Mastocytosis is actually a spectrum of rare disorders,
all of which are characterized by - not surprisingly - an increase in mast
cells. Most patients have disease that is localized to the skin, but about
10% of patients have systemic involvement. There is a localized, cutaneous form
of mastocytosis called urticaria pigmentosum that happens mostly in children
and accounts for over half of all cases of mastocytosis.
Clinically, the skin lesions of
mastocytosis vary in appearance. In urticaria pigmentosum, the lesions are
small, round, red-brown plaques and papules. Other cases of mastocytosis show
solitary pink-tan nodules that may be itchy or show blister formation. The
itchiness is due to the release of mast cell granules (which contain histamine
and other vasoactive substances).
In systemic mastocytosis, patients have
skin lesions similar to those of urticaria pigmentosum - but there is also mast
cell infiltration of the bone marrow, lymph nodes, spleen and liver. Patients
often suffer itchiness and flushing triggered by certain foods, temperature
changes, alcohol and certain drugs (like aspirin).
Histologically, mast cell infiltration
may be subtle (with ocasional spindle-shaped mast cells around vessels in the
superficial dermis) to dramatic (with tightly-packed mast cells in the upper to
mid-dermis - check out the photo above). Mast cells may be difficult to
distinguish in regular H&E sections; you may need a metachromatic stain
(toluidine blue or Giemsa) to really visualize the granules.
