Each physician who starts to deal with peripheral nerves – and particularly focal peripheral nerve pathologies – needs to be able to visualize in detail the macroscopic anatomical image of the surgical region; he or she has to be motivated to look up any remaining special questions in the anatomical atlas. We therefore avoid repetition of the complete and normal anatomy and subsequently restrict ourselves to the unexpected but important variations of nerve distribution in the human extremities. Special and clinically important anatomical details will of course be pointed out in Chaps.​ 8 and 9 when surgical aspects of focal nerve entrapments will be described separately.

The phylogenetic and ontogenetic development of humans explains how greatly nerve ramifications can vary; we must be aware of these variations, especially during surgical exploration, and diagnostic estimation and evaluation of imaging needs to be based on reliable knowledge covering their possible occurrence.

Rare cases exist with a brachial plexus consisting of only two trunks at supraclavicular level: the root C5 and C6 unite to the superior trunk, for instance, and roots C7, C8, and T1 unite to form a unique second trunk, the inferior trunk. A medial trunk is absent in these cases [1]. More frequently, C5, C6, and C7 unite to form one superior trunk, whereas C8 and T1 form the inferior trunk. Knowledge of these variations might be rather important in thoracic outlet surgery which will be described in detail in Sect. 8.​2.​1.

Likewise, variations of the ramification of the lateral cord into the musculocutaneous and median nerves are rare. This ramification may be displaced towards distal, so that the musculocutaneous nerve can arise from the median nerve at mid-upper arm level [2].

More frequently you will find ramification variants between the median and ulnar nerves at lower arm level: the well-known Martin-Gruber anastomosis consists of motor fibers which normally run downwards in the median nerve but leave this nerve at mid-forearm level and find a connection to the ulnar nerve. Sometimes these fibers use the anterior interosseous nerve as connecting structure. At a far distal level, these motor fibers have to rejoin nerve fibers which supply the thenar muscle. If this anastomosis arises from the deep branch of the ulnar nerve within the palm, it is referred to as Riche-Cannieu [3]. Knowledge of these anatomical variations becomes very important when a physician is confronted with a median nerve compression or pain syndrome presenting with sensory deficits but totally missing motor deficits. In particular, neurophysiological evaluations may occasionally be characterized by such special features. These cases and their evaluation need a correct assessment.

In the lower limbs, anatomical variations of nerve ramifications are less important. Comparable to the median and ulnar nerves of the forearm, the medial and lateral branches of the tibial nerve may vary concerning their level of ramification. As they run through a common canal (“tarsal tunnel”), both are involved when, for instance, they are compressed so that nerve variations become insignificant. The same holds true with the common peroneal nerve which may be entrapped beside the head of the fibula. Varying levels of ramification of the sciatic into the tibial and peroneal nerves are nonrelevant for diagnostic or surgical procedures.

Small variations of the lateral femoral cutaneous nerve course have to be taken into account: the nerve may run through a slit between parts of the inguinal ligament but may also proceed across the bone of the iliac crest [4]. In the latter case, the small nerve is quite likely to become damaged during any kind of surgery within this region, such as bone removal for fusion surgery and endoscopic laparotomy, and also during nerve decompression surgery itself.