Discussion
In this study, we demonstrated that there were two different pathways of
PSF and each pathway seemed to represent their origin. We first reported
two different routes of PSF, and these two arms presented each unique
characteristic. The inferior parathyroid gland and thymus that
originated from the 3rd pouch should migrate a long distance, passing
outside the thyroid cartilage, and the remnant of the pathway is the PSF
from the 3rd pouch, whereas the superior parathyroid gland and C cells
need not move a long distance behind the thyroid gland. These two kinds
of tract represent not only their pathways of development but also
clinical manifestations. The differences in histological and clinical
presentations of PSF between the 3rd and 4th pouch are presented in
Table 3. Given that the 3rd pouch generates the thymus and inferior
parathyroid gland, extrathyroid tissues around the PSFs the originated
from 3rd pouch is interesting. Since most cases penetrate the thyroid
gland, the fistulas may carry some thyroid tissues during migration.
However, we have a limited number of cases, and there are tendencies of
distinction between the 3rd and 4th pouch in the
clinical presentation. PSF from the 3rd pouch is frequently observed in
men, while that from the 4th pouch is commonly observed in women. Since
the 3rd pouch tends to run outside the inferior horn of the thyroid
cartilage covered by inferior constrictor muscle of the pharynx,
infections are limited locally. On the contrary, infections caused by
PSF from the 4th pouch with a tendency of routing inside are considered
to expand easily via loose connective tissue inside the superior pole
and isthmus of the thyroid gland. Regarding two routes of remnant by the
3rd and 4th pouch, our PSF with double tracts (3rd and 4th pouch, Case
10) previously reported was sufficiently probable.4The fistula with 4th pouch origin in this case and Case 9 did not have
epithelium in the lumen. Since both cases had severe deep neck abscess,
the epithelium may be exfoliated by severe inflammation and granulation.
To date, there is no analysis on PSF cases with complete fistulectomy
with one report by Madana et al. in 2011.10 They
reported 18 cases of fistulectomy, with no recurrence, and insisted that
15 cases had 3rd pouch origin and three cases had 4th pouch origin. This
proportion is quite different from our cases (5:7 vs. 15:3); however,
this previous report did not show any criteria or evidence for
distinction of 3rd and 4th pouch origin. They also did not show any
histological findings of accompanied tissues around the fistulas.
Additionally, a previous study10 and this study showed
no recurrence although palliative endoscopic cauterization showed
recurrence at 25%.9 Now, we have recommended
fistulectomy for not only understanding of its route and origin but also
prevention of recurrence.
This study has some limitations. We conducted 19 complete
fistulectomies, while we obtained only 12 pathological slides and two
calcitonin immunohistochemical stains, which were judged as having no
positive cells. Due to lack of calcitonin staining, we could not clearly
and exactly classify PSF generated from the 4th pouch. Only PSFs
accompanied by thymic and parathyroid tissues were defined as having
3rd pouch origin, and others should be regarded as
having 4th pouch origin. As a result, our classifications in histology
and clinical manifestation did not present a clear cutoff.