Postural Thixotropy: a Significant Factor in the Stiffness of Paralysed Limbs?

Postural thixotropy: a significant factor in the stiffness of paralysed limbs?

E G Walsh MD FRCP FRSE

Department of Physiology, University of Edinburgh, Edinburgh EH8 9AG, Scotland.

The term 'thixotropy' is not at present seen, indicating that it was a passive prop widely used in the medical literature. It was erty of the tissues and was not due to some introduced in 1927 by Peterfi who stirred the special neural control mechanism. cytoplasm of sea urchin eggs with a needle In other observations the wrist was oscil and noticed a change from a gel to a sol; lated with small rhythmic forces at about instead of being stiff there developed the 3 Hz. At a certain level of force a brief consistency of a fluid. On being left undis passive disturbance loosened up the system, turbed for some time the sol again became a and the motion thereafter, for the same gel. Thus, at it's origin the term had to do force, was increased to double or more. The with a stiffness which was reduced by loosened state persisted as long as the force motion, and the stiffness took some time to was applied. If however the force was cut be re-established; the material had a 'mem out for a period and then reapplied the ory'. The etymology is an origin from 2 motion was reduced to about the original Greek words '8t/;ts', thixis, touch and level. 'rpcJ7Tos', tropos, state. Observations were made at a number of For about 20 years I have sought to other joints of the body. In each example investigate problems concerning the similar phenomena were found. measurement and regulation of muscle tone Stiffness which is history dependent is in normal people and in patients with a wide characteristic of thixotropy. There are so1/ range of neurological abnormalities. The gel tranformations, of solutions of long studies have included, in recent years, some chained molecules. The change from a gel to measurements of tone in the muscles around a sol is induced by motion; left undisturbed the knee in paraplegic patients. The mater the system reverts to the gel state and ial has been published in a journal devoted stiffens up. Clays and some paints are to physiology. 1.2 I wish now to discuss a thixotropic and bentonite is added to cala possible factor which may be significant and mine lotion because of its thixotropic prop which has not, I think, been approached in erties. If the bottle is shaken the lotion can other laboratories. Where necessary appro be applied as a liquid, left to itself on the priate informed consent and ethical permis skin it stays in place as a paste. sion were obtained. Thixotropy has been shown to occur in My initial studies were concerned with the isolated muscle. 4 It may be a function of the wrists of normal people. Rhythmic forces cross-bridges between the actin and myosin were applied from a special motor. It was filaments which can be separated by a found that at a certain frequency the oscilla moderately large movement. tion was greatest. This 'resonant frequency' The stiffening with immobility occurs in 2 reflected the stiffness of the muscles and it phases. There is a very rapid change in the was observed to be higher, indicating first few seconds of rest followed by further greater stiffness, for small as compared with stiffening having a much slower time larger movements. This was both a puzzle course.5 and a lead.3 Observations were made on subjects who Observations of a similar type were made had the forearm cooled whilst the thixo on patients who, for surgical operations, tropic effects were observed at the wrist. were anaesthetised and treated with full With cooling loosening up did not occur or doses of a neuromuscular blocking agent. was greatly reduced. 6 The stiffening with small forces was again Paralysed limbs are immobile and often 114 Walsh cold. Both factors suggest that a high level of thixotropic stiffening will occur. I have not had, or made, the opportunity of mak ing extensive observations on this phenome non in patients with spinal injuries but report here observations on a tetraplegic patient with a complete lesion at C7. The arrangements are shown in Figure 1. The patient lay on his side and a printed motor concentric with the knee was coupled to the leg by a light alloy splint and bandage. The motion was recorded by a low friction Figure 1 The cylindrical printed motor is above potentiometer concentric with the axis of and concentric with the knee. It is controlled by the motor. a power amplifier in the current feedback mode One result is shown in Figure 2. There so that the input voltage from a waveform generator provides a corresponding current was some low level ongoing activity in the through the disc-like armature.! EMG of the quadriceps, whilst the ham strings were silent. It will be seen that a rhythmic torque initially produced a little The motion after the disturbance was in oscillation. The leg was then displaced by creased several fold. hand to generate a single bigger movement. A number of the procedures used by Thereafter the motion was clearly greater. physiotherapists will loosen thixotropic stif Another similar record is shown in Figure 3. fening. Thus passive and active motion, heat

Torque 1 10 Nm

Velocity

Quadriceps EMG H 1.0s K\llIlll1ll'll!llllIIlIlItlIIIIMlnl'll�I\lIIlIuIIlII�IIIIIIIJIIIIUtflllll!1'III"11IUt'11-- Mill t---W�III-ihllll,HIUIIII �IIUdJI't H�U\l1 t 1Iltil/lH1U11111 I I IIllUlt-I 1IU1111 IiPUlI'l lllq

Hamstrings EMG l'

Perturbation

Figure 2 The motor generates an oscillatory torque at about 1 Hz. The oscillations of the paralysed leg are greatly increased after the perturbation produced by moving the limb by hand.

Torque

Position

'1, • I' III .H" �t .' • bli rl�1 I 1'1\ ... 1.0s t-< Ham. emg - Perturb

Right knee

Figure 3 Another example of loosening by a moderately large motion at the knee of the tetraplegic patient. Postural thixotropy 115

and diathermy and perhaps ultrasonics may changes more fully, with a view to improved all disrupt the bonds which have formed. It quality control of the efficacy of different would be important to document these therapeutic manoeuvres.

References

1 Douglas AJ, Walsh EG, Wright WG, Edmond P (1989) Muscle tone around the human knee in paraplegia. Q J Exp Physioi 74: 897-905. 2 Douglas AJ, Walsh EG, Wright GW, Creasey GH, Edmond P (1991) The effects of neuromuscular stimulation on muscle tone at the knee in paraplegia. Exp Physioi 76: 357-367. 3 Lakie M, Walsh EG, Wright GW (1984) Resonance at the wrist demonstrated by the use of a torque motor: an instrumental analysis of muscle tone in man. J Physiol 353: 265-285. 4 Lakie M, Robson LG (1988) Thixotropy: the effect of stretch size in relaxed frog muscle. Q J Exp Physiol 73: 127-129. 5 Lakie M, Robson LG (1988) Thixotropic changes in human muscle stiffness and the effects of fatigue. Q J Exp Phys 73: 487-500. 6 Lakie M, Walsh EG, Wright GW (1986) Control and postural thixotropy of the forearm muscles: changes caused by cold. J Neurol Neurosurg Psychiatry 49: 69-76.