Synovial Biopsy, Synovium, Synovial Fluid & Arthrocentesis
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Gross – Tophi as refractile white clumps embedded in the synovium can fairly often be seen on the synovium surface at either surgery or autopsy. (3) (Fig.1). Crystal containing tophi may be seen in the tips or stems of villi. Most are in the tissue and are not removed by irrigation (4-7). A diffuse crust of urates over virtually the entire synovial and/or cartilage joint surface can also occasionally be seen. There is still some uncertainty as to whether this surface depositioncan be early or whether tissue deposits are always first.
Light Microscopy – During acute gouty arthritis an exudate containing PMN and fibrin adheres to the synovial surface. Synovial lining cells are proliferated in some areas (8). Diffuse and perivascular infiltration by PMN is seen (Fig.2), but in addition there are almost always also lymphocytes, macrophages and even clumps of plasma cells (Fig. 3). Vessels are congested and PMN may be marginated in the lumens of venules. PMN infiltration may be striking in one piece of synovium while an adjacent specimen may show mainly chronic inflammatory cells. Foreign body type giant cells have been seen near the surface.
Tophi are commonly seen in synovium, for example being found in 3 of 10 needle synovial biopsies in acute gouty arthritis (8) and in 4 of 6 biopsies in another series (9). Tophi are also clearly present in asymptomatic joints between attacks (5).
Tophi consist of monosodium urate, needle shaped, crystals arranged radially (Fig. 4). Crystals are dissolved out by formalin based fixatives (10) (Fig. 5) but alcohol fixation preserves them intact. Urate crystals can alsobe easily identified in frozen sections or acetone fixed tissue of stained sections examined with compensated polarized light. The DeGalantha stain or Gomori Methenamine silver stainalso can be used to stain the crystals (11) (Fig. 6). If crystals are dissolved out leaving only the amorphous purple stained matrix, the former sites of tophi, they could be confused with rheumatoid nodules (5) although clefts from which crystals have been dissolved can often be seen. The matrix material around the crystals has been shown to contain cholesterol and mucopolysaccharides (5). There is faint PAS staining and some metachromasia. Lipid stains show most lipid at the margins (5). Immunoglobulins have also been localized to tophi (12) (Fig. 7). Tophi may also occasionally calcify with deposition of hydroxyapatite.
The cells surrounding the tophus crystalsare fibrocytes, macrophages, other mononuclear cells and some foreign body giant cells. Acute inflammatory cells are strikingly absent around tophi. Many synovial tophi are very superficial and often rather thinly encapsulated with fibrous tissue. Fibrin like material has been seen between some tophi and the joint space (Fig. 8) (8).
Electron Microscopy - During acute gouty arthritis there is surface fibrin-like material and proliferation of lining cells with predominance of Type B and intermediate cells. Type B cells often have lipid deposits. The type A macrophage related cells have large vacuoles with granular material and cell debris and occasional definite crystal outlines in phagolysosomes (8). In experimental urate crystal synovitis crystal phagocytosis by lining cells has also been seen (13) (Fig. 9). Inflammatory cells are seen as described under light microscopy. Crystals are strikingly absent within PMN infiltrating the synovium or in synovial vessels. Phagosomes of macrophages contain cell fragments and intact PMN’s. Extravasated erythrocytes and cell debris are scattered through the interstitium.
Some small vessels are congested while the lumens of others are packed with platelets, RBC’s and WBC’s. There are gaps between venular endothelial cells with emigrating RBC and PMN, but very little endothelial necrosis. Two patients have been described with dramatic but not further characterized electron dense deposits lying between endothelium and pericytes (8) (Fig. 10). Vascular basement membranes are often multilaminated.
PMN in vessel lumens often have foamy clear patches in the cytoplasm and fewer granules suggesting intraluminal degranulation (14). Some free dense bodies and fragmenting intraluminal PMN can also be seen. Microtubules are easily seen in venular endothelium, pericytes, and lining cells in patients with or without clinically effective doses of colchicine.
