Practical Gastrointestinal Endoscopy

COLONOSCOPY AND FLEXIBLE SIGMOIDOSCOPY 125
Fig. 6.37–Fixed (iatrogenic) hairpin
bend at the sigmoid–descending
colon junction.
Fig. 6.38–The length of mesentery
and the extent of retroperitoneal
fixation determine the
acuteness of the sigmoid–
descending colon junction.
the sigmoid colon is long or elastic enough to make a large loop,
and the retroperitoneal fixation of the descending colon happens
also to be low in the pelvis (Fig. 6.38). Sometimes, when the sigmoid
colon is long an ‘alpha’ spiral loop occurs, which, blessedly
for both endoscopist and patient, avoids any angulation at the
sigmoid–descending junction. The ‘alpha’ describes the shape of
the spiral loop of sigmoid colon twisted around on its mesentery
or sigmoid mesocolon into a partial iatrogenic volvulus (Fig.
6.39). Formation of the loop depends on the anatomical fact that
the short inverted ‘V’ base of the sigmoid mesocolon twists easily—providing
that the sigmoid is long enough and that there
are no adhesions.
Mesenteric fixation variations occur in at least 15% of subjects
because of partial or complete failure of retroperitoneal fixation
of the descending colon in utero (see p. 109). The result is persistence
of varying degrees of descending mesocolon, which in
turn has a considerable effect on what shapes the colonoscope
can push the colon into during insertion; the descending colon
can, for instance, run up the midline (Fig. 6.40) or allow a ‘reversed
alpha’ loop to form (Fig. 6.41). Surgeons are well aware
that there is great patient-to-patient variation in how easily the
colon can be mobilized and delivered outside the abdominal
cavity; occasionally the whole colon can be lifted out without
dissection. A colon that is ‘easy’ for the surgeon to mobilize is,
however, often extremely unpredictable and difficult for the
endoscopist.
Fig. 6.39–An ‘alpha’ spiral
loop—a bene ficial iatrogenic
volvulus.
Fig. 6.40–The endoscope may
push a fully mobile distal colon
up the midline.
Recognizing loop formation
Recognizing that a loop is forming doesn’t take genius, but it’s
surprising how many endoscopists ‘push on regardless’ on the