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PATIENT HISTORY

When deciding whether a horse with epistaxis requires urgent veterinary attention, it is important to determine how much blood has been lost, how long haemorrhage has been ongoing and whether it has stopped. Profuse, arterial haemorrhage represents a veterinary emergen-cy, but many cases of epistaxis are mild or recurrent in nature and active haemorrhage will usually have stopped by the time the animal is examined. The nature of the nasal discharge may give some clues as to when haem-orrhage occurred. Blood that is bright red or dark red is suggestive of recent haemorrhage, whereas if the dis-charge is dark brown/black and also mucoid or purulent, haemorrhage probably occurred several days previously.

AETIOLOGICAL CLUESIn some cases, the cause of epistaxis is known, such as with recent trauma to the head, but in others it is less obvious. In addition to a thorough general history of the patient, the following specific questions may help to identify the potential aetiology: ■ When did the episode of epistaxis occur?■ Have any other episodes of epistaxis been noted before this?■ Was this a trickle of blood or more profuse haemorrhage?■ How long was haemorrhage observed for?■ How much blood has been lost?■ Was blood seen at one or both nostrils?■ Was this blood bright red, dark red or black/brown in colour?■ Has there been any known trauma to the head?

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EPISTAXIS – bleeding from the nostrils – is a relatively common condition in the horse. Haemorrhage may originate from a number of locations, including the nasal cavity, paranasal sinuses, ethmoid labyrinth, guttural pouches or lungs. There are a number of potential causes of the condition, some of which are mild and self-limiting, and others, such as guttural pouch mycosis, which are potentially life threatening and require surgical intervention. This article discusses how to distinguish emergency from non-emergency cases, and describes a range of investigations that will help in establishing a diagnosis. It goes on to discuss treatment approaches for the most important causes of epistaxis.

Differential diagnosis of epistaxis

in the horse DEBRA ARCHER

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■ Was haemorrhage associated with exercise?■ Has the horse undergone recent long-distance transport?■ Has a nasal discharge been noted between episodes of epistaxis?■ If so, what was the nature of this discharge?■ Has the horse had a recent respiratory tract infection?■ Has coughing/paroxysmal snorting been observed?■ Have any neurological signs been observed (eg, food or water draining from the nostrils)?

Guttural pouch mycosis should be suspected in horses with profuse arterial haemorrhage, particularly if several episodes of epistaxis have occurred in the pre-ceding days or weeks. In some of these cases, neurologi-cal signs such as dysphagia may also have been observed. Exercise-induced pulmonary haemorrhage (EIPH) is a common cause of epistaxis that occurs following intense

Debra Archer graduated from Glasgow in 1996 and worked in mixed and equine practice for three years before completing a residency in equine surgery at the University of Liverpool. She is now a senior lecturer in equine surgery at Liverpool. She holds the RCVS certificate in equine soft tissue surgery, the European diploma in equine surgery, and a PhD for studies on the epidemiology of colic.

Causes of epistaxis

■ Trauma (iatrogenic/external head trauma)■ Guttural pouch mycosis■ Progressive ethmoidal haematoma■ Exercise-induced pulmonary haemorrhage■ Neoplasia ■ Foreign bodies■ Clotting/bleeding disorders■ Pneumonia/pulmonary abscess■ Rhinitis/sinusitis■ Nasal amyloidosis/polyps■ Infected nasolacrimal duct

Exercise-induced pulmonary haemorrhage is one of various potential causes of epistaxis in horses. It can occur in any animal after short periods of strenuous exercise, but is particularly common in racehorses. Picture, Derek Knottenbelt

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exercise. Unilateral epistaxis usually suggests the ori-gin of haemorrhage is located rostral to the nasophar-ynx, whereas bilateral epistaxis is suggestive of a lesion caudal to the nasal cavities.

CLINICAL EXAMINATION

A general clinical examination should be performed and any evidence of pyrexia, depression, anorexia, nasal dis-charge, cough or abnormal findings on auscultation of the thorax noted. The head should be examined carefully for facial asymmetry or recent signs of external trauma, in particular for any evidence of cranial nerve deficits, such as dysphagia. Percussion of the paranasal sinuses should

be performed to assess for changes in resonance, and the guttural pouches should be palpated externally to assess for any evidence of pain or swelling in the region.

