30 Fistula
A fistula is an abnormal communication between two epithelial-lined surfaces. It is not to be confused with a sinus, which is a blind ending tract lined with epithelium. Fistulae may be congenital or acquired. Congenital fistulae usually occur as a result of a disorder of embryological development. Acquired fistulae occur as a result of infection, inflammation, malignancy, trauma or they may be iatrogenic.
30.1 Congenital Head and Neck Fistula
An understanding of embryology is required to manage congenital fistulae. They are usually due to a disorder of development of the branchial arch system. They have a range of expression and may present as a simple sinus (blind-ending, epithelial-lined pits), a cystic lump or a fistula with internal and external openings.
The branchial apparatus appears at about the fourth week of foetal development. It consists of six paired arches, separated by clefts externally (ectodermal) and pouches internally (endodermal), and is responsible for the formation of the lateral structures of the face and neck. The persistence of a cleft or pouch alone may lead to a simple sinus opening externally, or internally, respectively. Persistence of both can lead to the development of a fistula with both internal and external openings, joined by a fistula track.
30.1.1 First Branchial Arch Fistula
The first branchial arch is responsible for the formation of the malleus, incus, mandible and maxilla, amongst other structures. The first arch pouch forms the eustachian tube and middle ear, while the dorsal portion of the first arch cleft forms the external auditory meatus; the ventral portion is normally obliterated. When it persists, a first arch fistula will form. Although relatively uncommon (accounting for < 5% of branchial arch fistulae), they tend to be relatively large, have their superior opening in the external auditory canal and their inferior opening in the neck, somewhere between the tragus and the hyoid bone. They have a variable relationship to the facial nerve. Like all such fistulae, they carry a significant risk of infection and so are best excised. A full parotidectomy approach will be required, often after a prior fistulogram.
30.1.2 Second Branchial Arch Fistula
These are the commonest form of such fistula, accounting for approximately 95% of cases. The second branchial arch is responsible for the formation of the stapes, the stylohyoid ligament and the posterior portion of the hyoid bone. The second arch pouch forms the bed of the tonsillar fossa. The second arch also grows and extends inferiorly, over the second, third and fourth branchial clefts, which are normally obliterated. Where this obliteration does not occur, branchial cysts can form. If there is a connection to the skin, then a branchial sinus will form. If, in addition, there is a connection to the second branchial pouch, a fistula will be formed.
Such fistulae usually have a skin opening low in the neck, at the anterior border of the sternomastoid, and an internal opening in the tonsillar fossa. They usually pass between the internal and external carotid arteries.
Investigations usually involve a fistulogram, and may include a contrast computed tomography (CT) or magnetic resonance imaging (MRI) scan. Again, there is a risk of infection with such fistula and so surgical excision is warranted. The surgery will commence with an elliptical incision around the lower cervical opening but may require one or more further incisions, higher in the neck, to allow full visualisation of the fistula and to facilitate safe and complete excision. On rare occasions, an ipsilateral tonsillectomy may also be required.
30.1.3 Third and Fourth Branchial Arch Fistula
These fistulae are much less common than those described above. Where they occur, they tend to have a similar opening in the neck to the second arch fistulae, but the internal opening will be found in the pyriform fossa, or pharynx. Investigation and surgical excision follow the principles described earlier.
30.2 Acquired Fistula
30.2.1 Oroantral Fistula
An oroantral fistula is an abnormal communication between the oral cavity and the maxillary sinus. It can be caused by infection (tuberculosis, syphilis, leprosy), malignant neoplasms, phycomycoses, midline granuloma (a form of lymphoma), developmental clefts and cocaine abuse, but the most common cause of an oroantral fistula is tooth extraction. Maxillary first molars account for 50% of oroantral fistulae caused by extractions. It can occur as a consequence of infection of a bone graft placed during a sinus floor augmentation procedure. (Such procedures are performed through a lateral sinus window, not unlike a Caldwell–Luc approach. The sinus mucosa is elevated and some form of bone graft is placed in the floor of the sinus to create a thicker bony sinus floor to allow for the placement of osseointegrated dental implants).
Management
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