toms, occasionally also with the consequences of rhab-domyolysis (crushed kidney, electrolyte disorders).
As a rule, position-induced nerve injuries are to be diagnosed in a binding fashion on the basis of a careful study of the case history together with clinical and possibly neurophysiological examinations. In any case, due consideration must be given to possible causes independent of the surgical procedure and also other iatrogenic causes (O Table 10.1).
Differential diagnosis requires a good knowledge of the pathogenesis and symptoms of peripheral nerve injuries, because otherwise there is a risk of confusion. Medical literature repeatedly classified nerve injuries following heart operations with median sternotomy as position-induced injuries because it was not known that arm plexus paralysis is frequently inevitably caused by this surgical procedure itself.
It is advisable to consult a neurologist familiar with the problems in every suspected case. Failure to clarify the cause of the nerve injuries at an early point in time can often have a negative effect for the doctors in the case of open questions during a later dispute.
Position-induced nerve injuries are not treated any differently from other paralysis. As a rule, conservative therapy is required with active and passive movement exercises, and suitable measures to prevent secondary complications. Whether electrotherapy is effective is a subject for controversial discussion. The administration of so-called neurotropic vitamins can be considered useless.
The prognosis for position-induced nerve injuries is generally favourable. In Parks' cases , the paralysis receded completely within 6 weeks in 52% of the cases and within 6 months for another 40%. Only 8% of the patients still suffered from major deficits even after 12 months.
10.3.5 Special nerve injuries
The emergence of position-induced arm plexus paralysis is explained well in scientific terms following many clinical examinations and examinations of corpses [34, 37]. During operations in the supine position with the operating table in a horizontal position, the nerve injuries occur in the arm spread out [13, 20, 64]. The arm nerve plexus is fixed at the cervical spine and in the axilla. All movements enlarging the distances between the fixed points cause a more or less large stretching load on the arm plexus. The injury is usually caused by overstretching as a result of abducting the arm too far. The stretching effect is reinforced if the arm is retroverted, turned outside and/or supined at the same time, and also if the head is turned to the opposite side. Simultaneous abduction of both arms is problematical.
In the case of arm plexus paralysis in the Trendelenburg position with the head lowered, various different mechanisms are discussed. Some authors presume that the shoulder supports aiming to prevent the patient from sliding off the operating table can cause direct pressure injuries to the arm plexus if positioned too far in a medial direction. Stohr  on the other hand presumes for various reasons that in this position too, the cause is probably overstretching by moving the body and head against the fixed shoulders or arms.
In most cases the injuries consist of upper arm plexus paralysis with paresis of the movements in the shoulders and upper brachial muscle. Sensory deficits are frequently limited to the outside of the arm, but can also reach right down to the thumb. As a rule, there is no significant pain.
Where position-induced lower arm plexus paralysis is concerned, consideration must always be given to anomalies in the cervicothoracic transitional area. A hyperplastic transverse process C7 with fibrous ligament from here to the first rib is probably more dangerous than a fully formed cervical rib. The worst injuries can be caused if the arm falls off the operating table, particularly because tension forces are transferred to the nerves in full force with the muscles in relaxed state .
With regard to differential diagnosis, it is important to differentiate position-induced arm plexus paralysis from postoperative neuralgic shoulder myatrophy (time interval between anaesthetic and the onset of neurological symptoms, sharp pain, other distribution of the neurological deficits ). It is very rare for the positioning of central vein catheters to cause arm plexus lesions.
The large frequency of position-induced paralysis of the ulnar nerve results from its exposed position at the back of the elbow. Here the nerve runs under the surface in a bone furrow (ulnar nerve sulcus) so that it cannot evade influences from the outside. Its histological structure (nerve fibres in just a few fascicles, little epineurium) is also not very beneficial. The nerve is most frequently injured by pressure from the outside, for example as a result of inadequate padding. If the elbow is held very bent for a longer period of time, this can cause endogenous compression through aponeurosis of the flexor carpi ulnaris muscle .
The main symptoms of an ulnar lesion include par-aesthesia and sensory deficits in the fourth and fifth rays in the hand (typical restriction in the middle of the ring finger) and weaker movements in the hands and fingers, expressed particularly in clumsiness.
