In humans, the branchial (pharyngeal) apparatus appears during the fourth week of development, and consists of arches, pouches, and clefts. The neural crest cells of the first through sixth pharyngeal arches give rise to the bones and cartilage of the jaws, hyoid apparatus, and larynx, while the endodermal pharyngeal pouches give rise to structures and spaces such as the Eustachian tube, parathyroid glands, and thymus. The first pharyngeal cleft, which is a surface feature, develops into the external acoustic meatus, but the remaining clefts disappear into a shrinking structure called the cervical sinus. However, in some individuals, remnants of the clefts or the cervical sinus persist, and if these abnormal structures fill with fluid, they will appear as branchial cysts.
Most branchial cysts and sinuses (persistent pharyngeal pouch) in humans involve the second cleft or pouch. In a recent paper, David and colleagues (2008) reported a branchial arch abnormality in a Haflinger foal that was likely due to persistence of the third cleft and/or pouch. Haflingers are a small horse breed, developed in the Southern Tyrolean Mountains, known for their chestnut coat color and flaxen mane and tail. The “patient”, a 1-week-old Haflinger foal, was in some respiratory distress, and presented with a soft, non-painful mass ventral to the throatlatch region. The mass displaced the trachea and larynx, as well as the right hemimandible, and had been drained of fluid, milk, neutrophils, and bacteria by a veterinarian several times since the foal’s birth.
Figure 1, David et al., (2008) Branchial cyst in 1-week-old Haflinger foal
After careful computerized tomographical analysis, both the mass and an associated abscess were painstakingly removed by the veterinary surgeons. Histopathological findings were consistent with the diagnosis of branchial cyst or sinus, and the location, medial to the right carotid arteries and entering at the piriform recess, was consistent with an origin from the third pharyngeal pouch. It’s worth noting that the external landmark for the foal’s mass, just cranioventral to the sternomandibularis muscle, corresponds to the location of branchial cysts in humans, i.e. along the anterior border of the sternocleidomastoid muscle. In fact, this location allows differential diagnosis in distinguishing a branchial cyst from a thyroglossal duct cyst, as the latter is located in the midline of the neck. The authors emphasized the difficulty of locating the branchial sinus by endoscopy in an equine patient, and concluded that, as for human patients with this anomaly, complete surgical excision of branchial cysts is necessary. The colt in this report recovered completely from the surgery, and grew and developed at a rate normal for the Haflinger breed.
David, F., Savard, C., Drolet, R., Alexander, K., Pang, D.S.J., Laverty, S. (2008). Congenital branchial apparatus malformation in a Haflinger colt. Veterinary Surgery 37, 3-11.