Rib notching with coarctation:
- Inferior rib notching is seen mostly in adults (post-ductal coarctation) over age 20 and is not seen in infants who usually have pre-ductal coarctation.
- Location: ribs 3-9 (most pronounced in 3rd +4th ribs, less pronounced in lower ribs); 1st+ 2nd ribs do not participate because the posterior intercostal arteries originate from the costocervical trunk (not post stenotic Aorta).
- Site: central + lateral thirds of posterior rib.
Almost always Bilateral with exceptions:
- Unilateral on left side: left aortic arch with aberrant right subclavian artery below coarctation.
- Unilateral on right side:
-- normal aortic arch with coarctation proximal to origin of left subclavian artery
-- right aortic arch with anomalous left subclavian artery below coarctation.
Thorax - Blood Supply:
First two posterior intercostal arteries <--- from superior intercostal artery <--- from costocervical trunk <--- from subclavian artery.
Posterior intercostal arteries - 3rd and below are branches from thoracic aorta.
Anterior intercostal arteries receive their supply:
- upper 6: from internal thoracic artery
- lower 5: from its branch (muscolophrenic artery).
Internal thoracic artery comes off subclavian artery also.
What happens in Coarctation ?
Coarctation of thoracic aorta results in absent flow in posterior intercostal arteries (3rd and below).
However, Internal thoracic artery & musclophrenic artery sends blood to anterior intercostal arteries which continues into posterior intercostal arteries (3rd and below).
The 1st and 2nd posterior intercostal arteries receive blood from subclavian (unlike rest of posterior intercostal arteries).
Why are 1st two ribs spared ?
The high pressure first two posterior intercostals anastomose with the first two anterior intercostals which are also high pressured, so the anastomosis does not enlarge. (Hence no notching)
But 3rd posterior intercostals and those below it, are from descending aorta beyond the coarctation, so they have low pressure. The blood from anterior intercostals which are high pressured rushes into the low pressured posterior intercostals thus enlarging the anastamosis forming collaterals.