Exercise 1 discussion

Clinical history and questions

Split femurs from two adult cats of approximately equal size. One animal was severely and chronically infested with fleas, and one was chronically lame due to severe osteoarthritis of the stifle joint.

Can you relate these specimens to the conditions in these cats?

An interpretation of Diagnostic Exercise No. 1 – Roger Kelly

Of the two marrows, the one on the right is closer to normal than the one on the left.  Normal femoral marrow in an adult cat should have a pink-red zone of haemopoiesis immediately under the endosteum, while the central part is expected to be paler because it is mostly fat.  This bi-modal feature is clearly apparent in the marrow on the right, which would also respond to Archimedes’ test by floating in water or fixative  Thus the marrow on the left is abnormally uniformly dark red throughout.  This is consistent with erythroid hyperplasia of more than a week’s duration (it takes time for an adequate erythropoietic stimulus to cause enough hyperplasia of erythropoietic cells to replace the central fat).  It would also be consistent with a myeloproliferative disorder of the erythroid line, but that type of marrow neoplasia is exceedingly rare.  Since one of the cats is very likely to have been secreting higher than normal amounts of erythropoietin (chronic blood loss to the fleas), it would be perverse to attribute the appearance of the marrow on the left to anything other than erythroid hyperplasia in this animal.  A bit of fat metabolism due to the parasitism might also have helped this marrow look darker. Inflammation involving bone and its marrow tends to produce much more irregular changes than is seen in either of these specimens, while developmental disease could be invoked, but seems intuitively to be less likely than either reactive hyperplasia or neoplasia.

Compare the relative thickness of the femoral cortex in these bones.  The cortex is thicker in the femur on the left, and the history tells us that the cats were of similar age and weight.  Therefore either one femur is abnormally thick, or the other abnormally thin, or one is normal and the other either thicker or thinner than normal.  Since the history gives one of the animals a good reason for long-term non-weight-bearing (chronic stifle osteoarthritis), it seems reasonable to expect one femur in the lame animal to have a thinner cortex than normal (osteoporosis of disuse).  One could argue that the contralateral femur in the same animal might be thicker than normal because of extra weight-bearing, and I would accept that argument, since we are not told if the femur shown is from the lame or the sound limb.

With the above qualifications, the basic disease process is again either reactive hyperplasia/hypertrophy, or atrophy (a degenerative condition).  The abnormality is too uniform for it to be due to either neoplasia or inflammation, and developmental abnormality seems unlikely.

So it could be that the femur on the left was from the sound limb of a cat which was suffering from severe unilateral stifle osteoarthritis as well as from severe chronic flea-bite anaemia, while the femur on the right was from a normal cat.  OR, the femur of the left could have been from the anaemic cat with two sound pelvic limbs, while the femur on the right is from the lame leg of the cat with a normal haematocrit.  To the clever-dick that tried to argue that both femurs are from the same cat, I would point out that the marrow nearly always reacts as a unit, so such dissimilar marrows would have to come from different animals.

My assessment of answers to this problem would allot maximum grades to those answers which covered all the possibilities in this manner.

Of course, such a long-winded answer would never do, given the usual time restraints of examinations.  But something like the following would indicate that something like the above deductive process had been properly applied:

“The marrow of femur on the left is uniformly deep red throughout, while that of the other femur is paler, particularly toward its core.  Of the two, the marrow on the right is the more normal for the class of animal described, presumably due to its fat content, and I would expect it to float in water.  The marrow on the left, given the history, is likely to have been transformed by erythroid hyperplasia due to chronic blood loss, with possibly some fat metabolism contributing.

The cortexes of these bones are of dissimilar thickness.  Without scales or other references, the right femur could be abnormally thin (due to disuse osteoporosis in the case of non-weight-bearing lameness), or the cortex of the left specimen could be abnormally thick, as in the case of excess loading in the sound limb opposite the lame leg.”

I hope that people who have matters do discuss about this case will feel free to respond to it, either by communicating with me individually at Roger-Kelly@aapt.net.au or by opening wider discussion by posting their responses on the mailing list.

I also hope that people with nice case illustrations will give consideration to allowing them to be used in this way as part of a mode of continuing education.  I will be very happy to advise on techniques to maximise/optimise quality of images for this purpose, but the most important things to remember (after the quality of the original illustration) is that the original scan or photograph should be made/taken at maximum resolution, and saved preferably as a .TIF file, before making final adjustments to colour balance, brightness, etc with imaging software, then saving as a .JPG file compressed down to about 150kB file size.