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Standing Thyroidectomy HorseStanding thyroidectomy in 10 horsesMarco Marcatili1, Sarah J. Voss2 and Patrick J. Pollock31,2, Weipers Centre Equine Hospital, Division of Equine Clinical Sciences, School of Veterinary Medicine, University of Glasgow, 464 Bearsden Road, Glasgow, G61 1QH, Scotland1Pool House Equine Clinic, Crown Inn Farm, Lichfield, Staffordshire, WS13 8RD, UK. 3 The Royal (Dick) School of Veterinary Studies, The University of Edinburgh, Easter Bush, EH25 9RG, ScotlandCorrespondence: Marco Marcatili: marco.marcatili86@ABSTRACTObjective – To describe a surgical technique for thyroidectomy in horses with thyroid neoplasia under standing sedation and local anesthesia.Study design – Retrospective clinical study.Animals – Client owned horses (n=10).Methods – Medical records of horses with a history of thyroid enlargement were reviewed. Horses were included in the study if thyroid gland enlargement was treated surgically using hemi- or bilateral thyroidectomy with the horse standing and sedated. Data from follow up clinical examination, performance level, occurrence of regrowth and cosmetic outcome were evaluated.Results –Thyroid enlargement was unilateral in eight, and bilateral in two horses. Histopathological findings included adenomas (50%), adenocarcinomas (20%), cystic hyperplasia (20%), and C-cell adenoma (10%). No major complications were encountered during surgery or during the postoperative period. All the horses resumed their previous level of exercise within six weeks. One horse presented with evidence of recurrence after seven months and a second surgery was required. Recurrent laryngeal nerve neuropathy and seroma formation subsequent to surgery, were not recorded in any of the cases.Conclusion – Thyroidectomy can safely be performed with the horse standing and sedated with local anesthesia.Clinical relevance – Performing standing thyroidectomy does not increase intra- or postoperative complications and could be considered for horses presented with thyroid enlargement amenable to surgery.IntroductionThyroid enlargement can be related to goiter or neoplasia.1 Goiter refers to a non-neoplastic enlargement of the thyroid due to an excess2 or deficiency of iodine in the diet, or it can be due to hyperthyroidism.3-5 Neoplasia is the most common cause of progressive growth of the thyroid gland,5-20 with adenomas, adenocarcinomas, and C-cell tumors most commonly reported in horses.5-20 Less commonly mixed follicular C-cell carcinomas have been described.4 Thyroid tumors have been reported to occur in horses of all ages. However, they are most likely to be encountered in horses over 10 years of age, with pathological studies reporting an incidence of 30% for horses ≥ 10 years old and 75% in horses ≥ 20 years old.14,19Thyroid enlargement is associated with a variety of clinical signs including: localized enlargement in the throat region, respiratory distress/dyspnea, abnormal respiratory noise, esophageal obstruction (choke), and reduced or limited movement of the head and neck, particularly when ridden.21The treatment of choice for thyroid masses that result in clinical signs is surgical excision (uni- or bilateral thyroidectomy). 21 Hemithyroidectomy is an effective and safe procedure for cases where clinical signs associated with thyroid masses are present (i.e. dyspnea and dysphagia). The long-term prognosis after surgery is excellent.21 The most commonly reported surgical complications include seroma formation (29-33%) and recurrent laryngeal nerve neuropathy (RLNN) 0-50%.6,21 For this reason if a benign enlargement is detected during clinical examination, without any related clinical signs, a conservative approach can usually be recommended.1To date thyroidectomy (hemi- or bilateral) has been performed with the affected horse under general anesthesia (GA). 4-6,11,12,15,17,21,22 However, risks and complications associated with GA can be a limiting factor for the overall outcome23 with a mortality risk of 1% reported for healthy horses undergoing general anesthesia.24 Performing thyroidectomy under standing sedation would eliminate the risks associated with GA. Furthermore, we hypothesized that the standing position could improve identification of neurovascular structures and the esophagus during the surgical procedure as gravity would result in a progressive ventral displacement of the thyroid mass during surgery. To the authors’ knowledge there are no reports describing a standing technique to date. The aim of the study presented here was to describe a technique for thyroidectomy in standing horses and to report the outcome and complications.Materials and MethodsCase selection: The medical records of horses referred to two equine clinics, University College Dublin, and The Weipers Centre Equine Hospital for investigation and treatment of an expanding mass in the region of the proximal third of the neck between 2002 and 2016 were reviewed. The following information was recorded: age, sex, breed, history, ultrasonographic findings, upper airway endoscopy findings (pre- and post-surgery), and intra- and post-operative complications. Follow up information was obtained 6 weeks to 1 year after discharge by telephone conversation with the owner/keeper.Preoperative examination and procedures: