1: THE HEAD (INCLUDING THE SKIN)

1 THE HEAD (INCLUDING THE SKIN)



Clinical importance of the skin


The surface of the horse is clinically important. Any part is liable to trauma because of the activities in which a horse participates. For example, ponies collide with obstacles, thoroughbreds with fences, and hunters with any hazard in the field. Therefore, the skin of the horse can be easily damaged. Horses react with violence to stress, especially to the presence of other horses, so there is a real possibility of being kicked. Head trauma is always a risk; brain damage as well as skull fractures may result, and the neurological effects can be detected by a full neurological examination.


The skin is one of the largest and most important systems of the body in the horse. Physical examination of the surface of the horse will help to assess general bodily condition. Also, the skin itself may be examined and the distribution and sizes of lesions may be noted. Clinical manifestations may include alopecia (hair loss) and pruritis (itching). Physical examination may also detect signs of trauma in the form of wounds, bites, burns, thermal injuries, acute swellings (oedema), insect stings and bites particularly midge bites from Culicoides, and the consequences of any initial injury. There are many consequences of surface trauma and these include inflammation, infection, oedema, haematomas, chronic wound infections, lymphangitis (swollen lymphatics, particularly in the lower limbs) and purpura (immunological disorders).


The skin will also show evidence of generalized systemic disturbances. It may show dehydration, congestion, oedema, possibly jaundice etc. and will be visibly affected when the horse is suffering from any form of malnutrition. Many of these conditions will require treatment, and a thorough knowledge of anatomy will be important, particularly if diagnostic techniques (e.g. radiography, ultrasonography, diagnostic nerve blocks), treatments (bandaging, corrective farriery) and surgery are required. There are also diagnostic techniques that can be applied to the skin – these include skin scrapings, stains, microbiological culture and biopsy techniques.


One of the important truisms is that the horse has four feet on the ground; these are easily damaged. Always remember the old adage that the first place to look for the seat of lameness is in the foot. Puncture wounds are common, and these may progress to solar abscesses and other abnormalities discussed in the section on the foot. Piercing of the feet by nails from incorrect shoeing, or from losing a shoe traumatically, is also a relatively common accident.


Treatment of wounds is a whole subject in itself. There are four phases: inflammation and debridement, repair, maturation and wound healing. The approach to wounds is important. Wounds to the trunk often heal by contraction and are treated by daily cleaning (wound lavage and debridement), because they cannot be dressed easily. Wounds in distal parts of the limbs are often complicated by oedema, and often heal with ‘proud flesh’. ‘First intention’ wound healing follows suturing, but wounds that are either infected, or too extensive to suture, heal by ‘second intention’ and these repair, ultimately, by granulation tissue. Wounds require treatment and lots of time – in the horse, poor wound healing is a feature of a number of cases. There are many factors which delay wound healing, and these are beyond the scope of this introduction. Nowadays, a whole new field of equine surgery is possible, using skin grafting to repair these abnormalities of wound healing.


There is a multitude of skin conditions in the horse. Many of these are parasitic and include lice, mange of various types, and miscellaneous parasitic skin conditions including onchocerciasis, harvest mite infestation, equine ventral midline dermatitis and parafilariasis. Bacterial skin diseases are widespread and include specific exotic infections such as glanders (a zoonosis). Much more likely is one of a whole range of opportunistic bacterial infections including those involving Dermatophilus (rainscald); in many cases where there are pyogenic organisms involved, wounds progress to form an abscess. Mycoses include Sporothrix, histoplasmosis and, much more routinely, ringworm infections. A variety of hypersensitivity or similar reactions are also seen, including Culicoides hypersensitivity, urticaria pemphigus, and lupus erythematosus. Nodular skin disease and neoplastic papillomatosis may be seen, but are not common. The exception is probably sarcoids which are very common in all horses; they are a type of fibroblastic skin tumour. In addition, melanomas occur in grey horses. Other miscellaneous skin conditions include sunburn and photosensitization which may be of hepatogenous origin and associated with poisonous plants, especially ragwort (Senecio).



Clinical importance of the head


We will now consider the various regions and structures in the equine head, but disorders of the larynx and guttural pouch are discussed in Chapter 2.


