2: THE NECK

2 THE NECK



Clinical importance of the neck


Airway obstruction results from blockage at one or more of several sites. Choanal stenosis (at the nostrils) is the most obvious cause. Obstruction may also be caused by pharyngeal lymphoid hyperplasia, palatal cysts and defects, pharyngeal paralysis, neoplasia, pharyngeal cysts, congenital tracheal abnormalities and tracheal obstruction, and damage resulting from tracheotomy intubation. It is important to remember that lower respiratory tract disorders can become apparent as a nasal discharge or cough.


The larynx lies at the junction between head and neck. The cartilages of the larynx are locked into the caudal wall of the nasopharynx by the palato-pharyngeal arch of the soft palate in an artificial seal. This is disconnected for the act of swallowing, in which the palato-pharyngeal muscles participate.


In the condition of ‘roaring’ or ‘whistling’, the horse has an excessive inspiratory noise and poor exercise tolerance. More correctly, the condition is called recurrent laryngeal neuropathy. Damage to the cranial laryngeal nerve prevents the normal, symmetrical, abduction of the arytenoid cartilages during inspiration. Air turbulence in the larynx results. It is usually left-sided. The left recurrent laryngeal nerve is significantly longer than the right. The usual aetiological suggestion is that stretching or damage to the nerve results from its course around the aorta in the thorax. There are various gradations of the disorder. It is really a progressive neurogenic atrophy. Treatment is beyond the scope of this introduction. Several surgical procedures have been developed over the years including removal of the laryngeal ventricle (ventriculectomy), prosthetic ligature, and arytenoidectomy. All of these techniques require a detailed knowledge of the anatomy of the larynx. Other defects of the larynx may include epiglottic entrapment, sub-epiglottic cysts (these are usually derived from embryonic tissue from the thyroglossal duct and are often found in an entrapment), arytenoid chondritis and defects of the 4th branchial arch leading to congenital abnormalities.


In young horses, pharyngeal lymphoid hyperplasia, probably in response to an infection, is a common problem. In epiglottic entrapment, the cartilage of the epiglottis becomes enveloped by a fold of glosso-epiglottic mucosa arising between the epiglottis and the base of the tongue and extending laterally as the aryepiglottic folds. It is not clear why it occurs. Lateral oedema is not unusual. There can be paralysis or paresis of the larynx, particularly when the lymph nodes are damaged by infection with Streptococcus equi (strangles).


Obstructive airway disease may involve the pharynx, but can be associated with guttural pouch problems. Other causes of obstructive airway disease include emphysema, pharyngeal paralysis, palatal defects, or displacement of the soft palate.


Occasionally the trachea is the original source of an obstructive problem, as the horse is notoriously susceptible to allergic respiratory disease. The trachea can be palpated in the neck. Its superficial position facilitates transtracheal aspiration, by which uncontaminated samples can be collected from the lower respiratory tract. An incision is made through the skin in the ventral midline; the strap muscles are separated and an incision made between the cartilaginous rings, through the annular ligament, into the trachea. Then a catheter can be used to flush with saline for a tracheal wash or for bronchoalveolar lavage. Tracheal obstruction is not common but may occur following trauma or result from a dorsal mass pressing on the trachea. The obstruction is usually cervical, occasionally thoracic. Tracheal endoscopy is used for investigating dynamic airway collapse.


Blood can be collected from the jugular vein and from other veins, including the transverse facial vein, the superficial thoracic vein caudal to the elbow, and the cephalic vein in the medial aspect of the forelimb. The saphenous vein on the medial aspect of the hind leg is also available. For arterial blood-gas analysis, the transverse facial artery, facial artery and dorsal metatarsal artery can be used. The jugular groove is important. It contains the jugular vein, an important structure for intravenous anaesthesia, administration of medications and for obtaining blood samples. The cranial third of the groove is the most accessible and likely to pose fewer hazards. It is also used for jugular catheters. The vein normally has a visible jugular pulse in the lower third of the neck. Distension of the jugular vein, or an abnormal pulse, suggests primary cardiac disease, or heart compression caused by fluid or masses in the mediastinum or pericardial sac. Thrombosis of the jugular vein (septic or non-septic) may occur after venepuncture or catheterisation and the effect on venous return from the head may ultimately produce oedema of the muzzle. The thyroid gland is important clinically. Foals rarely suffer from hypothyroidism, even though there is no build-up of iodine stores. In old horses there may be thyroid adenomas causing palpable thyroid enlargement and hyperparathyroidism also occurs.


The cervical vertebrae are clinically important. Equine accidents during racing or hunting may cause cervical vertebral trauma, particularly at the atlanto-occipital articulation. The important malformation known as cervical vertebral stenosis is more colloquially known as ‘wobbler syndrome’. It occurs in 18- to 30-month-old thoroughbreds. It is probably of complex aetiology but may involve cervical vertebral displacement, developmental abnormalities of the cervical vertebrae, or cervical arthropathy. It is also possible, using ultrasonic guidance, to accomplish facet arthrocentesis for the joint spaces between the cranial and caudal processes of the vertebrae when neck pain is suspected. It is essential not to puncture the vertebral artery. The cervical musculature is readily accessible and not noticeably subject to adverse effects and so is most often the site for intramuscular injection in the horse. The cervical transverse processes can be felt, and a site 10 cm or less above the vertebrae in the middle third of the neck, a handsbreadth in front of the scapula and ventral to the nuchal ligament, is ideal. The pectoral muscles are sometimes used and also the lower half of the semimembranosus and semitendinosus muscles in the hind leg.


