Types o f stimulation

2.6.1. Mechanical stimulation

2.6.1.1. Innocuous mechanical stimulation

Innocuous stimuli were provided by calibrated von Frey hairs. A von Frey hair is a single nylon monofilament o f graded diameter (0.08-1 mm), which in this case is inserted coaxially into a 12mm strip o f perspex rod, measuring 2m m in diameter. Sets o f hairs are made according to a logarithmic scale o f the weight in grams applied when the hair is pressed downwards on a surface. There is a base-10 log difference o f 0,22 between adjacent hairs in the series. The weight o f von Frey hairs used in this study ranged from 0.0794g - 34.7g (see table 2.3 for the weights o f each hair).

The von Frey hairs used in the study were recalibrated at regular intervals during the period o f data collection using an analytical balance accurate to Img. The calibration is performed by pressing the end o f the hair downwards onto the balance in exactly the same way as it would be used on a subject and the maximum weight in grams measured. The hairs were also recalibrated whenever their environment changed, such as when they were subjected to extremes o f temperature and humidity, as encountered when testing infants receiving high levels o f intensive care (Andrews 1993). When not in use, the hairs were stored in their own box in a dry atmosphere and at room temperature. They were cleaned thoroughly with alcohol swabs between successive tests on infants. The cleaning procedure was agreed with the 'Control o f Infection' team at both

hospitals.

W hen using the von Frey hairs to elicit withdrawal reflexes, it was discovered that there w as an optimum tim e for exerting pressure on the surface o f the skin. Pressing for too long distorted the hair too much and made the pressure exerted inaccurate for the strength o f hair.

In order to enable the von Frey hair stimulus to be linked accurately to the flexion reflex EMG response, an instrument had to be devised which would detect the application o f the von Frey hair to the skin and trigger the sweep o f the 'Sapphire'. A s no such instrument was commercially available, Medelec Ltd was approached and asked i f they w ould be w illing to help with the design o f such an instrument. A specification o f my requirements w as written for them, and meetings held to clarify the design o f the device. A special hand-held piezo transducer device was made which could hold the von Frey hairs and trigger the sweep o f the EMG unit with a 5-volt TTL pulse (see plate 2.6). For a full account o f the device, including a block diagram, please refer to Chapter 3 on methods development.

W hen using the von Frey hairs and piezo triggering device, it was necessary to leave an interval o f at least 5 seconds between stimuli, and to hold the piezo transducer still for that time. This w as in order to allow the von Frey hair and the piezo transducer to return to their original positions, and for the hair to be applied with equal force. It was also important to m ove the piezo transducer towards the skin at as constant a speed as is possible with a hand-held device, and not too slowly, to prevent undue oscillation o f the von Frey hair and premature triggering o f the EMG sweep.

It was possible to display the output o f the piezo transducer on the 'Sapphire' screen, although it was not possible to measure the amplitude o f the transducer signal, only its duration, because the signal did not go through the amplifier. The piezo transducer signal was recorded on channel 2 o f the 'Sapphire' if recordings were being obtained only from biceps femoris.

2.6.1.2. N oxious mechanical stimulation

A frankly noxious stimulus in the form o f a heel lance was used to elicit flexion reflexes and compare them with those evoked by innocuous stimuli. A mechanical lancet, the 'Autolet' (Owen Mumford Ltd) was used to give the heel lance, as it has been found to cause less wakefulness and crying, and not to induce palmar sweating in full-term infants, compared with a manual heel lance which produced a greater than 100% increase in palmar sweating in this age group (Harpin & Rutter 1983).

In order to perform the heel lance, a lancet is placed in a spring-loaded cartridge which is held against the skin. When the spring is released, the lancet pierces the skin causing bleeding, and is immediately vdthdrawn. The 'Autolet' gives a fixed depth o f puncture (2.4mm), which decreases the risk o f calcaneal osteom yelitis, and is thus superior to a manual heel lance which gives no control over the depth o f heel puncture (Harpin & Rutter 1983).

Again, it was necessary to devise a way o f linking the stimulus to the response by enabling the 'Autolet' to trigger the sweep o f the 'Sapphire' EMG unit. Medelec were consulted again, and provided with a specification o f m y requirements. The 'Autolet'

w as fitted with a light-emitting diode (LED), which enabled it to trigger the sweep o f the 'Sapphire' when the heel lance was performed. For further detail concerning its' operation, please see Chapter 3 on methods development.

2.6.2. Electrical stimulation

In order to reduce som e o f the sources o f variability inherent in using mechanical stimuli, flexion reflexes were also evoked using small electrical pulses. These were delivered via the 'Sapphire' using a neonatal bipolar stimulator, specially-designed for the project to my specification by an engineer at M edelec Ltd (see plate 2.7 for illustration, and chapter 3 for a brief account o f the design). The skin overlying the sural nerve at its' retromalleolar pathway, just behind and beneath the lateral malleolus at the ankle, was cleaned vrith an alcohol swab but not prepared with ‘Nu-prep’, and the stimulator was strapped there using 'Mefix' tape, with the cathode sited proximally (see plate 2.8). It was necessary to clean the skin with alcohol to reduce the electrode impedance, since the ‘Sapphire’ would not allow the stimulator to deliver the stimuli if the impedance was above 8KQ. Trains o f 5 stimulus pulses were delivered with the aid o f a 'train stimulator box' which was again designed to m y specification by an engineer at M edelec Ltd (see plate 2.9 for illustration, and chapter 3 on methods development for a brief account o f the design). The box had to be designed specially because the 'Sapphire' is unable to deliver trains o f stimuli. It has been shown that trains o f 2 or more stimuli are more effective at eliciting flexion reflexes in the adult (Wilier 1983), which was why it was deemed important to use them in this study. The stimulus width o f the constituent single electrical pulses in the train was 0.1m sec, and the interstimulus interval (ISI) between each pulse in the train was 0.5msec.