Why bother with a problem based approach to cases?

What tools do you use to problem solve tricky cases? One of my favourite resources is a book called Differential Diagnosis in SmallAnimal Medicine. The second edition has just come out and I gave it to myself as an early Christmas present. In my teaching role I find that many students struggle with the concept of problem-based medicine (PBL) or problem-oriented medical management (POMM). Why compile a huge list of differential diagnoses when surely there’s one right answer? I discussed this with the authors of the second edition, Drs Alex Gough and Kate Murphy.

Putting together this book was a huge task. For the first edition, Dr Gough compiled a huge list of clinical signs (e.g. heart murmur), historical signs (e.g. seizures), radiograph signs (e.g. pleural effusion), laboratory findings (e.g. non-regenerative anaemia) and then for each of these signs scoured textbooks, conference proceedings and papers to complete the list. For the second edition, Kate revised, fact-checked and edited out some extraneous material. The end product is a brilliant resource.

Dr Kate Murphy at work.

What is your day job?

AG: Head of Medicine Referrals in a Multidisciplinary Referral hospital. [Ed - Alex sees referrals in medicine, cardiology and neurology; holds the Royal College of Veterinary Surgeons Certificate of Small Animal Medicine and Veterinary Cardiology and a postgraduate certificate in neuroimaging for research. He also wrote Breed Predispositions to Disease in Dogs and Cats].

Dr Alex Gough.

KM: My day job is balancing work and home life! At work I am the Clinical Lead for our Referral Hospital. I work in a busy Referral Hospital, which is also the main hospital for our First Opinion Practice. We have 13 branches sending in patients for elective investigation and surgery and then any acute cases, which need more care than the branches, can provide. As a result, there is never a “quiet” day. I am a Small Animal Internal Medicine Specialist and so as well as leading the referral team I am also actively seeing referred medical cases every day. Some of these are straightforward (e.g. gastrointestinal investigation) and then there are the others which is where POMM comes in!

What is POMM?

KM: POMM or “problem-oriented medical management” is a system which makes us concentrate on all the problems a patient has and forces us to avoid the problems of pattern recognition which not only can lead us to misdiagnosis but can also stop us from discovering new diseases or new presentations of old diseases.

There are so many differential diagnoses for some signs like polydipsia and polyuria, or forelimb lameness, yet we are taught common things occur commonly. Why bother thinking about all the potential differentials?

AG: If you fail to consider all the possibilities that may cause a particular sign, then you will not consider performing the range of testing that may be required to rule in or out all the differentials. Most of the time, if you only focus on the common diseases, you will get away with it, but there are a lot of uncommon and rare diseases out there, and uncommon presentations of common diseases are often seen. Taking into account all the possible differential diagnoses makes errors in diagnosis, sometimes catastrophic ones, less likely. For example, failing to consider that a dog with ascites may have a pericardial effusion has led to many dogs having unnecessary and dangerous exploratory laparotomies. That isn't to say you should blindly test for every condition on a differential diagnosis list - this is often suboptimal because of the invasiveness of some of the tests, as well as financial constraints. Rather, tests should be prioritised to give the highest diagnostic yield for the least cost (whether that is to the animal or owner), and be prepared to revise a diagnosis if test results don't support your diagnosis, or if an expected response to treatment is not achieved. 

KM: POMM does not stop us thinking about “common things occurring commonly” but it helps us when we are not seeing a common case to think logically and keep our options open.  It can help us to problem solve and then bring the most likely differentials together to aid the construction of a sound diagnostic plan based on information we have gathered and the finances of the client. Sometimes making assumptions leads us to select the diagnostics most likely to give us the answer we think we will find and we might be heading down the wrong (sometimes expensive track) because we were not prepared to “think outside the box”.

Why are students so attracted to a "pattern recognition" approach?

AG: The pattern recognition approach is quicker, and once it is learned, it requires less intellectual effort. It may also have some advantages, especially in simple cases, in reducing the number of tests a patient requires. For example, a limping cat with a swollen leg, pyrexia and a bite wound could be reasonably treated for a cat bite celullitis, and only if a good response to treatment is not achieved, starting testing for other problems. 

KM: Pattern recognition is easier than POMM and it is more innate. For example, a young dog with haematemesis, haemorrhagic diarrhoea and low white blood cells is assumed to have parvovirus infection. Such diagnosis reveals decent pattern recognition skills but in our clinic last year we saw patients who were not infected with parvovirus but with leptospira. Leptospirosis, would not be on many differential lists for haematemesis and haemorrhagic diarrhoea.

Is POMM still relevant for experienced practitioners, and why?

AG: Experienced practitioners will often use pattern recognition, and in short consultations, this may be the most efficient way of achieving a diagnosis. However, those cases that don't go according to plan, or have unusual signs, can be greatly helped by going through a logical thought process to get to the bottom of them. 

KM: POMM is used by specialist clinicians as well as experienced practitioners. You will probably find that they do not use it for every case in its written format, but in their heads they are running through the differentials for the presenting problems and then rationalizing into the most likely causes, which will then be investigated. When I see cases where I have no strong evidence for the cause, I return to the basic POMM approach to help me logically consider all the options.

Compiling differentials for every historical, physical, radiographic, sonographic, laboratory and electrodiagnostic sign or finding seems like a mammoth task - how do you do it?

The reality is that you do have to be a little discriminatory and this is where experience helps. So I do not write vast differential lists for non-specific signs such as inappetance and lethargy for example. I do keep those problems on my list so that if they do not appear to be secondary problems I push them back up my problem solving list. When interpreting clinicopathological data one has to get experience to decide which are significant results, which require problem solving and which do not. Sometimes it is the fact that whilst a result is not outside of the reference interval, it is an inappropriate finding in a “sick/collapsed patient”. For example: a collapsed dog with low normal glucose is an unusual finding; normally the glucose is at the top or slightly above normal and thus this could be an indication to consider an insulinoma a likely diagnosis. In a collapsed dog that does not have a stress leukogram, we should consider Addison’s disease as a differential.

Do we worry about a total white blood cell count that is 0.5x10⁹/l above normal – not really?

This skill is one of the most difficult to teach and when I was teaching undergraduate students we spent time discussing this aspect – however it is only by forcing oneself to look at the clinicopathological data and interpret it yourself before reading the clinical pathologist comments from the lab that will make the clinician a better problem solver in the future. The same for histopathology reports – read the description not just the final diagnosis. Why?….because we are the ones who have seen the client, examined the animal and now need to put it all together.

What is the best way to use a list of differentials?

KM: I use differential lists to guide me rather than to dictate me. Veterinary medicine is a science with some art thrown in. You can be an amazing POMM clinician but the art comes in to your conversation with the client, making sure they understand the jargon you are throwing at them, that they feel informed and involved in the decision making for their pet and that they feel their concerns have been listened to, respected and addressed. It is no good making the most amazing diagnosis if the client has lost trust because you have done loads of tests but explained nothing that they can understand or if by the time you make the diagnosis you have spent all their money and they can not afford the treatment. Honest, open discussion with a logical problem-orientated approach are the keys to my clinical practice.

Thank you, Drs Gough and Murphy, for your time. Their book, Differential Diagnosis in Small Animal Medicine, is available via Wiley Blackwell here.