All the horses underwent a complete clinical examination upon arrival. The lesion was classified as uni- or bilateral, and where indicated, the cranial third of the neck was clipped with a #40 blade and a bilateral comprehensive ultrasonographic examination of the thyroid performed. Ultrasonographic examination was performed using a 12MHz linear probe (Vivid i, GE?). The echogenicity, size (diameter) and presence or absence of a well-defined capsule were recorded. Where indicated (i.e. not performed before referral) a fine needle aspiration biopsy was performed. Endoscopic examination of the upper portion of the respiratory tract was also performed (Karl Storz?, Germany) in order to evaluate laryngeal function and to determine any effect of the thyroid mass on the airway lumen.Surgical technique: Procaine penicillin (22000 IU/kg IM) and flunixin meglumine (1.1mg/kg IV) were administered 60 minutes before surgery. The horses were sedated using a combination of detomidine hydrochloride (0.01mg/kg IV) (Medesedan? 1%) and butorphanol tartrate (0.01mg/kg IV) (Butador? 1%) as a bolus and maintained on a constant rate infusion (CRI) of detomidine (0.01 mg/kg/h) throughout surgery.25 The head and neck of each horse was extended and placed on a dental headrest. An area extending from the middle third of the mandible to the cranial third of the neck was clipped bilaterally with a #40 blade and the skin aseptically prepared. Prior to surgery the skin and subcutaneous tissues were desensitized using 10-30mL of mepivacaine hydrochloride (Intra-Epicaine? 2%). More local anaesthetic was added intraoperatively as required, either by direct injection, or by placing a gauze soaked in local anaesthetic solution into the surgical site and leaving it in place for approximately 1 minute. An adhesive surgical drape was positioned around the cranial third of the neck and head and a fenestration was created over the surgical site. When appropriate skin staples were used to maintain the position of the drape. A linear skin incision over the thyroid mass was made using a #10 scalpel blade. In cases where the thyroid mass was present bilaterally a ventral midline linear incision was made in order to access both thyroid lobes through one incision. In cases with a unilateral mass the axial edge of the sternomandibularis muscle was used as landmark for the skin incision. This allowed the skin incision to be located abaxially, in most cases overlying the mass. Thereafter dissection was used to avoid damage to the linguofacial vein. The subcutaneous tissue was bluntly dissected and the thyroid capsule identified. The fascial plane between the omohyoideus and sternomandibularis muscles was used to guide the plane of the dissection. Where required the omohyoideus muscle was divided along its fibers in order to exteriorize the thyroid gland. Blunt dissection was continued around the thyroid capsule whilst applying gentle traction to the mass in a ventral direction. After the connective tissue to the cranial and caudal pole of the thyroid were dissected it was possible to visualize the cranial thyroid artery (-ies) and vein (-s) of the thyroid dorsomedially and completely exteriorize the thyroid mass (Fig. 1). As the mass was mobilized, the effect of gravity led to progressive ventral displacement, providing visualization of the thyroid vasculature. The vasculature was identified and hemostasis was performed with either manual ligation (n=3), a LigaSure? device (Covidien Medical Products, Manufacturing LLC, Neustadt, Germany) (n=6) or an Enseal? G2 Super Jaw? (Ethicon, US, LLC, n=1). During the ligation (double circumferential technique) or sealing of the vascular structures, an assistant was asked to retract the skin incision using a handheld copper malleable retractor in order to facilitate visualization of the vessels (Fig. 2). After the hemostasis was achieved the mass was removed, the integrity of the capsule assessed (Fig. 3), and the surgical site inspected for any signs of hemorrhage. Where indicated the incised omohyoideus muscle was sutured using 2/0 USP glycomer 631 (Biosyn?) in a single continuous pattern. A simple continuous subcutaneous suture was then performed with the same suture material. In one case the subcutaneous suture was performed using a 3-0 USP PGA-PCL (copolymer of glycolide and e-caprolactone) unidirectional knotless suture (Quill?). In all cases the skin was sutured with 2/0 USP or 0 USP polypropylene suture material (Surgipro?) using an interrupted cruciate pattern (Fig. 4).The resected thyroid tissue was fixed in 10% formalin and submitted for histopathological examination. ResultsHorsesIn total 10 horses underwent standing thyroidectomy due to a rapidly growing mass. Breeds included; Thoroughbred (n=2), Sport Horse (n=2), Highland (n=1), Clydesdale cross (n=1), Welsh Section D pony (n=1), Arabian (n=1), Cross-bred pony (n=1), and Thoroughbred cross (n=1). There were 6 geldings and 4 mares. The median age was 14.5 years (range 7-26 years). A short term follow up period of 6 weeks was available for all cases except for one case that was lost to follow up. For 8 cases long term follow up was available (≥ 12 months).Clinical findingsIn 8 cases the mass was unilateral, of these, 5 were located on the left, and 3 on the right side of the neck. In the remaining 