Response to letter from Dr Lane

The authors of the report would like to thank Dr Lane for his letter to the editor of the EVE about our case report (Lane, 2023; Morelli & Rubio-Martinez, 2023). We are pleased to see that our report has caught Dr Lane's attention and that he has found the report very helpful. That is certainly most important to us as our intention has always been to provide information useful to equine clinicians for the benefit of our clinical practice and our equine patients. Both authors would like to thank Dr Lane for his comments, to which we are pleased to respond below. Both authors thank Dr Lane for this clarification regarding this quotation/misquotation. We would like to advise that readers of our case report refer to the publications as detailed by Dr Lane for a more extensive and accurate review. Both authors thank Dr Lane for his contribution to this aspect of the case report. We understand that presence of an abnormal articulation will make the action of a trans-articular muscle less efficient or totally inefficient. We therefore agree that an abnormal crico-thyroid articulation will negatively affect the function of the crico-arytenoideus dorsalis (CAD) muscle. Intra-operatively, it was obvious that tension on the suture placed into the rudimentary muscular process caused abduction of the left arytenoid cartilage, and hence the suture was retained. Horses affected with laryngeal dysplasia can have different degrees of abnormalities in the laryngeal anatomy, with varying sizes of gap at the cricothyroid articulation (Barakzai, 2016). We would like to suggest that depending on the degree of abnormality and gap present, the efficiency and abducting effect of the prosthesis may vary amongst horses with laryngeal dysplasia. In the horse reported in our case report, the degree of abnormality may be considered mild to moderate and, subsequently, a partial abducting effect was achieved by placement of the prosthesis. The stabilising effect of a healed left-sided ventriculocordectomy on a previously unstable left arytenoid cartilage has been previously discussed. Two experimental studies have reported that left-sided or bilateral ventriculocordectomy restored sound levels to close to baseline or pre-neurectomy levels in horses with grade 4 RLN (Brown et al., 2003; Robinson et al., 2006). However, this has been challenged as it is expected that in those horses the left arytenoid cartilage remained unstable and therefore would still collapse during exercise (Barakzai et al., 2019). A clinical study on draught horses with naturally occurring grade 4 RLN, found that bilateral ventriculocordectomy significantly reduced the respiratory noise (Cramp et al., 2009); however, those horses were exercised only at trot postoperatively. Collapse of an unstable arytenoid cartilage is more likely at high speeds because of greater trans-tracheal negative pressures when increased respiratory effort is required, and therefore the stabilising effect of surgical ventriculocordectomy on highly unstable arytenoid has been challenged (Barakzai et al., 2019). Objective investigation of the effect of unilateral laser ventriculocordectomy on horses with naturally occurring RLN has been investigated in one study (Barakzai et al., 2019). The study involved seven horses and included only two horses with grade 4 RLN. The results of that study indicate that the stabilising effect of unilateral ventriculocordectomy on the left arytenoid cartilage was subjective and, if present, very limited in horses affected with highly unstable arytenoid cartilages, as pre and postoperative left-to-right arytenoid angle quotients were very similar (Barakzai et al., 2019). The authors then concluded that ventriculocordectomy is unlikely to stabilise the left arytenoid cartilage sufficiently and suggested that combining it with laryngoplasty may be a better option in those horses (Barakzai et al., 2019). The position and dynamics of the left arytenoid cartilage in the horse in our case report changed from highly unstable, fully collapsing preoperatively (figure 1) to stable, moderately abducted postoperatively (figure 4) during cantering exercise. Based on the evidence available to date, it is the authors' opinion that explaining the postoperative stable degree of abduction solely by the healed ventriculocordectomy remains speculative. Further studies on objective comparison of pre- and post-operative laryngeal function would help our understanding of the effects of laryngeal surgery. We thank Dr Lane very much for his contribution to this aspect and for sharing his ample expertise in this field. When performing palpation of the larynx on a horse, the intrinsic structures of the larynx are palpated through the externally located extrinsic laryngeal musculature; however, the aim of this palpation is centred on the intrinsic musculature, specifically the CAD, which when atrophied, allows the palpation of a more prominent muscular process of the arytenoid cartilage. In this report, the authors were referring to the intrinsic musculature, specifically the CAD muscle. We thank Dr Lane for his clarification and apologise for the inaccuracy. The authors do not disregard the role of palpation and would like to reiterate its value when examining the larynx in horses. Experience is certainly an asset when performing laryngeal palpation. In the case reported here, the palpable crico-thyroid articulation abnormality was mild compared with other horses with laryngeal dysplasia where the space between the thyroid and the cricoid cartilages can be substantial. This relatively small gap may not have been apparent as an abnormality to the less experienced clinician. Alongside palpation of the larynx, the authors would like to emphasise the remarkable complementary value of ultrasound, computed tomography or magnetic resonance when assessing the structure and function of the horse's larynx. Ultrasonography remains a readily available imaging diagnostic technique that should not be underestimated in cases with laryngeal conditions. L. M. Rubio-Martínez: Writing – review and editing. C. Morelli: Writing – review and editing. The authors have no conflict of interest.