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and the nuchal ligament (Carson Dunkerley et al. 1997). However, to the authors’ knowledge, there has only been one other report of a lipoma in a horse arising from a tendon sheath (Hammer et al. 2002). In that report, ultrasonography was used to delineate the mass and ascertain its relationship to surrounding structures, but the authors did not consider the appearance as typical of a lipoma. In our case, the radiographic and ultrasonographic appearance were thought to be suggestive of a lipoma. In man, lipoma arborescens (LA) is described as a rare,
200 µm
Fig 7: Histopathological appearance of a section of the mass showing mature adipocytes. Haematoxylin and eosin; 20 3 objective.
The filly made an uneventful recovery from surgery, the
incision healed without complication and at 6 months there has been no apparent recurrence.
Histopathology
The submitted tissue samples were grossly white to yellow in colour, were of fatty consistency and surrounded by a thick fibrous capsule. Microscopic examination of haematoxylin and eosin stained sections showed a tumour composed of
mature adipocytes and infiltrated by numerous, mostly fine fibrous bands (Fig 7). There were a few small intratumoral lymphocytic and plasmacytic aggregates. The overall appearance was consistent with a diagnosis of lipoma.
Discussion
In a 1975 abattoir survey of tumours in horses, it was reported that 11% (143) of 1308 horses had tumours of various types, and 20 of these were abdominal or mesenteric lipomas (Cotchin and Baker-Smith 1975). While this apparent over- representation of abdominal lipomas would not be a surprise to most equine surgeons, it is indicative of the fact that most lipomas in horses occur in older animals that are more likely to be found in an abattoir survey. It is noteworthy that the tumour in this filly occurred at a very young age, and that this could lead to speculation that it originated from embryonic fat tissue such as a lipoblastoma, which subsequently differentiated into a growth that more closely resembled a lipoma. A more accurate diagnosis can be made by looking for rearrangements in chromosome 8q13, which are said to occur in 90% of lipoblastomas, or by using immunohistochemistry to reveal reactivity for desmin in mesenchymal cells or S100 protein in adipocytes (Coffin and Alaggio 2012). In our case, immunohistochemical analysis revealed no reactivity for desmin (1:300 dilution) or S100 protein (1:5000 dilution). Other reports in the veterinary literature have
documented lipomas in horses of various ages and in less common locations including the pericardium (Baker and Kreeger 1987; Hamir et al. 1994), the meninges (Anderson and King 1988), subcutaneous and intramuscular tissues (Blackwell 1972; Bristol and Fubini 1984; Lepage et al. 1993)
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benign lesion consisting of a subsynovial villous proliferation of mature fat cells, most commonly reported to occur in and around joints, and there are also sporadic reports of LA occurring in tendon sheaths (Senocak et al. 2007; Babar et al. 2008; Kamaci et al. 2014). Primary LA is thought to be idiopathic and to occur in younger patients, while a secondary type is more likely to be associated with an underlying chronic irritation or inflammatory process (Sanamandra and Ong 2014). In man, the typical ultrasonographic appearance is one of multiple finger-like globular and villous hyperechoic fronds that extend from the base of the mass – hence the term arborescens or tree-like appearance (Learch and Braaton 2000) – and in radiographs there is usually a characteristic lucency typical of internal fat content (Babar et al. 2008). These were not features seen in the current case where the tumour appeared as a solitary, encapsulated mass that originated from the lining of the extensor sheath over the tarsus. There is a feeling amongst veterinary pathologists that
the well-differentiated cellular component of lipomas in animals often makes it difficult to call them true neoplasms, and the term nodular hyperplasia or adipose tissue consistent with lipoma might be more appropriate (Robbins 1974; Stannard and Pulley 1978). Similarly in man, there is some conjecture about the exact nature of the tissue and the term nonspecific synovial fatty proliferation has been used (Sanamandra and Ong 2014). As in man, surgical removal by way of careful synovectomy is the current treatment of choice.
Authors’ declaration of interest No conflicts of interest have been declared.
Ethical animal research
This report describes a case for which the owners had given their full consent
Source of funding No internal or external funding was provided for this report.
Antimicrobial stewardship policy The horse was given appropriate perioperative antimicrobials.
Authorship All authors contributed to one or more aspects of the overall case description and manuscript preparation. C. Owens and B. Hilberg contributed to the surgery aspect, A. Young contributed to the imaging aspect, and P. Loukopoulos
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