If epistaxis is evident at the time of clinical examin-ation, the priority is to estimate how much blood has been lost, the likely cause of the bleeding and whether the horse requires urgent first aid. However, in many cases, epistaxis is not evident at the time of examination and the only clue may be the presence of some dried blood at the nares.

A single episode of epistaxis that is non-arterial in nature and where only a small amount of blood has been lost without any evidence of other abnormal clini-cal signs may not merit immediate further investigation. Owners should be asked to observe the horse closely,

Spontaneous, bilateral arterial epistaxis (above left) and fresh arterial blood mixed with mucus and feed material (above right) in horses with guttural pouch mycosis. The animal on the right also had concurrent dysphagia. (below left) Unilateral epistaxis following external trauma to the right nasal bones. (below right) Dark red/black bilateral nasal discharge, suggesting that haemorrhage may have occurred a few days previously. Pictures, Derek Knottenbelt

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with the animal being examined again if a further epi-sode of epistaxis occurs or if other clinical signs, such as a purulent nasal discharge, become apparent. However, a single episode of profuse arterial epistaxis or multiple episodes of epistaxis should be investigated further.

DIAGNOSTIC PROCEDURES

ENDOSCOPY Endoscopic evaluation of the upper and lower airways is the most useful adjunctive modality in cases of epistaxis. The procedure may enable the lesion responsible to be directly visualised or the location of haemorrhage to be determined (eg, in the lower airways or paranasal sinuses), particularly if recent epistaxis has occurred. Even if only unilateral epistaxis has been observed, both nasal passages should be examined, to ensure that bilaterally occurring conditions (eg, ethmoidal haematomas) are not missed.

The endoscope should be advanced carefully up the ventral meatus and evidence of any blood or lesions on the nasal mucosa noted. The ethmoid region and sinus drainage angle (nasomaxillary aperture), which lie at the caudal aspect of the middle meatus, can be examined by

angling the tip of the endoscope dorsally as the entrance to the nasopharynx is approached. Evidence of blood emanating from the sinus drainage angle indicates that further assessment of the paranasal sinuses using radi-ography and possibly sinoscopy is required. The endo-scope should then be advanced into the nasopharynx so that the ostia of the guttural pouches can be evalu-ated for any evidence of haemorrhage draining from the pouches.

Endoscopic assessment of the guttural pouches can also be performed at this stage (see box on pages 24 to 25). The larynx and soft palate should be examined. Evidence of laryngeal paralysis or persistent dorsal displacement of the palate may be seen in some cases of guttural pouch mycosis and other inflammatory conditions of the guttural pouch.

Finally, the endoscope should be advanced down the trachea to the carina, looking for signs of blood in the lumen or abnormal lesions on the mucosa. Any evidence of haemorrhage emanating from the lungs, and whether this is observed from one or both mainstem bronchi, should be noted. Where indicated – for example, if the horse has a history of coughing or epistaxis after exer-cise – a tracheal wash and/or broncho alveolar lavage may be performed at this time, and the samples sent for cytology and bacteriological culture.

If a mass is observed, a bi opsy can be taken using endoscopic biopsy instruments and sent for histopathology.

RADIOGRAPHYRadiographic examination of the equine head is indi-cated where blood has been evident emanating from the sinus drainage angle, where there is evidence of facial swelling/deformity or if the horse has sustained an exter-nal traumatic injury to the head. If the origin of epistaxis has not been determined on endoscopic examination, radiographs can be taken to identify any lesions within the paranasal sinuses. Standard radiographic views of the head – lateral, dorsoventral and lateral oblique – should be obtained, centred over the areas of interest (eg, paranasal sinuses or guttural pouches).

Radiography of the thorax is less frequently performed, but is indicated, for example, in cases of sus-pected pulmonary neoplasia or pulmonary abscessation.

Blood draining from the right guttural pouch ostium of a horse with guttural pouch mycosis. Blood from other sources can occasionally be aspirated into the ostia during swallowing and give a false impression of haemorrhage emanating from the guttural pouches

(above) Endoscopic view showing blood emanating from the sinus drainage angle (arrow). (below) Fresh blood in the trachea of a horse following exercise-induced pulmonary haemorrhage. Pictures, Derek Knottenbelt

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Endoscopy of the guttural pouches

Endoscopic examination should be undertaken with care to avoid damaging the vascular and neural structures that traverse the guttural pouches. The procedure is most easily performed in a sedated patient, provided the animal’s cardiovascular and neurological (in the case of traumatic injuries to the head) status is normal. A small diameter endoscope (<13 mm) and a guttural pouch probe or closed bi opsy forceps are used to elevate the ostia, which act as a guide when advancing the endoscope up the auditory tube and into the guttural pouch. An extra person is required to rotate the endoscope.

