Dynamic Laryngeal Collapse Associated With Poll Flexion: diagnosis, pathophysiology and treatment

Dynamic laryngeal collapse (DLC) associated with poll flexion, defined as bilateral vocal fold collapse with some degree of concurrent bilateral arytenoid cartilage collapse, is a newly described dynamic upper respiratory tract (URT ) obstructive disorder diagnosed most frequently to date in Norwegian Coldblooded Trotters (NCT).1,2 The disorder has also been diagnosed in Standardbreds, Icelandic horses and other breeds of gaited horses3 which perform in poll flexion with a “high” head carriage. The disorder is often accompanied by other dynamic URT abnormalities including dorsomedial deviation of the epiglottic margins and/or aryepiglottic folds.1,2 Complaints of abnormal URT noise and poor performance are associated with periods of enforced poll flexion during training and racing. Diagnosis can only be established by high-speed treadmill videoendoscopy (HSTV) or overground endoscopy with periods of induced poll flexion. This is achieved by examining horses with full headgear and driving or riding them into the bit.4 Resting endoscopy and HSTV without enforced poll flexion are within normal limits.

DLC associated with poll flexion can vary in severity among affected individuals, but in certain racehorses can cause a moderate to marked inspiratory obstruction. Petsche et al. demonstrated that induced poll flexion results in mild inspiratory obstruction in normal Standardbred (STB) horses exercised at maximum heart rates. 5 Another study compared tracheal pressure measurements in a group of 5 elite (E) NCT racehorses to 6 affected (A) individuals during alternating phases of free head carriage and poll flexion. Both groups of horses had similar baseline inspiratory pressures during the initial phase of free head carriage (E -30.66 cm H20; A -31.28 cm H20). Both groups had significantly greater negative inspiratory pressures during poll flexion relative to respective free head carriage measurements; however, the affected group had much greater negative inspiratory pressures (-58.4 cmH20) relative to the elite group (-39.4 cm H20). 6

The cause of DLC associated with poll flexion appears to be a more rostral positioned larynx, which becomes externally compressed in a “narrow” intermandibular space, during periods of enforced poll flexion with the head held high. 7 In NCT racehorses only 12 degrees of poll flexion is necessary to induce DLC in affected individuals.8 Other breeds may require more restraint into the bit (poll flexion angle) to induce the disorder. A recently published histopathology study did not support a neuromuscular cause of the disorder. 9 The author has unintentionally induced this disorder in several NCT racehorses treated with Tie-forward surgery for DDSP.

There is no satisfactory treatment to date for the disorder. A modified checkrein to limit poll flexion in racehorses has shown promise,8 but is not in practical use in Scandinavian racing. Bilateral ventriculocordectomy2 and unilateral prosthetic laryngoplasty3 have been attempted to salvage horses for athletic use.

References:
1) Strand E et al. Equine Vet Educ 16: 242 – 254, 2004.
2) Fjordbakk CT et al. Vet. Surgery 37: 501-507, 2008.
3) McCarrel & Woodie. Vet Clin N Amer- Equine 31: 13-26, 2015.
4) Strand E et al. Equine Vet J 44: 518-523, 2012.
5) Petsche et al. Equine Vet. J., Suppl. 18, 18-22, 1995
6) Strand E et al. Equine Vet J 41: 59-64, 2009.
7) Fjordbakk et al. Equine Vet J. 45: 705-10, 2013.
8) Fjordbakk et al. Equine Vet J. 44: 207-13, 2011.
9) Fjordbakk et al. Equine Vet J. 47:603-608, 2015.