Synonym: Autonomous area
Definition: These are the regions where single nerve roots supply distinct and non-overlapping areas of skin. By their nature the “autonomous zones” represent only a small portion of any dermatome and only a few nerve roots have such autonomous zones. The size of autonomous zone for a particular nerve is variable from individual to individual.
Sensory zones of a Peripheral nerve:
- Maximal zone: Maximal area supplied by a peripheral nerve
- Intermediate zone: Area of overlap of the maximal zone of different peripheral nerves
- Autonomous zone: Area exclusively supplied by a particular peripheral nerve
Note:
- Maximal zone = Intermediate zone + Autonomous zone
- With the interruption of a sensory nerve, all modalities of cutaneous sensation are lost only over the autonomous zone.
- Within a few days of complete interruption of a sensory nerve, the autonomous zone shrinks and the intermediate zone enlarges, which may be due to actual ingrowth of fibers from adjacent normal nerve.
Autonomous zones of various nerves:
B. Median nerve
C. Ulnar nerve
D. Common peroneal nerve
E. Sciatic nerve
- Radial nerve: 1st dorsal web space of hand (Anatomical snuff box)
- Median nerve: Distal phalanx (tip) of index finger (2nd finger)
- Other: Tip of thumb
- Ulnar nerve: Distal phalanx (tip) of little finger (5th finger)
- Common peroneal nerve/Common fibular nerve/External popliteal nerve/Lateral popliteal nerve: Central strip on dorsum of foot
- Deep peroneal nerve: 1st dorsal web space
- Posterior tibial nerve: Sole of foot
- Sciatic nerve: Mixed pattern of Common peroneal nerve and Posterior tibial nerve
Note: According to some authors, radial nerve and common peroneal do not have autonomous zones although complete transection of the nerve results in sensory loss over the above mentioned regions.
MRC grading of sensory recovery tested on autonomous zone:
- S0: Absense of sensibility in the area
- S1: Recovery of deep cutaneous pain
- S2: Partial recovery of protective sensation (superficial cutaneous pain and touch)
- S3: Recovery of protective sensation with accurate localiztion; sensitivity (and hypersensitivity) to cold is usual
- S4: Recovery of 2 point discrimination (In hand < 8 mm)
- S5: Normal sensation or Complete recovery