There are considerable differences of opinion about how ulnar paralysis occurs and can be avoided in the direct time context of an operation - referred to in literature as perioperative ulnar paresis. Some opinions claim that perioperative paralysis of the ulnar nerve constitutes a special case and cannot be avoided [18, 63]. An appropriate appraisal requires a thorough differentiation of position-induced ulnar paralysis from nerve injuries caused by other means with similar symptoms, particularly impending ulnar paralysis, lower arm plexus paralysis (typical consequence of heart surgery) and ulnar paralysis caused after the operation while the patient is confined to bed and repeatedly supports himself on his elbows [51, 69, 73].
The special susceptibility of the peroneal nerve comes, like that of the ulnar nerve, from its exposed position near to the surface behind the fibular head, together with a disadvantageous histological structure. In a typical case, compression is caused in the lateral or half-lateral position as a result of inadequate padding on the operating table, in the supine position from a lack of leg holders . In the supine position, nerve injuries can also be caused by pressure, if the legs are positioned with extreme outer rotation, for example as part of removing the long saphe-nous vein during vascular surgery .
A lesion of the peroneal nerve is expressed in paralysis of the foot and toe elevator muscle and the elevator muscle for the outer edge of the foot (drop foot). The sensory deficit is apparent on the medial back of the foot and front outside of the distal lower leg.
Paralysis of the peroneal nerve must be differentiated from lesions of the sciatic nerve with a peroneal emphasis or lesions of corresponding branches of the lumbosacral plexus, as can occur for example during operations in the lithotomy position (7 see below).
10.3.6 Lesions of the lumbosacral plexus and its branches in the lithotomy position
The various forms of the lithotomy position are a relatively problematical kind of positioning with regard to peripheral nerve injuries. Why lesions occur to parts of the lumbosacral plexus has still not been completely clarified. Pathogenetic mechanisms involved here can include pressure, overstretching and also ischaemia .
The best explanations are given for injuries to the femoral nerve following gynaecological operations. Experiments on corpses have verified that excessive flexion and outer rotation of the legs press the femoral nerve against the unyielding inguinal ligament [2, 33, 57, 64]. This causes paralysis of the knee extensor muscle and impaired sensitivity at the front inner side of the thigh and calf. In isolated cases, involvement of the hip flexor and obturator nerve has been verified, indicating a stretching effect at the same time. Occasionally isolated obturator lesions are also caused .
The lithotomy position can also cause overstretching of the sciatic nerve, usually affecting its peroneal part [7, 17, 47]. The sciatic nerve is fixed in the area of the major sciatic foramen and in the knee and is stretched by bending and outer rotation in the hip joint. This is particularly disadvantageous when the knee is stretched at the same time.
Injuries to the branches of the lumbosacral plexus must always be differentiated from more distal lesions of the leg nerves.
Operations on the extension table apply pressure to the perineal region and thus to the pudendal nerve against the rod of the table. This is an example for injury to a nerve which has to be accepted to a certain extent in order to bring the operation to its required conclusion. According to studies by Brumback et al. , pudendal lesions can be expected in about 10%. This results in sensory deficits in the perineal region and genitals, with erection and ejaculation disorders in men. The principles for preventing such nerve injuries consist of distributing the pressure to the largest possible area of soft tissue (wide counter bar between genitals and uninjured leg), with a restriction on the duration and intensity of traction to the leg.
10.3.7 Compartment syndrome following operation positioning
During longer operations, compartment syndrome can be caused by the positioning. Most relevant publications refer to the lithotomy position [29, 50, 60] or the semi-lateral lithotomy position . This major complication also oc curs very rarely in operations in the knee-elbow position  or in the lateral position . Most cases involve the compartments of the lower leg. The mean operating time in the published cases was about 7 h. All age groups can be affected.
The crucial increase in pressure in the compartments for the development of compartment syndrome and the corresponding drop in the arteriovenous pressure gradients is caused on the one hand by persistent pressure from the body weight. In addition, in the lithotomy position the raised position of the legs and possibly additional position-induced compression of arteries and veins can result in additional impairments to the circulation. A general drop in blood pressure during the operation can be just as disadvantageous as impending arteriosclerosis.
In contrast to position-induced isolated nerve injuries, the earliest possible detection of the development of compartment syndrome is crucial with regard to the necessary therapy (fasciotomy) in order to prevent permanent injuries.
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