Often TOS patients describe arm symptoms that follow a path down the medial arm to the fourth and fifth fingers.

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In thoracic outlet syndrome, the pattern of symptoms reflects which part of the brachial plexus is compressed. When the lower trunk (carrying C8–T1 fibers) is affected, the sensory distribution follows the ulnar nerve: it travels medially down the arm into the forearm and hand, with numbness or paresthesias in the little finger and the ring finger. So symptoms that run along the medial arm and extend to the 4th and 5th fingers fit lower-trunk involvement and the ulnar distribution, making this the best match. Other patterns point to different nerve routes (for example, lateral forearm to the thumb corresponds to other branches, while dorsal hand patterns align with radial distributions), which don’t align with the described medial-to-ulnar path.