APRIL KUNG, DVM ♦ January 27, 2018

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During my early days of working as an ER vet, when I was still within my six month mentorship window, a dog was brought in for abnormal behavior. My medical director went to talk to the dog’s owners, but sent the dog back to the treatment area for me to examine. The dog overreacted when I tested his pupillary reflexes, and his pupils were enlarged and slow to respond to my flashlight. More worryingly, he was having difficulty walking and was dribbling urine as he faltered about.

I racked my brain for what could be causing these signs. I rifled through the quick reference notebook (a.k.a. my medical bible) that I started creating when I was a fourth year vet student. The page I consulted is pictured below.

Okay, let’s see. The nerves to the urinary bladder and the external and internal urinary sphincters are the pelvic nerve, the hypogastric nerve and the pudendal nerve. Both the hypogastric and pudendal nerves cause constriction of the sphincters – so there could be something wrong with those nerves, and that would explain why urine was dribbling out. Those two nerves emerge from the sacral and the lumbar spine. Could there be an abnormality in the lower spine affecting both these nerves?

“There must be something terribly wrong. A brain tumor perhaps.”

But what about the dog’s difficulty walking? It’s not confined to just his hind legs. All four of his legs are affected. If the problem is related to the spinal cord, it would have to be way up in his neck, in his cervical spine, in order to explain deficiencies in all four limbs. But neither of those explanations are consistent with his slow pupillary reflexes. Therefore, the problem must be in his brain. What part of the brain would have to be affected in order to cause all these different symptoms?

I was very worried about this dog. There must be something terribly wrong. A brain tumor perhaps. But where? The abnormal pupillary reflexes indicated a problem in the forebrain or the midbrain, but bladder control could be affected by the forebrain or the hindbrain. Neurological abnormalities causing deficiencies in all four limbs could also be attributed to the brain. What was it the Neurology Professor said in vet school? The more abnormal the gait, the further back in the brain the lesion is likely to be?

“Didn’t he care that something awful was wrong with this dog? Why was he so relaxed?”

As I was standing there, my eyes growing wider as my conviction increased that something horrible must be affecting multiple areas of this dog’s brain, my medical director came back to the treatment area. As we both watched this poor animal stumble around, dripping urine all over the floor, I told him of my physical exam findings and gave him a brief recap of my diagnostic reasoning. He chuckled. I was shocked. Didn’t he care that something awful was wrong with this dog? Why was he so relaxed?

He turned to me and asked, “What would you say if I told you this dog was brought in by two teenage boys who are both acting strange?”

I shook my head back at him. I had no idea.

“This dog is high,” he said, chuckling some more. “He got into their marijuana.”

Aha! Marijuana effects multiple areas of the brain including the forebrain (which could explain the slowed pupillary reflexes and loss of urinary control) and the part of the brain involved in balance and coordination (which explained the dog’s difficulty walking).

“Did they tell you that?” I asked, sure that was the only way he could have solved this case so quickly.

“No, but I’ve seen this a dozen times.”

“Pattern recognition is the cornerstone of clinical expertise.”

This, in a nutshell, is the difference between a new veterinarian and an experienced veterinarian. Since I had never seen a case of marijuana toxicity before, I was using an analytic approach that drew on what I’d learned of “elaborate causal networks” of “pathophysiological processes.” This is a perfectly respectable method for diagnostic reasoning. It just makes you feel as slow as a turtle, and it has the potential to send you down a few blind alleys – which makes you feel like an even slower turtle.

My medical director, who had “seen this a dozen times,” had used pattern recognition, which is much faster. He wasn’t thinking about the pathophysiological explanations at all. He didn’t have to. Thanks to his previous experiences with similar cases, he instantly recognized the pattern of clinical signs that manifest in dogs exposed to marijuana. This is one justification for making interns and medical residents work such long hours and see so many cases. “Extensive exposure to many different cases may be the critical factor in developing expertise.”* Pattern recognition is the cornerstone of clinical expertise.

But if you look back at the analytical reasoning I did on the case of the stumbling, urinating dog, you’ll realize that I actually did come to the correct conclusion. The dog’s problem was indeed in multiple areas of his brain. And after my medical director revealed that marijuana toxicity was the cause, I never again forgot that toxicity should be at the top of my list of suspicions whenever symptoms of diffuse brain abnormalities are present. And I’ve certainly never forgotten what a dog who’s high on marijuana acts like. That case greatly enhanced my arsenal of pattern recognition, and I solved subsequent similar cases much more quickly.

