Diagnosis is dangerous. A mistaken diagnosis means wrong advice and management; and failure to identify the realities and respond correctly.
A diagnosis also tends to inhibit thought, especially if a senior doctor has made it.
When there is uncertainty we may speak of a provisional diagnosis, or a working diagnosis, but, being human, all too often those warning adjectives are forgotten.
The obvious diagnosis is especially risky, quickly made, an easy case, time for tea. But the obvious may conceal the real, perhaps much more sinister.
Here are two examples: the first shows the dangers of a wrong diagnosis, the second the danger of the obvious.
Jane was in her early 60's. I was asked to review her at home. Her general practitioner met me there, with a bulky folder of notes.
At the age of 25 she had lost vision in her left eye, but recovered after several weeks. The doctor had recognised retrobulbar neuritis, an inflammation of the optic nerve. In about half of cases retrobulbar neuritis is a first sign of multiple sclerosis; equally, in about half of cases it is not.
During the next two or three years she had seen doctors on several occasions with recurring, transient minor symptoms: numbness and tingling in one or other hand, headaches, an episode of difficulty in writing, and another of double vision. None of these episodes had lasted more than a day, and none had left any disability.
But a diagnosis of multiple sclerosis was made, and discussed with her. To be fair, multiple sclerosis is often a difficult diagnosis, but easier now we have MRI brain scanning.
Over the years she had lived in the expectation of progressive disability and an early death. Her minor symptoms continued, but no demonstrable physical disability developed. She had never married, and lived in a small flat on invalidity benefits. She was active in charities for disabled people, indeed she held a national post in one.
She had received little formal medical treatment, but was on a complicated vegetarian diet, and a number of mineral and vitamin supplements, including regular injections of vitamin B12.
On examination she was tense and talkative, rather obsessive about herself, wearing blue-tinted spectacles, but well nourished and apparently healthy. I found no physical abnormalities, in particular I found no signs of nervous system disease.
As a rough guide, a rule of thirds applies in multiple sclerosis. At 15 years from diagnosis, one third will be dead, one third will be disabled, and one third will be living reasonably normally. Jane was well some 40 years from diagnosis. Either her multiple sclerosis was exceptionally benign, or the diagnosis was in error.
Reviewing her case notes I concluded that the diagnosis was wrong: the original evidence was inadequate, and her subsequent course was not compatible with multiple sclerosis. [Yes, I had the benefit of hind-sight.]
But what to say to her? The belief she had multiple sclerosis had dominated her life for many years; could I kindly say it was all a dreadful mistake? I chose to say her case was exceptionally benign, and the disease had burned out. That might just have been the truth.
I learned later that my assessment had been reported with approval to her many friends, but she had continued much as before - perhaps the best outcome.
Pat was the second case, in her forties, mother of three, working part-time in a local hotel. She had been admitted the day before for a minor operation on her varicose veins. She should have gone home the same day, but persistent vomiting had set in after the operation. It had continued and my surgical colleague asked me to see her.
The consultation request was on my desk when I returned from my Friday morning out-patients' clinic. I had an hour for lunch and the 12 mile journey to another hospital where I was due at 2 pm.
Pat complained of nausea, but the sickness had subsided and she had been able to have some toast and tea. She was anxious to get home.
There was an obvious cause for her nausea: she was taking Distalgesic for pain control. Distalgesic combines paracetamol with dextropropoxyphene, which is a synthetic opiate, prone to cause nausea in susceptible people. It was a familiar problem with this drug.
So the prescription was easy: stop Distalgesic, take simple paracetamol if needed. Go home.
A week later my surgical colleague told me she had gone home, but had returned the next day because the nausea and vomiting had increased, despite stopping the Distalgesic.
Blood tests had shown her serum calcium value was 4.6 mmol/L - seriously increased above normal. Vomiting is a symptom of such a high calcium value. A chest X-ray showed her ribs had multiple translucent areas - the appearance of malignant secondary deposits.
Finally a proper clinical examination found a large hard lump in her right breast. Biopsy confirmed Pat had breast cancer, widely disseminated, a terminal disease.
Opportunity is fleeting; experience fallacious; judgement difficult.