1 - Stimulation Parameters [2]

NMES
TES
Intensity (mA)
Max tolerated with muscle
Sensory Threshold
contraction elicited
<10mA
Freqency (Hz)
35
35
Pulse Duration (µs)
300
300
On:Off (s)
7:12
7:12
Ramp Up (s)
2
2
Ramp Down (s)
1
1
Daily Session Time (Hr)
1
8
Treatments Per Week (Days)
5
5
Total Duration Of Treatment (Wks)
16
16

As seen in the table above, the only difference between NMES and TES is the intensity and daily session time. There is a much lower intensity required for TES as there is no muscle contraction elicited, but rather a sensory stimulation, where as NMES requires a larger amplitude to achieve a muscle contraction. Due to the lower intensity of TES, it requires a longer application time, hence the TES is applied overnight for 8 hours, five nights per week.

Note that in contrast to microcurrent modalities, NMES has an amplitude measured in milliamps (mA), and low frequency values are used in comparison to modalities used for treatment of pain (such as traditional transcutaneous electrical nerve stimulation).


References:

1 comment:

  1. Good opening contrast to set the scene Anika. Microcurrent can't cause muscle contractions so i think that the contrast you made there is a little bit of overkill. No matter. I know you'll focus on the rather obvious questions of how you get children to tolerate NMES and the additional safety considerations. cheers CY

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