EQUINE VETERINARY EDUCATION / AE / MAY 2015
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Clinical Commentary Infraspinatus and intertubercular (bicipital) bursae sepsis
W. H. J. Barker Newmarket Equine Hospital, Newmarket, Suffolk, UK. Corresponding author email:
will.barker@
neh.uk.com
Summary The case report by O’Sullivan et al. in this month’s issue of EVE describes an unusual case of synovial sepsis raising interesting points on synovial infection in general and more specifically the implication of haematogenous spread of bacteria from a remote site in the mature horse. The identification of synovial sepsis in this location is challenging due to the complex and unfamiliar anatomy in the shoulder region. This clinical commentary reviews the anatomy in this region and discusses the unusual presentation of synovial sepsis secondary to haematogenous spread.
Introduction
The intertubercular (bicipital) and infraspinatus bursae, like diarthrodial joints and tendon sheaths, are closed spaces with a similar mesenchymal synovial lining that produces and maintains a selective physical, cellular and biochemical environment. The physiological response to infection is similar in all synovial structures. Following colonisation of the synovial environment, a combination of bacterial pathogenicity and exaggerated host immune response leads to the release of proteolytic enzymes and free radicals, which result in marked synovial inflammation with synovial effusion, culminating in lameness. Synovial effusion also results in reduced tissue perfusion, further compromising the synovium and cartilage. If left unidentified, or if treatment is suboptimal or delayed, the destructive combination of bacteria and host immunity can permanently debilitate the synovial environment, in particular the articular cartilage, resulting in unresolvable lameness (McIlwraith et al. 2014). The terms contamination and sepsis or infection are
sometimes used incorrectly and interchanged when describing an acute inflammatory profile identified following laboratory analysis of synovial fluid. Contamination results from the introduction of microorganisms, be that by haematogenous spread of bacteria from a remote focus as reported in the case described in this issue of EVE by O’Sullivan et al. (2015), or more commonly from an inoculation from a wound or iatrogenic intervention. Following contamination, infection or sepsis ensues once microorganisms reproduce and colonise synovial surfaces. The ability of the microorganism to establish infection will depend on the virulence and number of bacteria present, as well as the immune system’s ability to overcome the initial contamination. The presence of foreign material or devitalised tissue will also increase the risk of bacterial colonisation by acting as a nidus for infection (Bertone 1996). When a penetrating wound enters a synovial cavity there is
a high (but not guaranteed) risk of contamination and subsequent infection. Therefore endoscopic surgical lavage and wound debridement accompanied with delivery of local antimicrobials is recommended to avoid the potentially devastating consequences of delayed treatment (Richardson
and Ahern 2012). Endoscopy also allows evaluation of the synovial structure to identify potential foreign material and secondary cartilage damage, both of which could affect outcome (Wright et al. 2003; McIlwraith et al. 2014).
Clinical diagnosis of sepsis
Clinical diagnosis of synovial sepsis is based on the results of clinical examination (heat, lameness, pain on palpation and synovial distension), synovial fluid analysis (total nucleated cell count, white blood cell differential and total protein content), bacterial culture and identification of bacteria on microscopic evaluation synovial fluid smears or cytospins.
Clinical examination
Clinical examination of the craniolateral aspect of the shoulder region is made difficult by the overlying broad brachiocephalicus muscle, which originates from the temporal bone of the skull, nuchal crest and 1st–4th cervical vertebrae and inserts on the deltoid tuberosity and crest of the humerus (Fig 1b). The bilobed bicipital tendon can be palpated deep to this muscle over the cranial aspect of the proximal humerus where it passes over the intertuberal grooves of the humerus (Fig 1c–e). The latter is created between the lateral and medial tuberosities and their dividing intermediate ridge (Fig 1a and e). The intertubercular bursa provides lubrication and smooth passage for the tendon over these prominences. Distension of the intertubercular bursa is difficult to appreciate. Commonly palpation of the overlying tendon is resented and manipulation of the distal limb in a caudal direction so that the shoulder joint flexes and the elbow extends is also met with marked discomfort. Horses will generally have a reduced cranial phase to the stride and a variable degree of lameness at the walk and trot depending on duration of inflammation and additional trauma. The infraspinatus bursa is also located deep to the brachiocephalicus muscle and cranial to the protuberant deltoideus muscle (Fig 1c). The infraspinatus bursa lies deep to the infraspinatus tendon immediately proximal to the muscle’s point of insertion on the caudal prominence of the lateral tubercle of the humerus (Fig 1a–f). The bursa provides lubrication and protection to the tendon as it passes over the caudal prominence of the lateral tubercle of the humerus (Fig 1a and f).
Diagnostic imaging
Radiographic evaluation of the shoulder region is limited by the joint’s close proximity to the trunk. O’Sullivan et al. (2015) indicate that the most useful radiographic projections are mediolateral and craniomedial-caudolateral oblique of the scapulohumeral joint and neighbouring tubercles of the humerus. When suspicious of intertubercular bursa pathology, the cranioproximal-craniodistal skyline projection allows
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