The mucous membranes of the eye, mouth and nose give a good assessment of the cardiovascular system and general bodily state. They may reveal cyanosis (blue colour), which is a failure of oxygenation, pallor (anaemia), or even yellow as in jaundice, of which there are various causes in the horse. The ear is often involved in surface injuries but may also suffer from infections. There may be pain on handling, discharges, swellings and abnormal head/ear carriage. Specific diseases include parasitic infections such as Otodectes and also lesions from small biting flies. Sarcoids (fibroblastic tumours) are found throughout the body. They occur in many forms on the skin and are often found in the ear. They may be occult, verrucose, nodular, fibroblastic, malevolent or of a mixed type. Otitis media (inflammation of the middle ear within the tympanic cavity) may result in rupture of the ear drum and may be associated with extension of guttural pouch disease or haematogenous spread of pus-forming bacteria.


The eye is often examined to give a general assessment of health. The ‘sunken eye’ (loss of the periorbital fat from the eye socket) may be caused by dehydration or by wasting and emaciation. It is often one of the first abnormalities detected at a full clinical examination. The eye may also be discoloured. Pale ocular mucous membranes may suggest one of several types of anaemia. There may be excessive red cell destruction, resulting from renal or hepatic failure. Neonatal isoerythrolysis, in which foals have acquired immunity to red blood cells, is also a possibility. The mare produces antibodies to the RBCs of the foal, which are then concentrated in the colostrum and the foal receives these antibodies when suckled.


Examination of the eye is performed quite frequently as part of a pre-purchase examination. The eye is often involved in fractures of the orbit and periorbital region, associated with trauma in racing or hunting. Young horses falling over backwards may fracture the basisphenoid or basioccipital bone and, occasionally, the petrous temporal bones. There is a good prognosis for these fractures if the eyeball itself is not involved in the process. There are several developmental abnormalities of the eye that may be found, including microphthalmos, anopthalmos and endophthalmos up to the sclera. The sclera is involved in pion. Orbital neoplasia can also occur, but is very rare. The eyelids are often affected by trauma (eyelid lacerations are quite common) which causes inflammation. Entropion and ectropion (in-turning and out-turning of the eyelashes on the abnormal lids) are also seen in the horse.


There are a whole variety of eye conditions, most of which require specialist ophthalmological investigations and are beyond the scope of this chapter. More straightforward diagnoses include inflammation of the conjunctiva (conjunctivitis). Foreign bodies may be found in the conjuctival sac. The lacrimal duct may be narrowed or blocked. Corneal problems may include inflammation (keratitis), ulceration and foreign bodies. Bacterial, viral and mycotic keratitis may occur. The uveal tract (iris, ciliary body and choroid) may be involved in disease processes such as acute or recurrent equine uveitis. Neoplasia, such as squamous cell carcinoma of the third eyelid, is rare. Congenital and acquired cataracts are not common, neither is glaucoma. The fundus of the eye of the horse is subject to a considerable range of variations. Pathological changes may include retinal detachment, retinal haemorrhage, retinal atrophy and inflammation. Optic nerve neuropathy, optic neuritis and optic nerve atrophy are rarely diagnosed.


The mucous membrane of the nose should be inspected in a general assessment of health. Discharges at the nostril can be very variable and include serous fluid, mucus, mucopurulent or purulent discharges and frank haemorrhage. They can originate from the nose itself or from any part of the respiratory tract, including the sinuses and the guttural pouches or even from the lung.


The nostrils should be examined, as they may show a variety of changes including hypertrophy of the alar folds, atheroma of the nasal diverticulum (false nostril) or trauma and necrosis of the alar cartilage. Foreign bodies and nasal polyps are commonly found, but amyloidosis, ‘wry nose’, fungal diseases and atheroma are rare. At the nostrils a specific haemorrhage, called epistaxis (idiopathic), may be seen in racehorses. It is associated with racing. Exercise-induced pulmonary haemorrhage (EIPM) is also seen at the nostrils. The nostrils are a common site for nasal lacerations.