The eccentric positioning of the biopsy channel through which the probe/forceps is passed means that the endoscope can be trapped by the cartil-aginous ostia, preventing advancement. The probe/forceps must be positioned so that it is as axial as possible in relation to the rest of the apparatus.

Careful endoscopic examination can rule out lesions within the pouches, particularly if laryngeal paralysis and/or persistent dorsal displacement of

the soft palate are evident. A history of trauma, together with evidence of blood emanating from one or both guttural pouches, may suggest rupture of the ventral straight muscles of the head, which can be confirmed endoscopically.

Eccentric positioning of the biopsy channel in the endoscope

(A) The endoscope is advanced up the ventral meatus and the tip is positioned ventral to the guttural pouch ostium. If the endoscope has been passed up the middle meatus, it may be difficult to obtain the ideal position, so the apparatus should be withdrawn and carefully advanced up the ventral meatus

(B) The probe/forceps should be inserted via the ostium into the auditory tube for a distance of 6 to 8 cm but no further, in order to prevent trauma to the structures within the pouch. The probe/forceps is positioned axially on this side, so rotation of the endoscope is not required

(C) and (D) The endoscope is advanced carefully up the auditory tube and into the guttural pouch. The probe/forceps is then withdrawn

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(F) The floor of the guttural pouch is examined for any haemorrhage pooling ventrally. The endoscope can be retroflexed to examine the roof and rostral aspect of the pouch to assess for evidence of haemorrhage/swelling in the ventral straight muscles of the head (see ‘External trauma’, page 27)

(G) Steps (A) to (F) are repeated to examine the contralateral guttural pouch. However, on this side of the horse the biopsy channel is positioned abaxially with respect to the rest of the endoscope. The probe/forceps should be inserted via the ostium into the auditory tube as before

(H) and (I) The endoscope is then rotated 180° so that the probe/forceps is now positioned axially. The endoscope can subsequently be advanced into the auditory tube without becoming caught up on the ostium

If blood is evident draining from a guttural pouch ostium or both ostia, and particularly if there has been no history of trauma, endoscopic examination of the guttural pouches may be inadvisable unless performed at a surgical facility. This is in case dislodgement of a clot results in the recurrence of active, uncontrollable haemorrhage from a mycotic lesion overlying the internal or external carotid artery.

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(E) The medial and lateral compartments of the guttural pouch are examined. The stylohyoid bone (SH) divides the guttural pouch into a larger medial compartment (MC) and a smaller lateral compartment (LC). The internal carotid artery (ICA) and external carotid (maxillary) artery (ECA) should be examined for evidence of overlying lesions

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HAEMATOLOGY AND CLOTTING FUNCTIONAbnormalities of blood clotting function may be sus-pected where haemorrhage fails to subside as expected in cases of iatrogenic trauma to the nasal passages, or where there is no obvious cause for repeated episodes of epistaxis. A simple test that can be performed in the field is to collect a sample of blood from the jugular vein into a plain blood tube. The tube should be kept at 37°C (for example, by holding it under the upper arm) and rotated every 30 seconds. The blood should clot within 15 minutes; longer than this indicates some form of clot-ting abnormality, in which case further blood samples should be placed in an EDTA tube for haematology (to rule out thrombocytopenia) and a citrate tube for further analysis of blood clotting function.

SINOSCOPY Sinoscopy – arthroscopic or endoscopic assessment of the paranasal sinuses – may be indicated in cases in which blood has been seen emanating from the sinus drainage angle but no lesions have been identified on radiographic examination of the sinuses. If radiography reveals lesions within the paranasal sinuses, sinoscopy enables the lesion to be visualised directly and biopsy samples can be taken for histopathological examin-ation. This technique may also enable the intralesional treatment of some masses.

Sinoscopy can be performed in the standing, sedat-ed horse and involves the creation of a portal for inser-tion of a rigid arthroscope or a flexible endoscope into the frontal, caudal maxillary or rostral maxillary sinus. A good knowledge of the anatomy of the equine head, including the anatomic landmarks for entry into each sinus, is required before undertaking this procedure.