A tophus by electron microscopy after standard processing with paraformaldehyde shows parallel arrays of electron lucent crystal outlines lying in a finely granular material (Fig. 11). Cells adjacent to the crystals are fibrocyte-like with large lipid deposits. Occasional crystals seem to be in phagosomes of these cells. No intracytoplasmic crystals have been seen (8).
Scanning electron microscopy - These images show crystals on the synovial surface and occasionally within synovial lining cell phagosomes (3) (Fig. 12).
Findings in joint fluid can vary widely during apparent acute attacks of gouty arthritis. Ropes and Bauer (15) described leukocyte counts of 1,000 to 70,000 WBC with a mean of 13,317. PMN were 80-95% (mean of 71%) and therefore always increased during attacks.
MSU crystals are seen intra and extracellularly during attacks of gouty arthritis. They may vary in size from large crystals that extend through a PMN like a stick through an olive to tiny birefringent dots without a long axis. Some PMN are dead with pyknotic nuclei (16,17). Fragments of synovial tissue or cartilage can be found floating in gouty joint fluid. Either may contain masses of crystals. Fragments of synovium in the fluid have been illustrated with dense microtophi containing radially oriented arrays of crystals ( Fig.13).
Rarely crystals are not found in what otherwise seems to be typical acutegouty arthritis (18). Uric acid concentrations in synovial fluid are almost always identical to serum levels although dissolution of crystals can show the increased total urate load (19). Crystals are also very commonly found in gouty synovial fluids n the interim between attacks (20,21). Untreated patients virtually always still have crystals in previously involved asymptomatic joints (20,22). Such crystals may be extracellular or phagocytized by mononuclear cells. With treatment by urate lowering drugs crystals can be depleted.
Neutrophils related to MSU crystals almost invariably show a striking patchy loss of cytoplasmic density in specimens fixed with standard techniques (23) (Fig.14). This is in contrast to PMN of pseudogout and RA. Urate crystal outlines are shown by dense proteinaceous material lying on the crystal surface with the actual crystal dissolved by the fixative (8,23,24). Many crystals are in phagosomes with dense bodies and finely granular material lying adjacent in the phagosome (Fig.15). Other crystals have been felt to be totally free in the cytoplasm (24) but there is a strong possibility that some of these are artifacts. Large defects are often seen in phagosome membranes. Crystals in macrophages and synovial lining cells are not associated with membrane defects or loss in cytoplasmic density. Very tiny crystal outlines are also seen (Fig.16). Synovial macrophages have been seen to phagocytize crystal laden PMN . Microtubules are easily seen in these large mononuclears.
IMPLICATIONS OF PATHOLOGY OF SYNOVIUM AND SYNOVIAL FLUID
Morphologic studies have so far shown no differences between the arthritis in primary or secondary gout of various kinds. Although not absolutely proved, most evidence suggests that tophi in the synovum antedate most attacks of gouty arthritis. These tophi are often thinly encapsulated and may well be the site from which crystals break free into the joint to initiate the acute arthritis. McCarty (25) and others (13) have shown that injection of synthetic MSU crystals into human or animal joints does produce an acute arthritis like that seen in human gout.
The possibility that the matrix in which the urate crystals lie may play a role in precipitating crystals or maintaining the crystals has been felt to be “not unreasonable” by many including especially Sokoloff (5), Hasselbacher et al (12) and others. Changes in mucopolysaccharides or other connective tissue materials in addition to serum uric acid levels and serum binding factors may determine local deposition of crystals. It is of interest that local tophaceous crystal deposits have occurred at sites of local trauma such as a burn on the finger (26) (Fig. 17).
PMN leukocytes are essential for the production of the full syndrome of acute gouty arthritis (27). Phagocytosis of MSU crystals by PMN produces rapid cell death with release of cell debris, enzymes and crystals that tend to perpetuate the inflammation (23). Crystal laden phagolysosomes seem to lyse in some synovial fluid cells at least in part due to the effect of the negatively charged crystal on the membrane (28,29). PMN in the synovial microvasculative and interstitium also seem to degranulate without direct contact with crystals. Crystals in mononuclear cells do not produce the same cell death.
