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.