COMPUTED TOMOGRAPHY/MAGNETIC RESONANCE IMAGINGEvaluation of the head by computed tomography (CT) or magnetic resonance imaging (MRI) is not usually neces-sary in cases of epistaxis. Few equine centres have these facilities in the UK and, until recently, general anaes-thesia was required for the procedures, increasing the expense and the risk to the patient. At present, one cen-tre in the UK can perform CT evaluation of the head in

the standing, sedated horse. However, radiographs of the equine skull can be difficult to interpret, and the advan-tage of CT and MRI lies in the ability to obtain cross-sectional images. Therefore, when assessing extensive masses arising within the paranasal sinuses (eg, masses that have invaded the cranium) or injuries to the head, the techniques are useful in order to determine the prognosis more accurately and to assist the planning of treatment.

CT or MRI examination of the head in cases of epistaxis may be indicated when all other tests are negative and haemorrhage originating from the lower respiratory tract has been ruled out.

LESIONS CAUSING EPISTAXIS

IATROGENIC TRAUMAAs the intranasal structures are well vascularised, trau-matic injury to these areas can result in profuse epistaxis. Iatrogenic epistaxis may occur during nasogastric intu-bation or endoscopic examination, particularly if the

(left) Lateral radiograph of the head of a horse with a progressive ethmoidal haematoma (PEH). A circular radiodensity can be seen ventral to the ethmoidal turbinate bones (black arrow), and fluid lines are evident secondary to haemorrhage from the mass (white arrows). (above) Dorsoventral radiograph of the head of a horse with a soft tissue mass (arrow) within the paranasal sinuses. This was also a PEH

Computed tomography image of the head of a horse with an extensive progressive ethmoidal haematoma growing within the paranasal sinuses (arrow)

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pro cedures have been performed repeatedly. The risk of epistaxis may be minimised by careful endoscopic examination or, in the case of nasogastric intubation, by thoroughly lubricating the end of the stomach tube and ensuring that the tube enters via the ventral (rather than the middle) meatus, to avoid trauma to the ethmoid labyrinth.

Haemorrhage can be profuse and alarming to the owner/carer, particularly if this complication has not been mentioned before intubation. However, the bleed-ing is almost always self-limiting unless the horse has concurrent clotting abnormalities (eg, thrombocyto penia or disseminated intravascular coagulation). The patient should be left quietly and, as a guide, epistaxis should abate within five minutes and completely stop within 20 minutes. If haemorrhage continues for longer than this, blood clotting time should be assessed.

EXTERNAL TRAUMATrauma to the bony structures overlying the nasal pas-sages and paranasal sinuses (eg, as a result of hitting a fixed object or a kick from another horse) can result in unilateral or bilateral epistaxis, depending on the struc-tures damaged. If haemorrhage has occurred within the paranasal sinuses, epistaxis may not be evident immedi-ately but altered blood may be seen at the nares a few days after the traumatic incident.

DiagnosisCareful external examination of the head and endoscopy will usually enable the source of the haemorrhage to be identified. Radiographic examination may be required if there is evidence of potential fractures (eg, an open wound, crepitus beneath the skin or obvious distortion to the facial outline), to define the extent of the injuries more accurately.

Rupture of the ventral straight muscles of the head (longus capitis and rectus capitis ventralis muscles) can occur subsequent to trauma to the base of the skull (eg, rearing over backwards), resulting in haemorrhage into the guttural pouch and subsequent epistaxis. Diagnosis in this situation is based on endoscopic and radiographic assessment of the guttural pouches.

GUTTURAL POUCH MYCOSISFungal infection of the guttural pouch can result in a fatal episode of epistaxis and should be ruled out in any horse that has suffered moderate/severe epistaxis or repeated episodes of epistaxis that are not associated with exercise or trauma, particularly if concurrent signs of cranial nerve deficits are evident.

Fungal plaques (usually Aspergillus species) most commonly localise in the roof of the medial compartment (overlying the internal carotid artery), but they can involve the lateral wall of the lateral compartment (overlying the external carotid [maxillary] artery). Usually, only one gut-tural pouch is affected, but bilateral lesions can occur. Fungal erosion of the arterial wall can result in spontan-eous epistaxis, which is evident as arterial haemorrhage from one or both nares. This is followed by a dark red/brown, mucoid discharge over the following few days. The first episode of epistaxis rarely causes death, but the final, fatal haemorrhage usually occurs within three weeks.

