Positioning injuries as seen by the neurologist

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H. Muller-Vahl 10.3.1 Introduction

Nerve injuries are one of the complications feared after surgery. They have a considerable negative effect on postoperative rehabilitation and cause lasting handicaps in many cases. Most iatrogenic nerve injuries are caused directly by the surgical procedure. But they can also result from other medical measures, including positioning during or after an operation.

While many cases of surgical nerve injuries also occur under optimum procedures, position-induced nerve inju ries can be avoided as a rule [13, 26, 52, 53]. The basic principles in the pathogenesis of position-induced nerve lesions have been clarified for a long time. Prevention of such lesions entails all doctors and nursing staff responsible for positioning being acquainted with these principles and being capable of recognising which positioning endangers the peripheral nerves.

10.3.2 Frequency

The frequency of position-induced nerve injuries is only known approximately. Most injuries of this kind recede rapidly and are in many cases not featured in the patient's records. In older retrospective studies, the frequency of position-induced nerve injuries amounts to about 0.050.3% [25, 53]. This has scarcely changed in the meanwhile. Cooper et al. [20] recorded 3 cases of position-induced arm plexus paralysis after 15,000 general anaesthetics. Following total hip replacement surgery, Posta et al. [56] observed nerve injuries in the arm with a frequency of 0.22%. The most frequent injuries are to the arm plexus, the ulnar nerve and the peroneal nerve [25]. In a group of altogether 134 doctor liability processes because of position-induced nerve injuries observed in recent years in our hospital, these nerves were affected in 46, 41 and 29 cases, respectively.

10.3.3 Pathophysiology

Position-induced nerve injuries are caused by pressure or tension. In addition, in very rare cases nerve injuries can be caused during long operations in the framework of a compartment syndrome.

The severity of nerve injuries caused by pressure depends on the corresponding extent and duration [49]. Brief, moderate pressure can cause a line blockage which is reversible within a few minutes, probably as an expression of a metabolic disorder following compression of intraneural microvessels. If the pressure persists, this can result in an intraneural oedema. The endoneural pressure increases. It can take a few days or weeks for the paralysis to recede. Even greater pressure causes histologically verifiable injuries to the myelin sheath of the nerve fibres. The axons remain intact (neurapraxy according to Seddon). Large-calibre myelinised nerve fibres suffer earlier and greater injuries from pressure than non-myelinised fibres. Regeneration of the injured myelin sheaths can take several weeks, in rare cases a few months. Even greater pressure injures the axis cylinders (axonotmesis). This leads to wallerian degeneration of the distal axons in the lesion site, marked in neurophysiological terms by the occurrence of pathological spontaneous activity in the elec-tromyogram. Regeneration entails regrowth of the axons in a distal direction from the lesion site. This happens at a rate of approx. 1 - 2 mm per day. Position-induced pressure only causes little change to the inner structure of the nerves so that there are good conditions for successful regeneration.

All physiological movements subject the peripheral nerves to a certain stretching load. Their histological structure prepares them well for such load. Sustained, extreme stretching load can cause nerve paralysis. When a critical stretching limit is exceeded, this causes occlusion of the intraneural blood vessels, initially the venules, then also the arterioles and capillaries. Greater stretching causes histologically verifiable changes to the myelin sheaths, axons and connective tissues. The extent of structural injuries depends on the strength and duration of stretching. Abrupt extreme stretching is not well tolerated. The most severe positioning injuries are caused when an arm or leg falls from a support during repositioning.

The risk of position-induced nerve injuries increases with the length of the operation. But positioning injuries can also not be ruled out during short operations. For example, Mitterschiffthaler et al. [48] described severe paralysis of the arm plexus after an operation lasting only 20 min. Position-induced nerve injuries occur preferably at anatomically predisposed places. Thin patients are at greater risk from pressure injuries than obese patients. The risk during the anaesthetic is related to the reduction in muscular tone (particularly when using muscle relaxants) and to eliminating the physiological protection reflexes. If a patient were awake he would automatically correct his position after a few minutes because of paraesthesia and pain. A special predisposition can result from anatomic anomalies (for example lower arm plexus paralysis in cases of cervical rib). The tolerance of peripheral nerves to pressure or tension is reduced in cases of latent or manifest polyneuropathies (most frequently with diabetes or alcoholism). Finally, arterial hypotension and hypothermia are also discussed as disadvantageous factors. But it is not possible to verify any predisposition factors in the l arge majority of patients with position-induced nerve damage.

10.3.4 Symptoms

An external sign of pressure being applied can include a pressure mark on the skin. In cases of pressure ulcers, these indicate extensive damage of the inner nerve structure, with a less positive prognosis.

Nerve injuries involve motor and sensory deficits to differing extents, together with vegetative malfunctions corresponding to the supply area of the damaged nerve. The motor malfunctions are generally greater than the sensory deficits. Position-induced compartment syndromes are accompanied by the classical local clinical symp-

OTable 10.1. Differential diagnosis of nerve injuries

ascertained after operations

No causal context with

Injuries already existed

medical procedures

Injuries occurred after the operation

Causal context with

Injection

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