Lipid composition of xanthomas of different types

Mild hypercholesterolemia impacts achilles sub-tendon mechanical properties in young rats

BACKGROUND

Hypercholesterolemia is associated with tendon pathology, but the reasons underpinning this relationship are not well understood. Cholesterol can accumulate in the tendon non-collagenous matrix which may affect both global and local tissue mechanics. Changes to the local strain environment within tendon may have significant implications for mechanosensitive tenocytes. Here, we investigated the association between elevated blood cholesterol and presence of tendon lipids in the Achilles tendon. We expected lipids to be localised in the proteoglycan-rich inter-sub-tendon matrix (ISTM), therefore we also sought to examine the impact of this on the biomechanical and viscoelastic properties of the ISTM.

METHODS

The Achilles tendons of 32 young wild-type (SD) and 32 apolipoprotein E knock-out rats (ApoE^-/-) were harvested at 15.6 ± 2.3 weeks of age. 32 specimens underwent histological examination to assess the distribution of lipids throughout sub-tendons and ISTM. The remaining specimens were prepared for biomechanical testing, where the ISTM between the gastrocnemius and soleus sub-tendons was subjected to shear load mechanical testing. A sub-set of tests were video recorded to enable a strain analysis.

RESULTS

ApoE^-/- serum cholesterol was double that of SD rats (mean 2.25 vs. 1.10 mg/ml, p < 0.001) indicating a relatively mild hypercholesterolemia phenotype. Nonetheless, we found histological evidence of esterified lipids in the ISTM and unesterified lipids in the sub-tendons, although the location or intensity of staining was not appreciably different between rat strains. Despite a lack of observable histological differences in lipid content between groups, there were significant differences in the mechanical and viscoelastic behaviour of the Achilles sub-tendon matrix.

CONCLUSION

Even slightly elevated cholesterol may result in subtle changes to tendon biomechanical properties and hence injury risk. The young age of our cohort and the mild phenotype of our ApoE^-/- rats are likely to have limited our findings and so we also conclude that the ApoE^-/- rat model is not well suited for investigating the biomechanical impact of tendon xanthomas on Achilles sub-tendon function.

Ocular lipid deposition and hyperlipoproteinaemia

In all species there are potential ocular manifestations when circulating lipoproteins are raised and these may be transient or permanent Many factors, both systemic and local, influence lipid influx and accumulation (progression) and lipid mobilisation and efflux (regression). In both humans and animals some types of lipid deposition will regress if the local and systemic factors involved in pathogenesis can be modified. There are inescapable parallels with the same phenomena in other tissues.Three types of corneal lipid deposition have been linked with hyperlipoproteinaemia. In corneal arcus, lipid is deposited preferentially in the warmest part of the cornea initially and, in people, the lipid remains almost exclusively extracellular. In animals, corneal arcus is associated with initial extracellular lipid deposition followed by the appearance of intracellular lipid and vascularisation, so that established corneal arcus tends to become more typical of lipid keratopathy. In humans, hyperlipoproteinaemia may be an associated systemic factor and early onset corneal arcus is a recognised feature of certain primary hyperlipoproteinaemias and their secondary phenotypes. In dogs, corneal arcus is always associated with hyperlipoproteinaemia. Corneal vascularisation is a ubiquitous feature of lipid keratopathy in all species and both necrotic fibroblasts and foam cells are common in progressive lesions. The extent and position of lipid deposition and the evolution of lipid keratopathy can be related to local ocular disease and circulating lipids and lipoproteins. Many aspects of the pathogenesis of lipid keratopathy are similar to those of atherogenesis. Hyperlipoproteinaemia, especially hypercholesterolaemia is the commonest systemic abnormality. In crystalline stromal dystrophy (Schnyder's crystalline stromal dystrophy) of the cornea there is no inflammatory element and no vascularisation. The dystrophy is associated with accumulation of lipid within the corneal fibroblasts, but typical foam cells are absent, the crystalline opacity involves the coolest part of the cornea, correlates with local fibroblast death, and is always bilateral. Hyperlipoproteinaemia, may be present, but this is not universally so.The objective of this paper is to evaluate the factors that may influence ocular involvement in hyperlipoproteinaemia. A comparative approach, utilising information available from studies of both ocular and non-ocular tissues, aids elucidation of the complex pathogenesis.

Magnetic resonance imaging of Achilles tendon xanthomas in familial hypercholesterolemia

The demonstration of tendon xanthomas is helpful in diagnosing heterozygous familial hypercholesterolemia, but in many patients lipid may accumulate without clinical abnormality being present. We investigated the possibility of detecting the lipid element with magnetic resonance (MR) imaging in seven patients with familial hypercholesterolemia and six controls. Although the mean relative signal intensities measured on long TR/TE spin echo sequences of the tendon were significantly higher in patients than in controls, the lack of such elevation does not rule out the presence of such lesions. In vitro measurements indicated that the signal intensity of triglycerides was quenched by cholesterolesters. The anatomic findings of MR imaging were compared with those of ultrasonography (US), showing excellent correlation in measurements between MR images and US [r(S) = 0.95 and 0.97 respectively]. MR imaging and US provide equal information on the anatomy of the Achilles tendon; as an abnormally increased signal intensity within the xanthoma on MRI was found in only a minority of our patients, the value of MRI in the demonstration of Achilles tendon xanthomas is limited when using conventional T1 and T2 spin echo sequences.

Thermal transition studies of a mature xanthelasma by differential scanning calorimetry

A large xanthelasma which had been present for at least 5 years was removed surgically from a normolipaemic female age 54 years, and examined in the fresh state by differential scanning calorimetry. Thermal transitions recorded over the range 30-40 degrees C suggest that the lipid present, predominantly esterified cholesterol, is not bound to protein or other tissue components, and that the chronicity of mature xanthelasmata as with ectopic lipid deposits at other sites is enhanced by chemical modification of lipid and effects on component phase behaviour, which are significant at local skin temperature.

Bilateral and extensive xanthelasma palpebrarum in a young man

A 25-year-old white man had been aware since childhood of yellow discolorations of the medial aspects of his upper lids. The skin changes progressively spread to involve all four eyelids, the inferior skin of the brow, and the lateral and medial canthal skin. The clinical diagnosis was extensive bilateral xanthelasmas . The patient had no manifest abnormality of lipid metabolism, although his father and grandfather had both developed xanthelasmas early in childhood. A snip biopsy of upper lid skin was performed and studied by both light and electron microscopy. The light microscopic appearance confirmed the diagnosis of xanthelasma, in that the upper dermis contained mono- and multinucleated foamy xanthoma cells beset with myriad cytoplasmic vacuoles. Cells displaced outwardly into the interstitium evidenced degradation phenomena within the vacuoles, consisting of lamellar bodies or fingerprint-like inclusions, presumably the result of fusion of lysosomes with the lipidic inclusions. Scattered non- lipidized mononuclear histiocytes with abundant profiles of smooth-surfaced endoplasmic reticulum possibly represented a reserve population of monocytes for conversion into xanthoma cells.