TreatmentIf haemorrhage is ongoing at the time of examin ation, in reality, unless the horse is at a surgical facility, there may be little that can be done other than to put the ani-mal in a stable, keep it quiet and avoid elevating its blood pressure in the hope that the bleeding will subside.

The quantity of blood that the horse has lost should be determined; loss of more than 4 to 5 litres in a 500 kg horse is clinically significant, and ongoing haem-orrhage will result in collapse and death. The horse’s heart rate and peripheral blood pressure should be assessed. If the horse is anxious, provided that it is not tachycardic (ie, its heart rate is <60 beats per minute) and it has good peripheral pulse pressures, a low dose of acepromazine (0·02 mg/kg) can be given intravenously to reduce anxiety and produce a degree of hypotension in an attempt to achieve haemostasis. Acepromazine should not be administered if the horse has evidence of hypovolaemic shock (ie, tachycardia, pale mucous mem-branes and poor peripheral pulse pressure) as it could precipitate collapse of the patient. As a final resort, if uncontrolled massive haemorrhage is ongoing, the common carotid artery can be ligated, but it may not be possible to do this quickly enough or safely before collapse and death occurs.

Iatrogenic unilateral epistaxis following nasogastric intubation. Bleeding can be profuse but is usually self-limiting

Mycotic lesion in the medial compartment of the guttural pouch, overlying the internal carotid artery. Two episodes of epistaxis had occurred before this horse was examined endoscopically

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The administration of intravenous fluids and whole blood is advocated by some clinicians to prevent fatal hypovolaemia; others believe that these are contra-indicated and result in further haemorrhage as normal blood pressures are restored. In the author’s opinion, if haemorrhage has subsided, fluid therapy should only be administered once the horse is at a surgical facility, and ideally only when surgical intervention has prevented further bleeding. If haemorrhage subsides, and where surgical treatment is an option, a surgical facility should be contacted for further advice.

PROGRESSIVE ETHMOIDAL HAEMATOMAProgressive ethmoidal haematomas are non-neoplastic, slowly expansive masses that originate from the sub-mucosa of the ethmoid labyrinth at the caudal aspect of the middle meatus. Depending on the location of the mass, as it grows it will extend into the nasal cavity, naso pharynx or, less commonly, the paranasal sinuses.

The aetiology is unknown, but haemorrhage within the submucosa results in stretching and thickening of the mucosa, creating a capsule. Repeated haemorrhage results in progressive expansion of the mass, and affect-ed horses most frequently have a history of recurrent episodes of low-grade epistaxis. Occasionally, horses will have a history of abnormal respiratory noise at exer-cise or at rest, or there will be signs of facial swelling (in the case of large masses). It is possible that the mass will infiltrate through the cribiform plate into the brain caus-ing signs of central nervous system disease, but this is a rare occurrence.

Diagnosis A provisional diagnosis is made by endoscopic visual-isation, with the mass within the nasal passages or nasopharynx appearing to originate from the ethmoid labyrinth. Masses can vary in colour from green/yellow to red/brown.

Ethmoidal haematomas that grow into the paranasal sinuses, resulting in evidence of haemorrhage from the sinus drainage angle, can be diagnosed by radiography, if they are large enough to be visualised, or sinoscopy. CT or MRI can be a useful adjunctive means of assess-ing the origin and extent of large ethmoidal haema-tomas before treatment. Definitive diagnosis is made by

histo pathological examination of a biopsy from the mass.

Bilateral lesions can occur in approximately 15 per cent of cases and so the nasal passages and para-nasal sinuses on the contralateral side should also be examined.

TreatmentTreatment of these lesions depends on their location and size, and includes the use of intralesional formalin, laser ablation, surgical resection or cryogenic ablation.

Ethmoidal haematomas reportedly recur in 14 to 45 per cent of cases, and so affected horses should be assessed every six to 12 months after treatment for evidence of regrowth.

EXERCISE-INDUCED PULMONARY HAEMORRHAGEEIPH occurs in horses that undergo short periods of strenuous exercise. It is therefore a common disease of racehorses, but can occur in all breeds following different forms of intense, fast exercise. EIPH should also be con-sidered in horses that show a sudden loss of speed during exercise, coughing and increased swallowing, but may only be evident in as few as 0·5 to 2 per cent of affected animals. Horses with atrial fibrillation may present with a history of exercise-associated epistaxis, together with other clinical signs such as exercise intolerance or weak-ness; identification of an irregularly irregular rhythm on cardiac auscultation warrants further investigation.