“If pattern recognition alone were enough, a veterinary assistant or technician with years of experience could be a doctor.”

This isn’t to say say, however, that once you become an experienced clinician you’ll never use the analytical approach again. Pattern recognition is a necessary ingredient of clinical expertise, but in and of itself, it’s not sufficient. If pattern recognition alone were enough, a veterinary assistant or technician with years of experience could be a doctor. Pattern recognition can help rule out unlikely diagnoses quickly and shorten the list of possible diagnoses, but it must still be combined with a doctor’s knowledge of patholophysiological processes to make a definitive diagnosis. When a medical case is straightforward, you can make a diagnosis based on pattern recognition so long as everything about the case is consistent with your knowledge of pathophysiological processes. But when a medical case isn’t straightforward, pattern recognition also has the potential to send you down blind alleys.

Consider this example. A patient presents with difficulty breathing. Pulse oximetry confirms abnormally low levels of oxygen in the blood. A heart murmur and crackling sounds are heard when a stethoscope is used to listen to the patient’s chest. Chest radiographs show a pattern consistent with fluid overload in the lungs. This could be a slam dunk case of heart failure.

Pattern recognition combines the following observations to make the diagnosis of heart failure:

1. Respiratory distress
2. Low blood oxygen content
3. A heart murmur
4. Crackling sounds from fluid in the lungs
5. Chest radiographs consistent with fluid overload in the lungs

After seeing this combination a few times, almost anyone would be able to diagnose heart failure – and in many cases, they’d be right. In many cases – but not all. If the patient responds to treatment for acute heart failure and breathing improves, pattern recognition has enabled a quick diagnosis and effective symptom resolution. But what if the patient is treated for acute heart failure yet breathing and blood oxygenation don’t improve? This is where continued dependence on pattern recognition ceases to be helpful and could even potentially be harmful. Only the slower, analytical approach using the doctor’s knowledge of pathophysiological processes has a chance of leading to the correct diagnosis.

“…it’s the analytical approach and knowledge of pathophysiological processes that make a doctor a doctor.”

Here’s a simplified summary of the pathophysiological processes underlying acute heart failure: A heart murmur can indicate underlying heart disease. If heart disease is present and the heart is too weak to do its job of pumping blood, blood backs up behind the heart, just like cars back up when a traffic light is broken. If blood can’t move forward through the arterial system like it’s supposed to, pressure increases in the veins behind the heart. Eventually enough pressure builds up to push blood plasma (the liquid portion of the blood that does not contain red blood cells) through the semi-porous membranes of the tiny blood vessels in the lungs – resulting in the fluid overload in the lungs that causes respiratory distress.

Acute heart failure is typically treated with an intravenous injection of furosemide, which is a diuretic. This drug causes increased blood flow to the kidneys and increased fluid excretion through the kidneys. Urination reduces the amount of fluid in the blood vessels, thereby reducing fluid overload in the lungs. If the diuretic fails to induce urination however, the doctor must consider whether there is something wrong with the kidneys. Perhaps the heart murmur is just a red herring. The kidneys filter blood and excrete excess fluid. If they aren’t functioning and the patient isn’t producing any urine, that too can cause fluid overload in the body – and in the lungs. If blood chemistry reveals abnormally elevated kidney values, the more likely cause of the patient’s fluid overload then becomes acute kidney failure. The doctor must then then use her knowledge of pathophysiological processes to determine what available medical treatments might be effective in reducing fluid overload if the patient can’t be made to urinate.

So you see, pattern recognition can make a doctor faster, but it’s the analytical approach and knowledge of pathophysiological processes that make a doctor a doctor. When you begin practicing, you’re going to feel like a turtle compared to your more experienced colleagues. You may even notice that support staff seem to come to conclusions more quickly than you do. New veterinarians often observe their comparative lack of speed and conclude there’s something wrong with them. They feel inadequate and proceed to doubt their worth as doctors. But now you’ll know that your lack of speed is no reflection of a lack of knowledge nor a lack or worth. It’s merely a lack of simple pattern recognition,  and that’s nothing to beat yourself up about because pattern recognition comes to all of us the same way – one case at a time.

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This is an excerpt from Book 3 of the On Being a Veterinarian series, Practicing Small Animal Medicine. If you’d like to be notified when the book is released, please subscribe to my email list.

REFERENCES

* A Cognitive Perspective on Medical Expertise by H.G. Schmidt, Ph.D.