One case of bilateral fusion of upper and middle trunks of
brachial plexus was reported by
6. An extensive study by
Uysal et al.,
7 showed superior trunk not being formed in 1%
of cases and inferior trunk not being formed in 9% of case. Our
case of unilateral variation is associated with phrenic nerve variation (passing anterior to subclavian vein and
communicating with C5 root) and double axillary veins.
This case report gains tremendous importance in context
of upper trunk brachial plexus injury (Erbs parlysis). Above
mentioned case will not manifest characteristic (waiter’ tip
position) or typical (porter’s tip) deformity 8 and 9. Lee Mc
Gregor’s book 8 described that abduction of shoulder is
dependent on C5, while flexion of the elbow is dependent on
C5, C6, on the contrary, adduction of at shoulder and extension
of wrist and fingers are dependent C6, C7, while extension of
the elbow is dependent on C7, C8. Hence the spectrum of
clinical manifestations produced by upper trunk injury in the
present case will be entirely different from the classic Erb’s
palsy and may mislead the clinicians. Phrenic nerve’s
communicating branch to C5 root may add pericardial and
diaphragmatic manifestations along with brachial plexus
injuries.
Kutiyanawala 2 reported double axillary veins, but that
case was not associated with trunk variation of brachial plexus.
The axillary vein variations are important in breast carcinoma
treatment, since venous drainage of the breast is mainly to
axillary vein, whereas the first part of axillary vein may be
used for venepuncture 8 and 9.
Embryological hypothesis:
The formation of the brachial plexus begins in early
development in the 4th week of gestation. In essence, as axonal
growth is directed by sclerotome, nerve formation follows the
dorsal rotation of the upper limb bud. The axons from the
ventral column motor cells start to grow towards the
sclerotome cell mass, thereby forming the ventral root.
Similarly the dorsal root forms by axons growing in the
opposite direction from the dorsal root ganglion cells. The
primitive capillary plexus of the flattened limb buds gives rise
to a peripheral border vein which serves as an early drainage
channel to blood brought in by the axial arterial vessels. The
border vein appears in the arm in the 6th week and the general
venous plan becomes outlined within the next two weeks of
intrauterine life. The radial extension of the border vein
atrophies but the ulnar portion persists, forming at different
levels the subclavian, axillary and basilic veins. The cephalic
vein develops secondarily in connection with the radial vein
but later it anastomoses with the external jugular vein, but
finally opens into the axillary vein, as in the adult 10.
As suggested by Sannes et al 11 that the guidance of the
developing axons is regulated by expression of
chemoattractants and chemorepulsants in highly coordinated
site specific fashion. Any alterations in signaling between
mesenchymal cells and neuronal growth cones can lead to
significant variations and probably in this case resulted in
absence of middle trunk and phrenic nerve communication
with C5 root along with double axillary veins. Once formed,
any developmental differences would persist postnatally.