Haemorrhage originates from the pulmonary vascu-lature in the caudodorsal lung fields, and a variety of aetiologies have been proposed, including stress failure of pulmonary capillaries.

DiagnosisEndoscopic examination of the lower airways should be performed in horses that have suspected EIPH, in order to confirm the diagnosis by identifying blood in the tracheo bronchial tree. However, even if there is no gross evidence of blood in this area, EIPH cannot be ruled out. A tracheal wash or bronchoalveolar lavage should be per-formed in order to identify erythrocytes and haemosider-in in macrophages, which may be evident in the samples for up to three months after a single episode of EIPH.

(above) Progressive ethmoidal haematoma (PEH) in the nasal passages of a horse experiencing repeated episodes of epistaxis. (right) Appearance of a PEH after two treatments with intralesional formalin. Further treatment resulted in complete resolution of the lesion. Pictures, Derek Knottenbelt

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TreatmentThere is no single treatment for EIPH, but rest, non- steroidal anti-inflammatory therapy and investigation and treatment of concurrent airway disorders should ini-tially be undertaken. In addition, an incremental training regimen, together with the use of nasal strips and furo-semide, has been suggested. However, EIPH has a high rate of recurrence in affected individuals, despite these measures.

OTHER PULMONARY CONDITIONSVarious pulmonary conditions other than EIPH may present with epistaxis, including pulmonary abscesses, neoplasia, pneumonia, trauma and foreign bodies. A history of thoracic trauma or recent long-distance trans-portation (in the case of pleuropneumonia) and the results of careful clinical examination may assist the diagnosis.

Clinical signs of epistaxis, together with pyrexia and/or weight loss (in cases of pneumonia, abscesses and neoplasia), ventral thoracic oedema (pleuropneumonia) and subcutaneous emphysema over the thorax (trauma), may indicate that haemorrhage originates from the lungs. Further investigation should include endoscopic examination of the lower airways, thoracic radiographs and ultrasonography of the thorax.

OTHER INFLAMMATORY CONDITIONSA variety of inflammatory conditions may result in epistaxis due to ulceration of the respiratory mucosa and subsequent haemorrhage. Fungal lesions can cause marked destruction of the underlying tissues, and are treated by removing any loose fungal plaques, followed by administration of a topical antifungal.

Nasal amyloidosis is an uncommon condition that can cause epistaxis. Lesions appear as nodular masses cov-ered by mucosa, and the diagnosis should be confirmed by biopsy. Treatment consists of surgical removal if the masses are accessible and not too extensive.

NEOPLASIANeoplastic conditions of the upper and lower airways may result in epistaxis, together with other clinical signs such as facial distortion, purulent nasal discharge and stertorous respiration. A variety of neoplasms can arise within the airways, in particular carcinomas such as adeno carcinoma/squamous cell carcinoma. Histopathology is used to confirm the diagnosis and to determine whether there are any treatment options. Generally, the prognosis is grave.

FOREIGN BODIESForeign bodies can occasionally become lodged in the upper and lower airways. Those that get stuck in the nasal passages may cause unilateral epistaxis, together with paroxysmal snorting and face rubbing. Diagnosis is achieved by endoscopic examination and, depending on the size and location of the object, removal may be possible using endoscopic instruments.

Exercise-induced pulmonary haemorrhage (EPIH): (left) bilateral epistaxis following intense exercise, (above) severe haemorrhage in the lower airways. (below) Bronchoalveolar lavage samples taken from a horse following EPIH. Pictures, Derek Knottenbelt

AcknowledgementThe author is very grateful to Dr Shaun MacKane for his helpful comments during the preparation of this article.

Further reading AUER, J. A. & STICK, J. A. (2006) Equine Surgery, 3rd edn. Philadelphia, SaundersBARAKZAI, S. (2007) Handbook of Equine Respiratory Endoscopy. Philadelphia, SaundersMcGORUM, B. C., DIXON, P. D., ROBINSON, N. E. & SCHUMACHER, J. (2007) Equine Respiratory Medicine and Surgery. Philadelphia, Saunders