A 6-month-old Morgan colt was evaluated because of a 10-cm right-sided retropharyngeal swelling. The swelling was soft and moveable on examination, and palpation did not elicit signs of
pain. Radiography revealed a large space-occupying mass ventral to the second cervical vertebra; ultrasonography revealed an anechoic fluid-filled structure with a well-defined hyperechoic
capsule. Fine-needle aspiration yielded a viscous
amber fluid. Cytologic evaluation indicated that the fluid was an exudate; anaerobic and aerobic bacterial culture did not yield any growth. Histologic examination of a portion of the
cyst capsule revealed a connective tissue wall lined by pseudostratified columnar to cuboidal epithelium, consistent with a
branchial cyst. The
cyst wall was marsupialized to the skin, and
iodine sclerotherapy was performed twice daily for 14 days, at which time
forceps were introduced into the
cyst and the
cyst lining was removed. The site was allowed to heal by second intention, but 10 days later, the swelling recurred. An incision was made over the previous marsupialization site, and residual remnants of the cauterized
cyst lining were removed with a
forceps. The foal did not have any other complications during the subsequent 2 years. Branchial arch
cysts are uncommon embryonic anomalies of horses, mice, cats, dogs, and cattle. Results suggest that marsupialization and
iodine sclerotherapy may be a viable alternative to surgical excision in horses with
branchial cysts; however, the entire
cyst lining must be removed at the completion of
sclerotherapy to prevent recurrence and
abscess formation.