The schematic overview in Fig. 3.2 illustrates the lining up of diagnostic decision-making processes.

After a trauma with assumed lesion of a peripheral nerve, it applies first to prove or to exclude the same. In the case of a proven nerve injury, its depth has to be documented. Careful anamnesis could often point on a nerve injury, e.g., a report of violent shooting in electrifying pain in the context of a surgical intervention. In the case of a traumatic accident, its course and primary clinical status are to be exactly documented, not only for insurance-legal reasons but also to answer the question whether the lesion has developed by the accident mechanism (e.g., avulsion injury) or the supply of a fracture (e.g., with a humerus fracture).

The severity level of the nerve lesion has to be classified as this is indispensable for the statement of the injury depth and the prognosis (see Chap. 1) but serves equally for the decision-making regarding the treatment strategy, in particular the indication for the operation. The Medical Research Council (MRC) scale (http://​www.​mrc.​ac.​uk/​research/​facilities-and-resources-for-researchers/​mrc-scales/​mrc-muscle-scale/​) and the Seddon and Sunderland classifications (with the additions of Millesi) are still commonly used (see Chap. 1). Especially in early stages after nerve lesion, it can be difficult or impossible to differentiate between neurapraxia, axonotmesis, and neurotmesis just on the basis of clinical symptoms and physical findings. Therefore, additional electrophysiological investigations are essential [17]. In early stages after a nerve injury, compound muscle action potentials (CMAPs) and motor units (MU) can be examined, which are, however, of only limited evidence. Recently, nerve sonography showed its superiority over the electrophysiological assessments [7]. But also nerve sonography has its limitations, as it is very much restricted to the examination of rather superficial nerves. Deep-running nerves can be judged, however, with the magnetic resonance neurography (MRN) (see Chap. 2). Already today, this modern method substantially affects diagnostic and therapeutic decision-making processes and in particular also the planning of the surgical intervention [8]. Ultrasound or standard MRN are unable to fully discriminate between neurotmesis and axonotmesis, in particular when the nerve remains in continuity. Diffusion tensor tractography (DTT) represents a recent development in MR imaging that may revolutionize this aspect of the diagnosis and monitoring of peripheral nerve trauma [19].

Sunderland Grade III lesions may often regenerate spontaneously (incomplete regeneration) and then result in better functional recovery than after nerve reconstruction. Both will result in incomplete regeneration, but the degree may vary considerably. Especially in this scenario, the decision for the correct procedure is very important for the further process. On the basis of novel diagnostic techniques, in particular related to the advances in peripheral nerve imaging, patients may be more correctly selected to receive prompt surgical intervention when indicated, and invasive procedures may be more correctly avoided when spontaneous regeneration is likely. Simon et al. [20] suggest that “further exploration of non-invasive strategies to augment nerve regeneration processes, such as modulation of central and axonal plasticity through repetitive stimulation and functional retraining paradigms, may provide further benefit for patients with moderate and severe nerve injury, including those patients in whom surgical intervention is not needed” [20].

In general, an indication for a surgical procedure results from the following reasons [4]:

Contraindications for a surgical intervention include bad general condition of the patient and risk of general or local sepsis, as well as uncertainty over the kind and extent of the injury (e.g., bullet or saw injury). A nerve reconstruction is not reasonable whenever no appropriate instrumental, machine-aided, or spatial conditions are present and whenever no experienced operation team is available. In certain cases also a primary muscle or tendon transfer can be the better choice, e.g., in the case of irreparable lesions of predominantly motor nerves (such as of the radial and common peroneal nerves).

In fact, the ideal case of a primary treatment of a nerve injury, i.e., the smooth uncomplicated disconnection of a nerve, is rather the exception. For the far more frequent secondary treatment, temporal limits have been specified, which vary depending upon the examiner (Fig. 3.3).

Acute peripheral nerve lesions require a differentiated proceeding. The strategic decision for an immediate reconstruction or a wait-and-see attitude depends on the type and the depths of the lesion. Generally a complete/total nerve transection injury can be assumed whenever an open cut or stab injury exists along the nerve trajectory and the loss of the nerve function has been proven clinically. During wound treatment the discontinuity of the nerve will be confirmed, and under optimal conditions, primary nerve reconstruction will be performed. This is not only the simplest decision to be taken by the nerve surgeon but also the ideal scenario for the patient with the best prognosis. Under ideal conditions the prerequisites are as follows: (1) smooth sharp transection and clean wound properties, (2) experienced nerve surgeon, and (3) appropriate technical equipment and surgery room. Certain types of nerve injury and their extent together with the extent of accompanying injuries and other concomitant factors, however, do often prohibit primary nerve repair.

Most examiners do not define a strict time limit for the primary coaptation. The transition to an early secondary treatment runs smooth, although an intervention within the first week (≤ 10 days) after injury is generally referred to as the optimum. A maximum time window of 6 weeks is accepted for secondary nerve repair. Such secondary nerve repair may not always have an inferior prognosis, because a highly experienced nerve surgeon performing a meticulous and conservative approach may compensate the disadvantages of the delayed treatment.

The decision to accomplish a reconstruction after more than 6 months must be discussed with the patient and depends on many factors like the age of the patient, the kind of nerve injured (motor or sensory), the proximo-distal height of the injury, concomitant lesions, etc.

The main principle is to perform tension-free and optimal adaptation of the nerve ends and fascicles. Contused nerve stumps have to be trimmed back to the healthy epineurium and a visible fascicular structure. During an immediate repair approach, it may be difficult to estimate exactly the length of necessary resection, especially in the case of more blunt dissection injuries. This specific condition may be better evaluated in a secondary approach or delayed repair procedure. The delay, however, should be kept as short as possible since nerve ends retract with time and this will consequently impair the coaptation of the nerve ends without significant tension. For combined lesions (cut and crush), decision-taking for immediate or delayed treatment is particularly difficult and responsible. The latter is also true for gunshot injuries in which typically a massive, diffuse destruction of tissue is present. Once, in these cases, a neuroma in continuity is found during the secondary care, the intraoperative recording of the compound nerve action potential (cNAP) can deliver reliable evidence whether nerve conduction at the lesion site is preserved or not. Such evidence facilitates very much the decision whether to remove the neuroma or not [13].

After the decision for nerve reconstruction has been taken, the appropriate reconstruction approach has to be considered. The intraoperative decision-taking process is illustrated in Fig. 3.4. In addition further important intraoperative determinations have to be made, like the length of the resection of the nerve stumps (how much to resect) and the anatomical allocation of the nerve bundle groups. The use of intraoperative motor and sensory nerve differentiation methods can diminish the risk of fascicular mismatch when grafting an autologous nerve. Available intraoperative methods for the differentiation between sensory and motor fascicles are the anatomic method, based on separate identification of groups of fascicles, the electrophysiological method, using stimulation of nerve fascicle in the awakened patient, and histochemical method, which is based on staining for enzymes specific to motor or sensory nerves. Both the latter methods are difficult to use, because the patient has to be highly compliant for the electrodiagnostic evaluation and because of the time gap that develops before sample tissue has been analyzed histochemically. Finally, the appropriate donor nerve has to be identified. Most of the time, the sural nerve is selected, but the cutaneous antebrachial and the saphenous nerve comprise good alternatives.