The first thing I think of when I hear “granuloma” is the Knack song My Sharona. It just fits so nicely in there, right where My Sharona would be…”Come a little closer, close enough to look at my granuloma.” Okay, it doesn’t make any sense, but maybe someone will come up with some nice memory device based on the song.

Silliness aside, you really need to know a few things about granulomas. There are a limited number of things that cause them – so when you see one, that narrows down your differential to a reasonably short list.

Granulomas are part of a special kind of chronic inflammation called, you guessed it, granulomatous inflammation. They represent the body’s attempt to contain a foreign agent that it can’t easily get rid of – like Mycobacterium tuberculosis, or talc. Histologically, they consist of a bunch of “epithelioid” macrophages (big, pink, activated macrophages that look like epithelial cells) surrounded by a collar of lymphocytes (and sometimes plasma cells). Sometimes these epithelioid macrophages fuse together, forming giant cells, with 20 or more nuclei inside (check out the giant cell in the granuloma above).

There are basically two kinds of granulomas: foreign body granulomas (formed in response to inert foreign objects, like talc or sutures) and immune granulomas (formed in response to any of a number of agents, like Mycobacterium tuberculosis or Treponema pallidum. Foreign body granulomas have the epithelioid macrophages described above, and sometimes you can see the foreign material (try using the polarizer and looking in the middle of the granuloma – foreign material is often refractile).

Immune granulomas can have a few different appearances, depending on their cause. Here’s a summary:

1.  Tuberculosis. Granulomas in TB are sometimes called tubercles. They are caseating, meaning they are “cheesy” in gross appearance. Histologically, there is a bunch of amorphous, granular, necrotic debris in the center of the granuloma. You should see some acid-fast bacilli in there too.

2. Leprosy. These granulomas are non-caseating, and an acid-fast stain should reveal bacilli.

3. Syphilis. Granulomas in syphilis are called gummas; they have central necrosis (but not really caseating, because you can still see cell outlines) and a plasma cell infiltrate.

4. Cat-scratch disease. These granulomas may be stellate in appearance. They contain neutrophils and some granular debris, but giant cells are rare.

5. Sarcoidosis. Granulomas in sarcoidosis are non-caseating, with a lot of activated macrophages.

6. Crohn disease. Sometimes you see non-caseating granulomas in the intestinal wall in patients with Crohn disease.

That’s it in a nutshell. Now if I could just get that song out of my head…