Parkinson’s disease is a common neurodegenerative disorder characterized by the loss of dopaminergic neurones in the nigro-striatal region of the brain. Loss of taste or smell can occur over a period of time in Parkinson’s disease but it may be so gradual as to remain undetected in many patients. Some studies have shown that impaired sensation of smell may occur in a PD patient even long before the development of motor symptoms. Disorders of taste (gustatory) and smell (olfactory) may occur normally with advancing years but suffering from a neurodegenerative disorder such as Parkinson’s disease seems to increase the chances of such disorders.
However, it must be kept in mind that not all people with the loss of smell or taste develop Parkinson’s and also not every patient with Parkinson’s presents with the loss of smell and taste. Researchers have concluded that these non motor symptoms associated with Parkinson’s disease which include constipation, sleep disorders, loss of smell and taste usually proceed the motor symptoms of the disease by several years and the increased focus on these symptoms can be used in the early diagnosis and treatment of this fatal disorder.
The disorders of smell and taste may be classified as :
- Anosmia – complete loss of sensation of smell or the inability to detect any odor.
- Hyposmia – decreased ability to detect odor.
- Dysosmia – distorted identification of smell.
- Ageusia – complete loss of sensation of taste.
- Hypogeusia – decreased ability to taste.
- Dysgeusia – distorted ability to taste.
Olfactory Loss in Parkinson’s Disease
Impairment of olfaction, which present as a decrease in the sensation, followed by the complete loss of sense of smell is the characteristic and early feature of Parkinson’s disease and more than 95% of the patients present with this sensory deficit. All the three olfactory qualities which include the threshold to the sense of smell, discrimination between two types of smells, and identification of a specific smell is either decreased or lost in the Parkinson’s patients. Therefore it is best to perform all the three sub tests to obtain a maximum of the reliable information for the diagnosis of Parkinson’s associated olfactory loss.
The patients are psychophysically assessed by the presentation of different odors and recording of the subjects response. This allows the rapid screening of olfactory function however more extensive tests offer reliable discrimination between hyposomic, anosmic, and normosmic patients.
Gustatory Loss in Parkinson’s Disease
The filter paper discs soaked in sucrose, quinine, citric acid and sodium chloride solution are used to evaluate various taste sensations like sweet, bitter, sour and salty sensation respectively in Parkinson’s disease. Other methods employed to assess the taste detection threshold includes electrogustometry.
Other treatable causes of the loss of smell and taste include mouth fungus, tumors in the nose, certain medications, depression, head injuries, other neurological conditions such as Alzheimers disease, Myasthenia gravis, endocrine gland diseases including diabetes, old age, and exposure to toxic chemicals
Causes of Taste and Smell Disorders in PD
- Age-related olfactory problems caused by epithelial changes in the olfactory system, such as decreased mucus secretion, hormonal changes, and changes in the epithelial thickness. With age, there is a reduction in the number of taste buds in the tongue, roof of the mouth, and throat, but more than this, the changes in taste cell membranes involving altered function of the receptors may be responsible for loss of taste.
- Although little is known about the causes of Parkinson’s disease, the loss of smell has been linked to the protein alpha-synuclein, which has been found to be clumped in such individuals. It has been hypothesised that the disease in such patients originate not in the substantia nigra region, but rather in the olfactory bulb (which is the part of brain to control the sensation of brain) and gastrointestinal tract. The alpha synuclein protein clumps initially formed in these regions are then migrated to other parts of the brain, where they lead to the dopaminergic loss.
This mechanism also possibly explains the appearance of these pre-motor symptoms in the form of loss of olfactory and gustatory sensation, long before the actual appearance of motor symptoms or long before the proper manifestation of Parkinson’s disease.
- The changes brought about by PD itself and the side effects of PD drugs could possibly have some relation to the smell and taste disorders seen in these patients.
- Problems with taste may also arise due to poor oral hygiene and dental problems seen in many PD patients.
- An alteration in taste or smell may be the primary feature or a secondary symptom of some other disease occurring concomitantly with PD, such as Alzheimer’s disease, nose or sinus problems, use of certain antibiotics or antihypertensives.
It is uncommon to suffer from complete loss of taste. Dysgeusia is the more common symptom likely to occur, especially with increasing age. Most taste disorders are actually, if not very closely related to, smell disorders.
Loss of the sensation to smell and taste and its relation to Parkinson’s treatment
The sensory loss of olfaction and gustation predilects an impending Parkinson’s disease. The early appearance these pre-motor symptoms long before the appearance of actual motor symptoms has been exploited by the scientists as a crucial warning sign. The appearance of this sensory loss of deficit mandates the need to investigate for the presence of clumped alpha synuclein protein in the olfactory area of brain or in the gastrointestinal tract of the patient.
However, it is often an overlooked symptom and is considered to be an early sign of PD demanding early initiation of treatment. If the alpha-synuclein clumps are detected and broken, before they reach the dopaminergic region of the brain, it might become possible for the scientists all over the world, to treat Parkinson’s even before the occurrence of major neurological damage.
Loss or impaired sensation of taste and smell may lead to :
- Dangers such as the inability to detect smoke in case of a fire, gas leaks or other dangerous fumes, which can be potentially life-threatening.
- Inability to identify spoiled food thereby leading to food poisoning.
- Increased salt or sugar intake in an attempt to make food taste better (may have disastrous consequences on a hypertensive or diabetic patient).
- Loss of appetite or less enjoyment of food, leading to nutritional deficiencies.
Loss of taste and smell can have a significant impact on the quality of life, not least due to impaired enjoyment of food.
An otolaryngologist should be seen in order to diagnose and detect the extent of taste and smell loss in a person. Although age-related loss of taste or smell cannot be stopped or reversed, some causes may be treated, such as taking care of dental problems, or changing or altering doses of medication which could be contributing to the problem.