2 cases the masses were bilateral. In 6 of the cases the presenting complaint was a rapidly growing mass (two of these cases also had respiratory signs (i.e. respiratory noise during exercise). In 3 of the cases the initial presenting clinical signs were respiratory distress (i.e. dyspnea) and in 1 of these cases there was also history of recurrent esophageal obstruction (choke). In 1 case the only presenting clinical sign was esophageal obstruction (choke). Ultrasonography was not performed in all cases, but did assist in pre-surgical planning when performed. Lesions subject to imaging (horses 8, 9 and 10) appeared as encapsulated homogenous masses, containing multiple discrete hypoechoic regions. All masses imaged were immediately adjacent to large blood vessels. Pre-surgical endoscopic examination was unremarkable in all cases.Histopathological findingsHistopathological findings were consistent with adenoma (A) in 50%, adenocarcinoma (AC) in 20%, cystic hyperplasia (CH) in 20%, and C-cell adenoma (CA) in 10% of the cases. Surgical outcomeThe horses were restricted to box rest until the skin sutures were removed (10-14 days). Oral anti-inflammatory medication (phenylbutazone 2.2 mg/kg once daily) was administered for 5 days postoperatively. All the horses were discharged between 24 and 48 hours after surgery. Short-term follow up (6 weeks) was available for all but 1 horse (case n° 7). All the horses were able to perform at or above their previous level of exercise. No major complications were encountered during surgery, and in all the cases it was possible to completely exteriorize the thyroid lobe/s. The amount of bleeding was minimal and hemostasis was easily performed under direct visualization. In cases with evidence of dyspnea and esophageal obstruction (choke), clinical signs ceased immediately after surgery. Endoscopic examination (at rest) was performed prior to discharge, there was no evidence of laryngeal asymmetry/dysfunction in any of the horses.Long-term follow up was available for 8 horses and there were no complications reported by the owner. In 1 of the 2 horses diagnosed with AC (case N° 8) the mass returned after 7 months. The horse underwent the same procedure and the mass was successfully removed. The histological findings were consistent with AC on both occasions. However, the horse was diagnosed with metastatic lesions of the regional lymph nodes and mandible two years later and was subjected to euthanasia.DiscussionThyroid neoplasia is the most frequent cause of progressive growth of the thyroid gland in the horse, 6 with prevalence increasing with age.14,19 The case series reported here included one case of metastatic adenocarcinoma, however most thyroid neoplasias are benign,26 and usually do not necessitate treatment unless impinging on adjacent structures. Surgical excision of the affected gland is the treatment of choice for cases that present with clinical signs related to the presence of the mass (e.g. dyspnea and dysphagia). Until recently this surgery was routinely performed under GA. The study described here reports a technique for thyroidectomy in standing sedated horses.The most common presenting complaint was the presence of a rapidly growing mass at the level of the thyroid region. Two of the 6 cases presented with clinical signs of dyspnea, whereas the remaining 4 cases did not show any respiratory abnormalities. Thyroid function testing was not undertaken in any of the cases described here. However, none of the horses showed clinical signs indicative of a functional thyroid neoplasia such as hypothyroidism (i.e. lethargy, exercise intolerance, poor hair coat) or hyperthyroidism (i.e. weight loss, tachycardia, tachypnea, hyperactive behavior, ravenous appetite, alopecia, and cachexia) 22,26. While every horse presented with a thyroid mass should be checked for the presence of these conditions, it should be noted that that the majority of thyroid masses in horses are believed to be non-functional.26 Ultrasonography was not performed in all cases, however ultrasound images were useful to provide pre-surgical planning. The use of color doppler allowed identification of the vascular structures associated with the mass (Fig.4). For this reason, and to confirm which structures were potentially involved (i.e. lymph nodes), ultrasonographic examination could be considered prior to surgery. In addition, if biopsy is being considered ultrasonography is recommended to direct sample acquisition.26 It was not possible to differentiate between adenomas and C-cell tumors ultrasonographically in the horses that were imaged. This is supported by previous reports that ultrasonographic appearance is poorly associated with histopathological findings.6 The absence of risks associated with GA, and the ability to perform the procedure in clinics where operating facilities are limited are advantages of standing surgery.27, 28 Previous reports have also suggested that standing surgical techniques can improve surgical access and surgeon comfort.27 Nevertheless, consideration should be given to performing surgery under GA in fractious or very nervous horses that may react unexpectedly or aggressively, when surgery is performed under standing sedation risking injury to themselves or the surgical team.27,28RLNN has been reported as a