Collapsing Trachea in a Miniature Horse

A three year-old miniature stallion presented to PHD Veterinary Services for the complaint of open mouth breathing and a "honking" sound during respiration. At presentation, the stallion was agitated and was continuously pawing at his muzzle with his mouth open. The stallion was mildly sedated for the endoscopic exam and interestingly, his breathing became more normal. The most proximal trachea (Figure 1) appeared normal in diameter however the lumen quickly became narrowed (Figure 2). The trachea was nearly completely collapsed for approximately 6 cm at the level of the thoracic inlet. Distal to this area, the trachea appeared normal (Figure 3).

Figure 1

Figure 2

Figure 3

Normally, the lumen of the trachea is continuous, extending from the larynx to the primary bifurcation of the trachea (Figure 4). Unfortunately, the miniature horse breed is predisposed to the condition known as collapsing trachea. The condition is often diagnosed shortly after birth or can develop later in life if the horse is allowed to gain excessive weight. A common clinical complaint is that of a "honking" sound during heavy respiration. The severity of the collapsing trachea depends on the location and the extent of the narrowed lumen. The closer the area of collapse is to the chest cavity, the worse the prognosis. There are reported cases of collapsing tracheas in miniature horses that were repaired via intra- and extra-luminal stents. Unfortunately, they are short term fixes and often do not provide a long term solution.

Figure 4

Keratoma in a horse!!

A 10 year-old gelding presented to PHD veterinary services for a history of recurrent abscesses in the left front foot. Over a period of 6 months, the gelding developed 3 distinct abscesses which ruptured at the coronary band. At presentation the gelding was mildly lame and there was no active drainage from the most recent abscess rupture. A radiographic exam was performed to determine if there was a radiographic explanation for the development of multiple foot abscesses in the same foot.

Figure 1

The most notable finding in the radiographic exam was evidence of coffin bone rotation (Figure 1 and 2). In Figure 2, the red dotted lines should be parallel. The red line on the left corresponds to the dorsal hoof wall and the red line on the right corresponds to the dorsal aspect of the coffin bone. The reason they are not parallel is because the red line on the right has rotated in a down ward direction approximately 10-12 degrees. Evidence of coffin bone rotation suggests a history of laminitis or founder and this might explain the recurring foot abscesses. In addition, the yellow lines in Figure 1 highlight the hoof wall defect which developed secondary to the recurrent foot abscesses. There are thin areas that appear radiolucent (black lines) which extend from the dorsal hoof wall defect (yellow lines) down towards the bottom of the foot. These radiolucent lines may correspond to remnants of draining tracts from the recent abscesses.

Figure 2

Figure 3

In Figure 3, the radiographic beam is aimed downward through the hoof and coffin bone. The image on the left is the left foot and the image on the right is the right foot. The yellow dotted circles correspond to the tip of the coffin bone that appears to be more radiolucent (less bone) in the left foot compared to the right foot. This would suggest some sort of pathological process affecting the tip of the coffin bone in the left foot. The possibilities for these radiographic changes include laminitis, chronic abscess with bone infection (osteomyelitis), and keratoma. An MRI study of the foot was strongly recommended to determine the cause and provide key information for likely surgical exploration.

Figure 4

The gelding was referred to the University of Florida for MRI of the front feet. A large area of concern was highlighted in the MRI that was either a chronic abscess or a keratoma. Subsequently, a partial hoof wall resection was performed by Dr. Andrew Smith and his surgical team at University of Florida's college of veterinary medicine (Figure 4). Once the hoof wall was removed, a large keratoma (yellow dotted lines) was identified and removed. A keratoma consists of  a benign tumor of keratin or horn-producing cells. The keratoma will grow between the coffin bone and the hoof wall causing distortion of the hoof wall, recurrent foot abscesses, and laminitis. Surgical resection is the only option but should be preceded by an MRI to identify the keratoma and its dimensions. Prognosis is good for a full recovery assuming the entire keratoma is removed at the time of surgery.

Special thanks to the Dr. Andrew Smith and the University of Florida's radiology department!