Smell tests have been widely touted as a diagnostic tool for Alzheimer’s disease (AD), under the theory that the nose knows more about the earliest signs of AD than any other bodily system. The level of difficulty people encounter in identifying certain scents—such as lilac, leather, lemons, strawberry, smoke, soap, menthol, pineapple, clove, and the sulphur in natural gas—has been postulated to be one of the first markers of increased risk for AD. However, new research suggests that this presumption may not be correct.
Scientists reviewed 32 studies conducted over the past several decades that have linked participants’ ability to identify scents with risk for AD. They could not find conclusive evidence that loss of smell is a predictor of the disease, according to a paper published in the May 2012 issue of the journal Laryngoscope.
“It is possible that the olfactory system of the brain may be a brain area affected by AD early in the disease process, and that symptoms may show up there many years before the onset of memory problems and other cognitive symptoms,” says MGH neurologist Mark W. Albers, MD, PhD, an expert in neurodegeneration and the olfactory system in AD. “But the researchers could only find evidence of an association between loss of smell and AD in many, if not most patients, and limited proof that loss of smell actually predicts AD.
“Most of the research conducted to date has consisted of cross-sectional studies that involved comparisons of performance on standard smell tests between a group of healthy people and a group of people with AD. It is already known that in people with AD, the ability to detect odors is diminished, along with the ability to identify, remember, or discriminate among odors. These studies are not predictive because they do not follow participants over time in longitudinal research that determines whether healthy people who perform poorly on smell tests go on to develop AD. We need longitudinal studies, and until they are done, the diagnostic value of smell tests is unknown.”
Smell deficits detected in AD studies are usually so minor that subjects are unaware of them, Dr. Albers says. And, in fact, subjective loss of smell does not usually correlate with the onset of AD. A decline in the sense of smell may be caused by a variety of factors other than AD, such as:
- Certain medications, such as beta-blockers and angiotensin-converting enzyme (ACE) inhibitors used for hypertension, antihistamines, certain antibiotics, some cholesterol-lowering drugs, certain antifungal medications, and vasoconstrictors in nasal sprays
- Medical conditions, such as allergies, sinus and other upper respiratory infections, hypothyroidism, diabetes, bronchial asthma, brain tumors, Parkinson’s disease, migraine, or cystic fibrosis
- Viruses, such as the flu (some can do permanent damage)
- Head or olfactory injuries
- Decreased hydration
- Thin nasal mucous lining
- Nasal polyps
- Vitamin or mineral deficiencies
- Exposure to toxic agents, such as solvents and insecticides.
Problems with smell may also increase with age. An estimated 25 percent of men and 11 percent of women aged 60 to 69 complain of smell disorders.
“Recent data has indicated that normal aging may involve much less functional decline than previously thought, so whether loss of smell is a normal aspect of aging or the onset of disease is still an open question,” comments Dr. Albers.
Dr. Albers recommends that people worried about deterioration in the sense of smell consult a medical care provider for assessment. The physician may suggest a treatment, such as an antibiotic or anti-inflammatory drug to reduce nasal swelling, or suggest a consultation with an otolaryngologist (ear, nose and throat doctor) who can help address the smell disorder.
AD not ruled out
Despite the findings of the recent Laryngoscope review, it is possible that further long-term research may eventually prove the diagnostic value of smell tests for AD, Dr. Albers says. In fact, he and his colleagues are currently conducting a longitudinal study of 144 older adults who are retested every year using brain scans, neuropsychological tests, and tests of olfactory performance to determine whether deficiencies in the ability to identify certain specific odors can predict AD.