complication in up to 50% of cases undergoing hemithyroidectomy under GA.6 In a more recent study there were no complications associated with the recurrent laryngeal nerve ipsilateral to the thyroidectomy.21 In the study described here, none of the horses developed RLNN and we believe that this may be due to the ventral displacement of the thyroid mass, as a result of the standing position, which results in better visualization of the surgical field. In the current report, none of the horses developed RLNN. The improved outcome could be attributed to the ventral displacement of the thyroid mass during dissection in the standing position, which results in better visualization of the surgical field and neurovascular structures. As ventral displacement is facilitated due to the effect of gravity on the mass, we recommend beginning the dissection dorsocranially for the unilateral approach. This facilitates ventral displacement of the thyroid lobe, exposing the neurovascular structures deep to the mass, and reduces the risk of damage to the nerves providing laryngeal innervation. In cases with bilateral thyroid enlargement, a midline incision is still required to facilitate access to each thyroid lobe. In these cases, tilting of the head (i.e. left for right hand side and vice versa) combined with skin displacement allows good visualization of the surgical field and neurovascular structures. The use of nasogastric intubation has previously been advocated as a useful method to improve identification, and therefore avoid damage to, the esophagus.6 However, nasogastric intubation proved unnecessary in the cases treated under standing sedation, and this complication was not encountered.Hemorrhage has been reported as a possible intraoperative complication when removing large thyroid tumors.21 The thyroid gland has a blood supply of 4 to 6 mL/min/g19 and it is therefore critical that the surgeon is familiar with techniques for hemostasis in order to avoid a large volume of blood loss intraoperatively. Intraoperative hemorrhage has been linked to seroma formation and previous studies have suggested that seroma formation affects 29-33% of horses undergoing hemithyroidectomy under GA.6,21 The use of surgical drains has been described, however this is generally considered to be unnecessary.6 None of the cases reported in this series developed a seroma, possibly due to improved visualization of the vascular structures when performing ligation or occlusion of the thyroid vasculature with bipolar vessel sealing instruments.Good long-term outcome was obtained in 7/8 cases with follow up available. One of the two horses diagnosed with AC was subjected to euthanasia following metastatic spread of the disease to the mandible and regional lymph nodes. Metastatic thyroid carcinomas are reported, with metastases affecting the uveal tract, heart, lungs, kidneys,15 and bone.10 Because the majority of thyroid tumors in horses are benign, 26 it is probably not necessary to routinely perform biopsy (FNA) of the regional lymph nodes. However, biopsy of the regional lymph nodes should be considered if there is a strong suspicion of malignancy or in horses presented for thyroidectomy where there has been recurrence following a previous surgery. Neither of the 2 horses undergoing bilateral thyroidectomy in this study showed clinical signs attributable to hypothyroidism following surgery. This could be explained by the presence of deiodinases (D1 and D2) in the peripheral tissues that contribute significantly to systemic T3 production and serum T3 concentration in humans and animals. 29 However, although compatible with life, bilateral thyroidectomy induced lethargy, hind limb edema and development of a coarse hair coat in mares undergoing bilateral thyroidectomy. 30 For this reason, monitoring triiodothyronine (T3) and thyroxine (T4) concentration should be considered in horses undergoing bilateral thyroidectomy. In conclusion, this pilot data, suggests that standing thyroidectomy is a safe, repeatable, technique and that the outcome is improved due to the reduced complication rate when compared to the previously described technique performed on recumbent horses. DISCLOSURE The authors declare no conflict of interest related to this study.Tab. 1 – Table of presenting clinical signs, diagnostic features and outcome after standing thyroidectomy in 10 horses. (TB= thoroughbred; G= gelding; M= mare; A=adenoma; AC=adenocarcinoma; CH=cystic hyperplasia; CA=C cell adenoma; NA= not assessed, Y= years)Case BreedAge (Years old)SexPresenting signsLocationMass diameter (cm)HistologyFollow up1TB17 GDyspneaLeft side 6A3 Y, no recurrence2Highland 12GExpansive lesion 6 months’ durationLeft side NACH Lost to follow up after 2 Y, no recurrence until then3Sport horse9MDyspnea expansive lesionRight side 22A5 Y, no recurrence4Sport horse22GExpansive lesionLeft side 12ALost to follow up after recheck at 6 weeks5Clydesdale cross9MDyspneaRight side 17A5 Y, no recurrence6TB7MExpansive lesion and respiratory noise during exerciseBilateral 6 left11 rightAC5 Y, no recurrence7Welsh section D pony19GExpansive lesionRight side 5CHLost to follow-up8Arabian10MDyspnea, coughing and recurrent chokeBilateral NAACNeoplasia recurred on left side 7 months after surgery. 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