Smell loss is most frequently caused by upper respiratory tract infection or head trauma. Most studies find that the return of smell function is more likely in the former case than in the latter. In medical jargon the conventional wisdom is that etiology (cause) determines prognosis (outcome).
However, a study published last year in the Annals of Neurology came to the surprising conclusion that etiology does not predict outcome. Richard Doty and his colleagues reviewed 542 patients examined at the University of Pennsylvania School of Medicine’s Smell & Taste Center. The patients received a comprehensive test battery on their first visit, including a 40-item smell identification test. At intervals ranging from three months to twenty-four years they were given a brief 12-item version of the same test. The researchers measured clinical outcome based on the difference in performance between the original test and the follow-up.
What determines who recovers and who doesn’t? Statistical analysis turned up several factors. One was the initial degree of dysfunction: the likelihood of recovery was better when the initial smell loss was mild, and worse when it was severe. Another factor was age: the older the patient was when the smell loss occurred, the less likely he was to recover. In particular, the odds of improvement drop substantially after the age of seventy-four.
Surprisingly, the cause of the smell loss did not predict whether patients recovered from it. Doty’s team concludes that
In other words, it’s the extent of the initial smell loss that matters, not what caused it.
head trauma patients with mild or severe initial smell loss have the same likelihood for functional recovery as patients with mild or severe smell loss due to other causes.
On the bright side, the study found that
some improvement occurs over time in one-third to one-half of patients with olfactory dysfunction.Improvement, however, doesn’t mean the patient regains full function. Far from it: even in patients whose initial loss was mild, only 18% regained absolutely normal smell function, and 23% regained function that was normal for age.
This study is not the last word on the topic—different patient populations, different smell tests, and different patient enrollment protocols might alter the exact nature of the results. Still, the study does put a big dent in the conventional wisdom about smell loss and recovery.