IMHA and ITP – Pathophysiology and Diagnosis – Part 1

Presenter – Christopher G. Byers DVM, DACVECC,DACVIM(SAIM) Diplomate American College of Veterinary Internal Medicine, Diplomate American College of Veterinary Emergency & Critical Care, Midwest Veterinary Specialty Hospital, Omaha, USA

The subject matter of this veterinary webinar left me feeling a little concerned that some of its content could make for challenging watching and, even worse, outsmart me. After happily sitting for an hour listening to Christopher Byers discussing this fascinating topic, I was clearly wrong.

Christopher brought clarity to complex concepts and made the whole discussion relevant to practice. This was probably helped by Christopher acknowledging that this particular webinar was likely to appeal more to the geeks amongst us but after spending an enjoyable hour I am very happy for my inner geek to come out and say hello.

Christopher started by discussing the pathophysiology of IMHA and explained that either intravascular or extravascular heamolysis can take place. Extravascular heamolysis is seen most frequently with red blood cells becoming coated in antibodies, binding to macrophages chiefly present  in the liver and spleen which go on to destroy the red blood cells. It is this condition where we are likely to encounter the icteric dog.

The less common presentation of intravascular haemolysis involves the rbcs becoming coated in immunoglobulin and complement which damage the cell membrane causing extravascular water to enter the cell, causing the cell to rupture. In these cases you would not expect to see an icteric animal but a patient producing port coloured urine. Christopher explained that differentiating between the two is important as they may need different treatments which will be explained in the next webinar.

Christopher also explained that the bone marrow will mount a healthy regenerative response in the majority of cases as antibodies are targeted towards adult red blood cells. However it is worth remembering that it can take between 72hrs and 7 days for the body to mount a healthy regenerative response. So if you see these cases acutely it may be several days before you see any regeneration. In about 33% of cases antibodies can be directed against marrow RBC precursors at any stage of development meaning you will see an in appropriate regenerative response OR the antibodies can target the marrow precursors ONLY and no peripheral haemolysis will be evident.

Christopher advised us to approach diagnosing these cases in a logical manner, confirming that there is firstly anaemia, secondly an immune mediated condition and thirdly haemolysis. Proving anaemia is the easiest task and Christopher always recommends sending blood to a reference lab for CBC and absolute reticulocyte count. Confirming the disease is immune mediated can be achieved by performing a slide agglutination test and if there is no macroscopic evidence of agglutination then it is always worth checking under the microscope.

Not every case of IMHA will autoagglutinate as Christopher explains further within the webinar so it may be necessary to perform a Coombs test. The presence of spherocytes provides evidence for haemolysis although IMHA cannot be ruled out if they are not present.

Needless to say I have only provided you with snippets of information from this veterinary webinar and Christopher also goes on to discuss ITP in depth. My inner geek thoroughly enjoyed this webinar and I’m keenly anticipating part 2 – management of IMHA and ITP.

The Stethoscope (MRCVS)

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