Cardiac afferents enter the cord at T1 to T4 and are misattributed to those dermatomes
Mnemonic
“T1 to T4 on the left”, which is why cardiac pain is felt where it is:
- Sympathetic afferents from the heart enter the cord at T1 to T4 or T5
- The same segments supply the chest wall, the medial arm and the ulnar forearm
- The brain cannot distinguish the two, so pain is referred to the chest, left arm, neck and jaw
“Convergence in the cord is the whole explanation.”
The vagus explains the rest: vagal afferents from the inferior wall converge with those from the upper gut, which is why inferior infarction commonly presents with epigastric pain, nausea and vomiting, and is mistaken for indigestion.
Jaw and neck pain occurs because upper cervical segments are involved through the same convergence.
Diabetic and elderly patients may have no pain at all, from autonomic neuropathy, which is why silent infarction is common in these groups and why the threshold for an ECG should be low.
Expansion
Visceral afferent fibres from the heart travel with the sympathetic nerves and enter the spinal cord at T1 to T4. They converge on the same dorsal horn neurones as somatic afferents from those dermatomes, and the brain, which receives far more somatic input over a lifetime, attributes the signal to the body wall.
Hence the typical distribution:
- Retrosternal chest
- Medial left arm and ulnar forearm (T1, T2)
- Neck and jaw, via convergence with upper cervical segments
Pain referred to the jaw involves ascending connections to the trigeminal nucleus, and epigastric pain reflects the lower thoracic segments, which is why an inferior myocardial infarction can present as indigestion.
Because the vagus carries afferents too, inferior infarcts also produce nausea, vomiting and bradycardia.