The upper respiratory tract is really important in equine practice. Horses that have run into objects, or have been kicked, may suffer from serious trauma to the nasal cavity. Many types of injury can occur and new diagnoses are being added all the time, particularly as a result of imaging techniques. These injuries include fractures to the walls of the paranasal sinuses or nasal passages; in these cases epistaxis will most obviously be a feature.


The nasal cavity provides a pathway through which the stomach tube or endoscope can be passed. The endoscope can be used to examine the sinuses, pharynx, larynx, Eustachian tube, guttural pouch, trachea and major bronchi of the lower respiratory tract, and also the oesophagus and stomach. It can be used to take pictures of anatomical abnormalities or lesions. It can be used to wash out structures and collect lavage fluids and tissues or discharges for culture, or cytology. It can also be used for biopsy. Nasotracheal intubation is a very useful technique for the investigation of respiratory disorders. Dynamic airway collapse can be revealed by tracheal endoscopy. The alar fold is reflected and the tube is passed along the ventral nasal meatus into the pharynx and then into the trachea, with the head elevated to prevent the tube being swallowed. For orotracheal intubation the tongue must be pulled out, a speculum is placed in the mouth, and the tube is advanced gently to allow swallowing. Radiography, ultrasonics and exercise tests including exercise endoscopy, are also used to examine these structures.


The head is obviously the most rostral part of the body and therefore particularly prone to trauma. In these cases there may be facial deformity or swelling. Jaw fractures occur reasonably commonly, especially to the body and the incisive region of the mandible. In the upper jaw, the incisive bone is most commonly affected. These fractures are often repaired surgically, possibly under nerve block and sedation, and may require braces, splints or pins and long screw fixation. These fractures may be accompanied by dental problems and other problems may follow extraction. Infections of the gingival margins may lead to dental alveolitis.


One of the key anatomical relationships in the head of the horse is between the nasal cavity, paranasal sinuses and teeth. The paranasal sinuses develop as progressive invasions of the diploë of the bones of the skull by the epithelium of the nasal cavity. The sinuses lighten the skull, insulate the brain from outside temperature variation and facilitate mucociliary clearance of the inhaled debris (look at the dust in a stable after the bedding or hay has been shaken out!). These sinus cavities are normally air-filled, resonant, and lined by mucoperiosteum. The presence of nasal discharge may be the first indication of a sinusitis that can subsequently be confirmed by radiography and endoscopy. Sinusitis may be associated with mycoses or neoplasia. The discharge may be mucopurulent, mucoid or serous.


Left and right sides of the head have paranasal sinuses that are separated by midline septa. On each side, two maxillary sinuses (rostral and caudal) extend progressively into the diploë of the maxilla from the middle nasal meatus through the nasomaxillary opening. The large caudal maxillary sinus extends caudally into the sphenoid and palatine bones to form the sphenopalatine sinus. It also extends through the frontomaxillary opening to form the frontal sinus. The frontal sinus extends into the caudal part of the first ethmoidal endoturbinate bone (the dorsal nasal concha) to form the dorsal conchal sinus. The frontal sinus also extends into the second ethmoidal endoturbinate bone to form the middle conchal sinus. The smaller rostral maxillary sinus also extends caudally into the caudal part of the ventral nasal concha through the conchomaxillary opening to form the ventral conchal sinus. (The anatomy is complicated by the presence of the conchal bulla which lies ventral to the conchomaxillary opening).


The frontal sinus has a conchal part and a frontal part; drainage goes into the caudal maxillary sinus through the frontomaxillary foramen. The ethmoidal and sphenopalatine sinuses also drain into the caudal maxillary sinuses and thus into the middle meatus. The rostral maxillary sinus has a drainage pathway into the middle nasal meatus through the nasomaxillary opening. This sinus is divided into a lateral bony compartment and a medial conchal compartment (within the ventral concha). These compartments are separated by the infra-orbital canal and a sheet of bone joining it ventrally to the roots of the teeth. In the young horse, the lateral component is almost entirely occupied by the roots of the cheek teeth. With age, the roots recede towards the floor of the sinus and the sinus increases in size. The ventral conchal sinus is accessible for surgery through the floor of the concho-frontal sinus.