Dysphagia (difficulty in swallowing) may involve the mouth (oral foreign body) or pharynx, but is perhaps more likely to be associated with the oesophagus. It can also result from oral irritation, brain stem disease or cranial nerve damage, rabies, or ‘grass sickness’ (now known as equine dysautonomia). Oesophageal obstruction may be caused by a variety of factors, including foreign bodies, dental problems and retro-oesophageal abscesses such as guttural pouch empyaemia. Simple intraluminar oesophageal obstruction may be caused by food, woodchips or bedding and may be indicated by salivation, retching and coughing. Passing of a nasal tube will indicate the site of ‘choke’ and then the material can, hopefully, be flushed onward or flushed retrograde. Oesophageal perforation may result from oesophageal obstruction, foreign body perforation, external wounds, repeated intubation, extension of infection or injury, or even corrective surgery. Oesophagotomy should be carried out over the site of obstruction. It requires an 8 – 10 cm longitudinal skin incision in the ventral midline. The positions of the longus colli, sternocephalicus, and sternothyrohyoideus muscles, jugular vein, and the brachiocephalicus and omohyoideus muscles should be noted to carry out oesophagotomy successfully. The common carotid artery, recurrent laryngeal nerve and vagosympathetic trunk must be treated with the utmost care on both sides.


The lymph nodes of the neck are valuable clinical indications of pathological changes in their drainage areas. The lateral retropharyngeal lymph nodes are supposedly superficial but are not usually palpable unless enlarged with fluid or inflamed or neoplastic. The medial retropharyngeal nodes are not palpable but the deep cervical chain of lymph nodes may be palpable if enlarged. The superficial cervical (formerly pre-scapular) lymph node takes drainage from caudal neck, shoulder and most of the fore limb. It is one of the largest lymph nodes in the horse’s body, and it takes drainage from a very wide area, including all of the front limbs. If often extends to a chain of accessory nodes and is palpable in thin horses, even when not enlarged.


The major clinical features of the caudal part of the head, bordering the neck, are the paired diverticula of the auditory tubes (guttural pouches). They lie ventral to the cranium and dorsal to the pharynx and oesophagus. The volume of each pouch is approximately 300 ml. Left and right pouches are in contact medially (mid-line). The architecture of each pouch is complicated by the stylohyoid bone which divides each pouch into lateral and medial compartments. The pouch is lined by pseudostratified ciliated epithelium and is therefore secretory. The drainage points of the pouches into the auditory tubes are not, unfortunately, at the lowest point of the pouch. These openings are patent during swallowing, and gravity aids drainage when the head is down. The ostia of the auditory tube are slits, under cartilage flaps, that open onto the dorsolateral wall of the pharynx. The clinical problem with the guttural pouches lies in their relationships to surrounding structures. The medial compartment is crossed by cranial nerves IX to XII and by the internal carotid artery, whereas the lateral compartment is crossed by the external maxillary artery and vein, and the facial nerve lies on the dorsal surface. Thus, any infection in the guttural pouches can potentially erode and damage these structures leading to a wide variety of clinical signs. Also, the pouch is a natural reservoir for Streptococcus equi, the cause of strangles. The simplest condition affecting the pouch is tympany which occurs mainly in foals, is often bilateral and can be felt and seen as swellings caudal to the mandible. In this condition, the ostium acts as a non-return valve; air enters but does not escape. It can be relieved by creating a fistula with the pharynx, or with the opposite pouch, or by dilating the ostium. There may be chronic inflammation (guttural pouch diverticulitis); mucus accumulates in the pouches and this may lead to chronic infections and the presence of solid concretions or chondroids. Far more serious is guttural pouch mycosis. This is an invasive process in the pouch, particularly in the upper part of both lateral and medial compartments (close to the vessels). The erosive process may therefore damage the internal carotid artery (medial compartment) or external maxillary artery (lateral compartment). This may result in a severe haemorrhage as the first clinical sign, which may rapidly prove fatal. In addition, the close relationship with the nerves and other structures may cause pharyngeal paralysis, laryngeal hemiplegia or facial palsy. The erosion may even extend into the temporomandibular joint. The complications can be assessed by endoscopy, ultrasonography and radiography. Treatment can be achieved through a variety of approaches such as; a) an incision cranial to the wing of the atlas; b) through Viborg’s triangle which gives only restricted access; c) a parapharyngeal approach with the horse in dorsal recumbency in which a ventral midline incision is made, permitting an approach lateral to the larynx, trachea and cricopharyngeus muscle; or d) a technique known as the ‘modified Whitehouse approach’ which involves an incision ventral to the linguo-facial vein.


Synovial bursae deep to the funicular part of the ligamentum nuchae may become enlarged and infected. At the arch of the atlas vertebra (C1), the cranial nuchal (atlantal) bursa may give rise to ‘poll evil’. Over the supraspinous processes of T2–T4, between the funicular and laminar parts of the ligament, lies the supraspinous bursa, which may give rise to ‘fistulous withers’. These are serious conditions requiring antibacterial therapy and, possibly, surgical drainage. Brucella abortus has been incriminated in both conditions.

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Jul 8, 2016 | Posted by in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off on 2: THE NECK

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