The article linked below has a little more information about man in a barrel syndrome. In summary, “The internal carotid artery supplies blood flow to the anterior two-thirds of the brain via the anterior cerebral artery (ACA) and the middle cerebral artery (MCA). The most distal branches of the ACA and MCA supply brain tissue, including upper extremity motor fibers. In the setting of inadequate blood flow to both sides, the brain, these most distal branches, or watershed zones, do not receive an adequate amount of oxygenated blood, resulting in the death of these cells. Based on the neuroanatomy of these anterior circulation watershed zones, damage to these areas can result in bilateral upper extremity weakness.”
Of note, the literature explains man in a barrel syndrome differently than how it was taught to me. Rather than being marked by proximal UE and LE weakness with preservation of distal strength, man in a barrel syndrome is generally accepted as simply bilateral upper extremity weakness with preservation of strength elsewhere.