Why care about nerve anatomic variation?
• Affects findings on physical examination.
• Helps determine viable treatment options.
• Important to know for surgical planning and avoidance of errors.
This section will not cover all of the anatomic variations that are described but we will highlight a few. The important thing is to keep variation in mind when examining and treating patients.
• For the most part, you will find the nerve anatomy to be “as classically described” in textbooks and an anatomic variation should not be the first thing used to explain away a constellation of symptoms that don’t make clear sense on the surface, but rather a final option to consider.
• When operating, slow and careful dissection and an open mind to anatomic variation will hopefully help prevent errors and minimize unexpected outcomes.
• Relationship of proximal median nerve and musculocutaneous nerve:
• The median nerve gets contributions form the medial (motor fibers) and lateral (sensory fibers) cord from the brachial plexus.
• Sometimes, this lateral cord contribution will travel within the musculocutaneous nerve from the plexus to upper arm level. It will then join the median nerve at the upper arm level.
• Awareness of this branching pattern is important to exploration at this level and may explain what might initially be thought of as a confusing branch pattern.
• Position and take off of the median nerve motor branch at the carpal tunnel:
• It may be at the radial aspect or located more centrally and superficial to the remainder of the median nerve sensory component.
• It can travel through it’s own fibrous tunnel to the thenar muscles or can penetrate the transverse carpal ligament at its midportion or even takeoff from the ulnar aspect of the main median nerve and then course radially
• This branch can be inadvertently transected if it is not avoided by making incisions for carpal tunnel release/median nerve exploration at the wrist at the ulnar most aspect of the carpal tunnel and being vigilant for these variations
• Brachialis muscle innervation:
• Primary innervation is from the musculocutaneous nerve.
• However, it often receives a contributing branch from the radial nerve as well.