Inflammation of the sinuses can occur by extension from the other mucosal surfaces. If they become filled with mucopurulent exudates these must be drained by trephination because the natural drainage ostia are not at the lowest points of the sinus system. Treatment of sinusitis involves trephining a hole in the bone over the sinus. The frontal sinus, dorsal and medial to the orbit, can be trephined along a line from the medial ocular angle (canthus) to the mid-line, about 1 cm caudal to the mid-point of this line. The caudal maxillary sinus is trephined 3 cm lateral to the medial canthus and 3 cm dorsal to the facial crest. The rostral maxillary sinus can be trephined at a point half way along the line from the medial canthus to the rostral extremity of the facial crest. Through these trephination sites, samples can be collected from the sinuses and can be cultured for bacterial examination and antibiotic sensitivity testing. The sinus can then be flushed out and treated. Sinus cysts, sinus polyps and neoplasia occur infrequently.


Four other conditions involving the head require some knowledge of anatomy. Occasional horses may show acute salivation (idiopathic sialo-adenitis). Even more rarely do we find occlusion of the salivary ducts and possible mucocoele. Head shaking is also an equine phenomenon. ‘Crib biting’ is a habitual neurotic vice which can produce extreme wear in the incisor teeth. It can suggest an abnormality of diet. It fulfils a functional digestive need in that it helps to meet the demand for unsatisfied foraging behaviour. Lastly, there may be masseter myopathy. In this, there is atrophy of the muscles, probably caused by damage to the masseteric nerve, which is superficial and easily damaged in accidents.


The lymph nodes are important. The streptococcal infection known as strangles (caused by Streptococcus equi) is a particularly severe purulent infection of the submandibular nodes, but may extend to other nodes of the head and neck and even into the thorax. The former epizootic disease (glanders) caused by Pfeifferella mallei also severely affects the lymph nodes of the head. Any local infection may cause swelling of the parotid, retropharyngeal and submandibular lymph nodes.


Alimentary tract disorders affecting the oral cavity are less common than disorders of the teeth or nasal cavity. Traumatic injuries to the tongue may or may not include glossal nerve damage. Paralysis of the tongue may indicate botulism. Viral stomatitis and oral ulceration are also sporadically diagnosed.


The teeth of the horse are hypsodont (high-crowned), composed of enamel (hard and brittle, 98% inorganic), dentine (slightly softer, 70–80% inorganic), cementum (with structural affinities to bone) and the pulp (soft). Deciduous incisor teeth erupt soon after birth. The central incisor teeth (i1) erupt at less than 2 weeks, the second incisors (i2) at about 6 weeks, and third incisors (i3) at around 6–9 months (you can remember 8 days, 8 weeks and 8 months more easily). Two-year-olds then shed the central deciduous incisors, 3-year-olds the second (lateral) incisors and 4-year-olds the third (corner) incisors. If all permanent incisor teeth (I1, I2, I3) are in wear, the horse is over 5 years old. Foals have three deciduous premolar cheek teeth. The permanent premolar teeth (P2, P3, P4) are usually called ‘cheek teeth 1,2 and 3’ in clinical work. They erupt at approx. 2.5, 3 and 3.5–4 years. The molar teeth (M1, M2 and M3) are usually called ‘cheek teeth 4,5 and 6’ in clinical work. They erupt at 1, 2 and 3.5–4 years of age.


In the young horse, developmental disorders of the teeth may include retained deciduous teeth, displacement of cheek teeth, and dentigerous cysts which are usually manifest as a discharging tract at the base of the ear. Teeth may also acquire disorders such as sharp edges (which require rasping – an energetic sport for the young and fit!), undulating occlusal surfaces (‘wave mouth’), or loss of enamel ridges (‘shear mouth’ or ‘18-month mouth’). In cases of tramautic injury, teeth may be split.


Dental diseases may show as ‘quidding’ (drooling of the food from the corners of the mouth), facial distortion, facial swelling and a nasal discharge. Overgrown teeth occur if the opposing tooth is missing. Periodontal disease is also possible. Occasionally, cheek teeth may be displaced and there may be supplementary teeth. Dental caries may occur, accompanied by alveolar periosteitis, as a result of bacterial fermentation leading to erosion of the enamel.


