Anatomy:
Left pulmonary artery sling (LPAS) describes the aberrant path of the
left pulmonary artery, instead of arising from the main pulmonary
artery, it takes its origin from the postero-superior aspect of the
right pulmonary artery. The aberrant left pulmonary artery then courses
between the trachea and oesophagus (retrotracheal) to the left hilum,
causing tracheobronchial compression at the level of the distal trachea
and carina. The trachea is caught between the main pulmonary artery
abutting anteriorly and the left pulmonary artery bordering the
posterior aspect. Wells et al classified LPAS in two types based on
tracheobronchial anatomy, Type 1 (normal bronchus) and Type 2 with a
bridging bronchus (Table 1).8,9 In type-1A LPAS, the
carinal anatomy is normal and bifurcates into left and right mainstem
bronchi, usually with mild airway compression (figure 1 b). In type-1B
LPAS, the carinal position is normal (T4/5 level) however the right
upper lobe bronchus is tracheal (arising from the trachea) above the
carina and supplies either a single subsegmental apical right upper lobe
bronchus (RUL) or rarely the entire RUL (figure 1c 1d) . The eparterial
tracheal RUL bronchus may be malacic and can also have a blind ending as
a diverticulum.10 Type-II LPAS has an abnormal
tracheobronchial tree without a well defined carina and is generally
associated with tracheal stenosis distal to the tracheal RUL bronchus
takeoff. This tracheal narrowing leads to a lower pseudo carina (T6)
from which the left main bronchus originates (LMB), figure 2.
Additionally the RML and RLL are supplied by a single bridging bronchus
that crosses the midline from the left (Type-IIA). Type-II B is
characterised by a complete absence of the right bronchial tree, with
the right lung being supplied entirely by a bridging bronchus from the
left mainstem bronchus and mostly, the right lung is
hypoplastic.11,12In some case series type-II lesions
predominate.8 The narrowed tracheal section often
contains complete tracheal rings (CTR) with an absent trachealis
muscle.13
CTR have been reported in approximately two thirds of
LPAS.3 This combination is often described as
‘sling-ring complex.5,6 The number of complete rings
may vary from one or two rings (focal stenosis at the LPA site) to long
segment tracheal stenosis involving the proximal trachea as well. CTR
may extend distally into the bridging bronchi, creating significant
surgical challenges for repair. The trachea may also taper from proximal
to distally in a funnel shape as the “rat’s tail”
trachea.14 CTR are associated with genes encoding
cartilage signalling pathways including Sonic Hedgehog and
Wnt.13 These
genetic embryonal developmental pathways are well described in animal
models but the exact association of complete tracheal rings and LPA
sling remains unknown in humans.9