Anosmia in geriatric age group is really common. Statistically speaking about 15% of geriatric population are anosmic. Anosmia in geriatric age group has disastrous consequences, including safety hazard. It affects the food preferences and nutritional status of the already compromised population. The absolute odor sensitivity as well as sensitivity to suprathrehold odors decreases in aging population. Loss of smell sensation could be a component of degenerative disorders like Alzeimer's disease and Parkinson's disease which commonly affect elderly age group.
Anosmia in elderly could very well be caused by pathologies involving either the transport of odoriferous molecules to the olfactory cleft or central processing mechanisms.
Applied anatomy & Physiology of smell:
Odors usually reach the olfactory epithelium present in the olfactory cleft orthonasally (anteriorly) via the nose and retronasally via the oropharynx. This retronasal pathway is essential for appreciating flavor of food consumed. In animals this pathway is essential to maintain the vital sense of smell even while feeding since it is very important for their survival. Odorants are appreciated when these molecules bind to the olfactory epithelium found in the roof of the nasal cavity. The most important feature of these olfactory receptor cells is the presence of non motile cilia which are the primary receptors of olfaction. These cilia are endowed with a large number of olfactory receptor proteins to which the odoriferous molecules bind. When these odoriferous molecules bind to the olfactory epithelium the G protein and cyclic AMP pathway is activated causing depolarisation of the olfactory receptor. The signal from the depolarized receptors are carried by the olfactory fibers to the primary olfactory cortex present in the uncus. The function of these receptors are dependent on the composition of mucous blanket which covers them. This mucous blanket is secreted by Bowman's glands and sustentecular cells. The quantity and quality of this mucosal blanket drastically changes in older age group blunting their ability to smell. Since the cilia over olfactory epithelium are nonmotile, the mucosal blanket clearance depends on the normal ciliary motility of the nasal epithelium.
The quality of the mucous blanket over the olfactory epithelium changes
Pattern of airflow in to the nasal cavity plays an important role in olfaction. Studies have shown that major air flow occurs through the floor of the nose, next comes through the middle meatus. Only 10% of the inspired air traverses through the roof of the nasal cavity (olfactory area). It is this superiorly directed air flow that determines the acuity of olfaction. In elderly individuals due to atherosclerotic changes of submucosal vessels the turbinates dont congest and decongest automatically, causing the air flow through the nasal cavity to be laminar in nature. Laminar air flow always occurs through the floor of the nasal cavity depriving the olfactory area's exposure to the inspired air. Hence elderly individuals resort to sniffing in order to perceive smell. Alar muscles of the nose must be acting normally in these persons for eddy currents to develop in the inspired air. Eddy currents ensure atleast a portion of the inspired air travels through the roof of the nasal cavity. In elderly individuals the alar muscles are weak and periodical sniffing acts leaves them really tired and drowned hence they avoid making this consious effort to perceive smell.
The olfactory epithelial layer is endowed with the presence of progenitor cells. These cells on maturation can progressively replace degenerated olfactory receptor cells. The number of these progenitor cells undergoes progressive reduction as the patient ages. Hence the regenerative ability of olfactory epithelium is highly restricted in geriatric age group compounding the problem of anosmia.
Olfactory receptor cells: These are bipolar neurons which are not only constantly exposed to odoriferous molecules but also to the insults heaped upon by viral infections, inflammations, and inhaled toxins. They undergo regular death by a process known as apoptosis. Their population is continuously replaced by the maturing progenitor cell pool. In nasal and sinus disorders the level of enzyme capsase 3 increases to alarming levels. This enzyme stimulates apoptosis (programmed cell death) of the olfactory receptors.
Importance of mucosal blanket over olfactory cleft in maintaining the function of smell:
Olfactory epithelial changes that occur due to aging:
As the individual grows older the olfactory epithelium gets progressively replaced by respiratory epithelium causing a diminition in the sensation of smell. Studies have shown that significant amount of olfactory epithelium gets replaced by respiratory epithelium by the time a person reaches the age of 70.
Damage to olfactory bulb and neuronal olfactory pathways due to degenerative neurogenic disorders are common in old age group. This is one important cause for blunting of sensation of olfaction in a patient above the age of 80. Alzeimer's disease is the most common degenerative disorder affecting this group of patients. It not only causes dementia in this age group, but also blunts their olfaction. Diminition of olfactory sensation is seen in early stages of Alzeimer's disease.
Appreciation of flavor of food:
This is dependent on retronasal olfaction. This is commonly affected in elderly individuals who use palate covering dentures. This is one of the common cause of loss of flavor of food stuffs seen in geriatric patients. In addition healthy oral cavity is a must for perception of flavors. Dry oral cavity commonly seen in elderly also cause loss of perception of flavor of food.