The rostral cheek teeth can be removed by oral extraction, principally by lateral buccotomy. This involves removal of alveolar bone, taking care not to damage the facial nerve. Retropulsion, with a punch, is also used for removing the mandibular and maxillary teeth. The upper cheek teeth 4 and 5 (M1, M2) are taken out through the maxillary sinus and 6 (M3) through the frontal sinus. In this you have to trephine (make a hole in the bone) over the tooth to be extracted and then punch it out. Care must be taken to avoid damage to the adjacent teeth. Complications do occur, including collateral damage, dental sequestration, alveolar bone sequestra and oronasal fistulae. In the mandible, retropulsion needs to be performed carefully because the parotid duct runs along the ventromedial edge of the body of the mandible, with the facial vessels. They turn laterally and dorsally close to the roots of the molar teeth.


Peripheral nerve or cranial nerve damage is not common. There are probably two exceptions: damage to the facial nerve and damage to the nerves associated with the guttural pouch (i.e. those traversing the foramen lacerum). The facial nerve can be damaged by trauma and this leads to paresis and paralysis. All three branches can be affected together at the nucleus level (in the pons) but the auricular, palpebral and buccal branches may be damaged separately on the face. The site of the damage on the face determines which nerve, and therefore which muscles, are affected. The buccal branches are probably the most exposed, over the body of the mandible and the masseter muscle. Damage to the facial nerve at any point will lead to abnormalities of facial expression. All of the branches will be affected if the lesions (such as fractures of the hyoid bone, or the cranial floor, or damage to the guttural pouch) occur near the base of the brain.


Local nerve blocks may be performed on the head, principally for facilitating wound repair (e.g. auriculopalpebral, lacrimal, zygomatic or infratrochlear nerves). In the past, mental and mandibular nerve blocks for dental surgery were also performed but general anaesthesia is now a much safer option. Occasionally, both are used together. A detailed analysis of the disorders of the central nervous system, especially the infectious diseases, is beyond the scope of this clinical introduction but a brief list would have to include Japanese B encephalitis, equine protozoal encephalitis, rabies, Western, Eastern and Venezuelan encephalitis and equine herpes virus infections and now, of course, West Nile virus infections.


Recently, a specific protozoal myeloencephalomyelitis (associated with Sarcocystis neurona) has become a problem in the USA. Generalized non-specific disorders such as vasculitis, epilepsy, ataxia and equine degenerative myeloencephalopathy should be listed. Equine motor neuron disease, botulism, tetanus and organophosphorus poisoning should be included in the list.


Pharyngeal lymphoid hyperplasia is an inflammatory condition of the pharyngeal mucosa. Pharyngeal trauma, lacerations and foreign bodies may be seen. Dorsal displacement of the soft palate occurs normally during swallowing, but can occur abnormally during racing or fast exercise; it is diagnosed by endoscopy. In this condition, the free border of the palatal arch becomes dislodged from its normal, sub-epiglottic, position. Unsupported soft tissue is then inhaled into the rima glottidis causing acute respiratory obstruction. It may be caused by fatigue or by disorders of the soft palate itself, or by disorders of the epiglottis and conditions that cause mouth breathing and pharyngeal discomfort. Remember the horse is an obligate nose breather.


The temporomandibular joint is of considerable veterinary importance. It may be fractured, leading to displacement, but disease in this joint can follow bacteraemia or septicaemia. If infected, it can be palpated; quite often there is swelling in the joint capsule and the fluid can be aspirated. Clinically, there is considerable difficulty in swallowing (dysphagia) leading to ‘quidding’ (dropping of food) and possibly asymmetry of the masseter muscles.


The larynx and the auditory tube, including the guttural pouch, are discussed in the section dealing with the neck and the clinical problems of airway obstruction.

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Related posts:

  1. 2: THE NECK
  2. 9: DIAGNOSTIC IMAGING OF THE HEAD, WITHERS, MANUS AND PES
  3. 4: THE THORAX
  4. 7: THE FOOT

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Jul 8, 2016 | Posted by in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off on 1: THE HEAD (INCLUDING THE SKIN)

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