Symptoms – Loss of Sense of Smell (Hyposmia)

The loss of sense of smell, or hyposmia, is one of the non-motor symptoms of Parkinson’s disease (PD). While the symptoms of PD vary between individuals, most people with PD experience a reduced sense of smell. Losing the sense of smell can also change the ability to taste foods, as these two senses are closely linked. The loss of the sense of smell can include impairments in odor detection, the ability to differentiate between odors, and the ability to identify odors.1,2

Researchers believe that loss of sense of smell may be one of the earliest signs of PD, however, it is often overlooked. PD is most known for its characteristic motor symptoms, and loss of sense of smell can be caused by a number of reasons, including allergies, nasal polyps, viral infections, head trauma, or smoking. While both PD and Alzheimer’s disease can cause hyposmia, most patients are unaware of their decreased sense of smell prior to being tested.1,3

The development of Parkinson’s disease

Due to the research of Dr. Heiko Braak, PD is believed to begin in the gastrointestinal system and the olfactory bulb, the part of the brain that controls the sense of smell. This is called Braak’s hypothesis. The accumulations of the protein alpha-synuclein, called Lewy bodies, are found in all patients with PD. These protein clusters are believed to begin in the olfactory bulb, affecting the sense of smell, and in the gastrointestinal system, where they may cause constipation or other digestive issues. These early, non-motor symptoms may be experienced years before the motor symptoms of PD occur.1,4

The link between loss of sense of smell and dementia

Research has identified that the loss of the sense of smell may be a predictor of future cognitive problems, such as dementia, in people with PD. In a study of 44 people with PD, 24 were found to have severe hyposmia at the beginning of the trial. Three years later, those with hyposmia had a greater rate of motor, cognitive, and visual perceptive impairment than the patients who did not have hyposmia. In addition, 42% of those with hyposmia had developed dementia at the three-year mark, while none of the patients without hyposmia had developed dementia. The presence of hyposmia put people at a nineteen-fold increased risk of developing dementia within three years. Imaging of the brains of these patients also showed that those with hyposmia had more widespread loss of brain tissue than those with PD who did not have hyposmia, and this damage to the brain tissue was seen at both the beginning of the trial and at the three-year mark. Cognition and smell are strongly associated. In addition to being able to detect an odor, identifying odors requires good memory and language skills. Researchers believe that people with severe cognitive impairment cannot perform well on tests that measure smell because of their cognitive abilities.5

Treatment

Hyposmia in people with PD does not respond to PD medications, including dopaminergic agents, dopamine agonists, or anticholinergic compounds. Currently, there is no known treatment that can improve the sense of smell in PD patients.3

Written by: Emily Downward | Last reviewed: March 2017
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