PMN contain myeloperoxidase that can digest crystals but the exact method for crystal clearing and factors producing subsidence of attacks are not yet clear. Alterations of proteins bound to the crystal surface (30,31) can be a factor in resolution as can changes in cytokines related in part to naturation of macrophages (32).
The presence of chronic inflammatory cells in synovium even early in an acute attack is followed by persistent mild chronic inflammation even in asymptomatic intervals between attacks. This may play a part in the eventual joint destruction. Chronic inflammation has also been produced by injections of synthetic crystals into dog joints (13).
The possibility that immunologic mechanisms might be involved in gouty arthritis is suggested in part by the dense deposits in vessel walls noted by EM. Gouty synovial fluids may have complement derived chemotactic factors (29). Immunoglobulins and complement have been identified in PMN phagosomes in synovial fluid in gout (33).
Synovial biopsy in gout may be useful in diagnosis between attacks if a tophus is identified. Don’t forget that the synovium must be fixed in absolute alcohol and processed without formalin.
BONE AND CARTILAGE
Tophi in subchondral bone are associated with collapse and resorption of trabeculae producing a punched out like space as seen in x-rays (Fig. 18) and tissue specimens. Subperiosteal marginal MSU deposits have been illustrated (4,5,34,35). Crystals are phagocytized by adjacent mononuclear and giant cells. Bone sclerosis and osteophyte formation are typical adjacent to the tophi.
Cartilage surfaces in chronic gout may appear as diffusely dusted with white crystal deposits or speckled with nodular deposits (34). Such crystal deposits have been felt to begin in superficial cartilage and are accompanied by chondrocyte loss and fibrillation. In these cartilage sites and in the helix cartilage of the ear cellular reaction is absent or minimal (34). Later cartilage can be focally eroded first with adjacent grossly normal areas. Pannus is not seen.
Synovial fluid cartilage fragments containing MSU crystals have been studied by polarized light by Pascual and Ordonez (36). Sheets of crystals were arrayed along apparent collagen fibers suggesting a role of the collagen matrix in the crystal localization.
Renal involvement in gout has been documented for years with early findings by Charcot and others (37). Nephrosclerosis and renal vascular disease are common. Deposits of MSU can form microtophi in the interstitium with variable inflammatory reaction including giant cells. Concentrations of urate are highest in the papillae so MSU tophi are more frequent there (38,39). Uric acid stones can form in the collecting system in acid urine in patients with increased uric acid excretion. Intraluminal casts of amorphous or crystalline uric acid have been demonstrated (38,40). There is some suggestion that renal tubular cells can phagocytize uric acid (41). Gouty patients also have an increased incidence of calcium oxalate stones. Lead nephropathy can show interstitial urates but is not specific histologically (39). There are other potentially important histologic changes including neutrophilic infiltrates from secondary infection, tubular atrophy, atrophy of loops of Henle, glomerulosclerosis and hyalinization of arterioles (40,42).
OTHER SITES OF EXTRAARTICLUAR TOPHI
Gouty tophi have been demonstrated in connective tissues in many parts of the body. These include subcutaneous tissue, tendons, heart valves, larynx, sclera and cornea, middle ear, nasal septum, tongue, bronchi, buttocks, carpal tunnel and even the penis (40).Histopathologic findings at these sites have been limited but generally show findings similar to those in the synovium and without acute inflammation (40). Gross appearance in skin often shows tophaceous masses with no overt inflammatory reaction (Fig. 19).
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More References Related to Gout
Agudelo C, Schumacher HR, Phelps P: Effect of exercise on urate crystal induced inflammation in canine joints. Arthritis Rheum 15:609-616, 1972.
Agudelo C, Schumacher HR: The synovitis of acute gouty arthritis; A light and electron microscopic study. Human Path 4:265-279, 1973.
Schumacher HR: The pathology of the synovial membrane in gout. Light and electron microscopic studies. Interpretation of crystals in electron micrographs. Arthritis Rheum 18:771-782, 1975.
Hasselbacher P, Schumacher HR: Immunoglobulin in tophi and on the surface of monosodium urate monohydrate crystals. Arthritis Rheum 21:353-361, 1978.
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