A loss or impairment of the sense of smell is known as hyposmia, olfactory deficit or olfactory dysfunction.
Loss of sense of smell (hyposmia) and Parkinson's
Most people with Parkinson’s have some loss of their sense of smell. Although Parkinson’s is generally thought of as a movement disorder, a number of non-motor features – including loss of the sense of smell, constipation, depression and sleep disorders - are frequently missed when doctors diagnose the condition.
Motor symptoms appear only when 50-60% of the dopamine-producing cells are lost in the substantia nigra area of the brain. Studies have shown that the pre-clinical or pre-motor phase (the time before motor symptoms appear) lasts around five years or more and it has been estimated that in some cases smell loss may occur up to 10 years before Parkinson’s is diagnosed. It is during this pre-clinical phase that early changes in the brain could potentially be detected if very early symptoms such as loss of the sense of smell were identified. It is therefore thought that screening, using reliable smell tests before typical motor symptoms are obvious, could help with the early detection of Parkinson’s or identifying those at higher risk of developing the condition.
Early detection of changes in the brain is important as this can help scientists understand the causes of Parkinson’s and develop better treatments, including neuroprotective strategies which aim to slow or prevent the development of the condition. In order to maximise the potential of such strategies they need to begin as early as possible in the course of the disease, so a reliable smell test could be invaluable in contributing to early diagnosis and prompt neuroprotective treatment.
Exactly why sense of smell is impaired in Parkinson’s is not completely understood. Many believe that it is related to the protein alpha-synuclein which clumps in the brain of everyone with Parkinson’s. These clumps have been found to occur in the area where smell is first processed in the brain (the olfactory bulb) as well as in the gastrointestinal system. In fact one study1 concluded that the olfactory bulb is where alpha-synuclein clumps first appear before migrating to other parts of the brain. If this theory is found to be correct then researchers may be able to find clumps and break them up before they reach the brain and cause major damage.
Treatment and management
There is currently no medication to help regain losses in the sense of smell in Parkinson’s, although one study2 in people who had received deep brain stimulation showed a slight improvement. Researchers think that the small improvement is probably because the stimulation affects the cognitive processing of information to do with smell.
Researchers are keen to establish whether or not a smell test is a reliable indicator for identifying individuals with pre-motor Parkinson’s, that is, who currently show no signs of Parkinson’s but will go on to develop the condition. In the same way that detecting heart disease can help to prevent heart attacks, it is hoped that knowing there is a high risk of Parkinson’s developing will allow time for preventative, neuroprotective action to be taken before it is too late.
- Staging of brain pathology related to sporadic Parkinson's disease - Braak H, Del Tredici K, Rüb U, de Vos RA, Jansen Steur EN, Braak - view abstract
- Olfactory function in patients with idiopathic Parkinson’s disease: effects of deep brain stimulation in the subthalamic nucleus. Hummel T, Jahnke U, Sommer U, Reichmann H, Mueller A.J Neural Transm 2005;112:669-676 - view abstract.