As the name suggests panniculitis
is inflammation of the fat layer of the skin.
As it is so deep you can surmise that the lesions are going to present
as smooth, red and inflamed lesions without any epidermal involvement. The commonest one you are going to come
across is erythema nodosum and clinically it is going to present as tender
painful nodules on the anterior shins., occasionally also on the extensor
surfaces of the arms but primarily on the shins, with a little bit of surrounding bruising. The other disorder you will see will be
erythema induratum or nodular vasculitis.
This tends to affect the calves primarily and it may present even as an
ulcerated lesion. Other forms of
panniculitis are relatively rare. You
may see pancreatic panniculitis where someone has an inflamed pancreas or
pancreatic cyst and pancreatic enzymes leak into the circulation and cause
damage to the fat lobules. Lastly there
may be a lupus panniculitis. It will
present in any part of the body but again particularly on the legs and extensor
surfaces of the upper arms, as an indurated plaque. There may or may not be overlying skin
involvement with lupus. Another rare
presentation would be cold injury to the skin which tends to occur over fatty
areas which insulate the skin, particularly seen on horse riders on the
buttocks or in young premature neonates.
As we said this is the commonest
presentation of panniculitis that you are likely to see in dermatological
practice. It characteristically follows
a streptococcal throat infection or women coming off the contraceptive
pill. It also can be a manifestation of
underlying sarcoidosis and it is prudent always to do a chest X-ray. Histologically the picture that you will see
is a septal panniculitis because of its involvement of the vessels that run in
the septa between the lobules. Usually there is no necrosis of the lobule
itself. In an acute phase you will see
neutrophils in the septum but in the chronic phase generally mononuclear cells
and occasionally granulomatous inflammation.
Erythema nodosum can also be seen with inflammatory bowel disease and
rarely after infections such as Coccidioidomycosis and Yersinia infection. There is a chronic form of erythema nodosum
that may show a granulomatous histology in the septa with fibrotic lesions.
Pancreatic panniculitis is a
lobular panniculitis. The released
pancreatic enzymes attack the liver lobules and destroy them. There are lymphocytes, foam histiocytes and
multinucleated giant cells and the fat necrosis leaves ghost like fat
cells, sometimes with deposits of calcium cells.
Again there is necrosis of the
fat lobule and there may be fibrous rims around the fat and nodular lymphoid
infiltrates. Occasionally this is
mistaken for a subcutaneous lymphoma. Sometimes
the overlying epidermis will show the typical features of discoid lupus and the
dermis may be sclerotic.
Erythema induratum (nodular vasculitis) View Clinical is another cause of red nodules, but they typically occur in the
calf and at one stage were thought to represent a tuberculid, in other words
an idiopathic reaction to the tuberculin or tuberculosis organism. Most cases nowadays are usually
idiopathic. Occasionally in this
condition you will get ulceration and also a lobular granulomatous
panniculitis with a mixed infiltrate including lymphocytes, histiocytes, plasma
cells and giant cells. Occasionally
caseous necrosis will occur and there may be a vasculitis involving the
arteries or small veins. Polyarteritis
nodosa has been considered as a significant differential of erythema induratum.
Subcutaneous fat necrosis of the newborn is really quite a rare condition where there are indurated subcutaneous
nodules. Generally this is self limited
and they sometimes suffer from hypercalcaemia. Histologically you get characteristic needle shaped clefts within the
fat cells and as well as the fat necrosis, the granulomatous infiltrate and
sometimes calcification reflecting the hypercalcaemia.
This condition is not
uncommon. We see it regularly in the
elderly particularly beginning on the medial aspects of the ankles and
gradually encircling the ankle. The skin is indurated, thickened and sometimes
tender. There is usually associated venous insufficiency and stasis dermatitis
and this is reflected in the epidermal changes, but there is fat necrosis,
marked sclerosis with collagen deposition and lymphocytic infiltrates into the
fat tissue in a lobular pattern.
Another rare but characteristic
histologically form of panniculitis is cytophagic histiocytic panniculitis. This again is a lobular
panniculitis but it is characterised by bean bag cells which are histiocytes
that are full of engulfed inflammatory cells and erythrocytes. This condition may follow infection or can be
induced by lymphoma.