Differential Diagnosis – The Crucial Trigger for Thinking Better

The Isabel project was inspired after my own daughter’s near fatal misdiagnosis in 1999. Last week I experienced my own misdiagnosis. Although far less serious, the episode is probably as instructive for the lessons it teaches. It showed me how easy it is to happen, how it’s often nothing to do with lack of time but just down to sloppy thinking. The discipline of doing a differential diagnosis (even for what seems a blindingly obvious, straightforward diagnosis) is crucial to avoid the sloppy thinking traps.

It was the last day of my vacation, skiing with the family. In the white out conditions we had experienced most of the week, we started down a run that the day before had had wonderful snow but this time had become slush that had turned to ice.  My left ski got knocked off as I hit a lump of ice and I tumbled over. As I fell, I put my hand down and my ring and little fingers must have taken the brunt of the fall. It was extremely painful and I felt slightly nauseous, reminding me of the feeling I got when I fractured my knee a few years earlier.  By the afternoon, when we were starting to travel home, the hand had completely swollen and was about 30% larger than my good hand.

We arrived home very late so I went to see my family doctor the following day to have it checked. I wasn’t in continuous pain but with the amount of swelling that hadn’t reduced with ice and anti-inflammatory drugs, it seemed sensible to have it looked over. The doctor asked what had happened. His first question that was also asked by all the subsequent doctors I saw was “where had I been skiing!”

He seemed to examine it carefully and thought that I had fractured the bone in the hand at the 4th metacarpal (ring finger). I agreed that it hurt there but said that it also hurt on the soft area (dorsal tenderness) just around the 5^th. He referred me for an x-ray that was carried out shortly afterwards at the attached small community hospital. The radiologist and nurse practitioner there both agreed that I did have a fracture of the 4^th metacarpal. I asked to look at the x-ray and was impressed when they showed me the fracture as it seemed nothing more than a slight difference in shading and difficult to spot. I felt reassured by the diagnosis and went home.

Two days later the hospital called to say that a senior radiologist had reviewed my x-ray and that I had, in fact, dislocated the 5^th carpometacarpal joint. The orthopaedic surgeon at the main city hospital said that I should come in as soon as possible as the joint had now been dislocated for 3 days; I wasn’t to eat anything and should expect to be operated on that night. This came as a bit of a jolt!  When the orthopaedic surgeon at the main hospital examined my hand, (after establishing where I had been skiing!), he pointed out that my little finger was considerably lower relative to the ring finger than it was on my other hand.  Wow! that was so obvious now it was pointed out, how was it missed?

I have been chewing over this episode over the last few days and a number of points have struck me:

• I was angry with myself for not asking the doctors the key question “what else could it be?” We had all fallen into the trap of finding one easy explanation (the fracture of the 4^th metacarpal) and been satisfied and not looked any further.
• The doctors said this was an unusual injury and is hard to pick up on x-ray. However, the real clue was the physical examination that showed my little finger was at a different position relative to the ring finger. Examination of both my hands side by side would have highlighted this.  The degree of swelling and the tenderness was also a clue as it was more than would have been explained by a minor fracture of the 4th metacarpal.  The community hospital that did the the first x-ray was told that I needed an x-ray for a suspected fracture of the 4th metacarpal but not to consider dislocation of the 5th. This meant that when they found what had been suspected they were satisfied and didn’t look for anything else.   Even though the x-ray clearly showed the dislocation right next to the fracture site and the misalignment of the finger joints! There is a very apt adage in radiology that says “what is the most commonly missed fracture on x-ray?” Answer: “the second one”.
• Although we often attribute diagnostic errors to the pressures of time, this mistake was not due to lack of time as neither my doctor nor the radiographer or nurse practitioners were particularly rushed during the consultations.
• I concluded that the only thing that could reduce the chances of this type of relatively simple mistake happening again was if my doctor was forced to record in the medical notes a full differential diagnosis of possible causes or maybe even 3 possible diagnoses. This would have acted as a thinking trigger and reduced the chances of the classic premature closure trap.  By ruling in and out the diagnoses on the differential list, he would have been more likely to reach the two diagnoses I was eventually diagnosed with: (i) fracture of the 4th metacarpal (ii) dislocation of the 5^th carpometacarpal joint.  Alternatively, or in addition, I (the patient) could have acted as the thinking trigger by asking the doctor what else this could be.

In the end this was, I admit, a fairly minor medical error but if dislocated joints are left unrepaired for several days there can be long-term implications. However, because it was so minor and such an easy a trap to fall into it seems more important to examine it closely and try and understand how these minor mistakes happen and can be avoided. Their minor nature may make them seem not worth addressing when we should be focusing on life threatening diseases. However, the root cause is the same so, if we can fix these seemingly minor events, then the serious ones will be fixed as well.