The five basic human senses are touch, sight, hearing, taste, and smell.
There are other senses in addition to the basic five including balance and spatical awareness or proprioception.
Touch- Touch is thought to be the first sense that humans develop. Touch consists of several sensations including pressure, pain, light touch, vibration and temperature.
Sight- Sight may be the most complex of all the senses. The cornea of the eye focuses light then passes the image to the lens which bends the light and focuses on the retina. The retina is full of nerve cells called rods and cones. Cones translate light into colors, central vision and details. Rods translate light into peripheral vision and motion. Rods also aide in dark environments with limited light. The impulses from the rods and cones are sent to the brain via the optic nerve.
Hearing- Sound is channeled into the human ear via the auditory canal. The wave then reaches the tympanic membrane and this thin membrane vibrates. Vibrations are received in the middle ear where the auditory bones vibrate and transfer the energy into the organ of Corti. This spiral organ is the receptor for hearing and the vibrations are sent to the brain.
Balance is also retained by the vestibulocochlear nerve which carries sound and equilibrium impulses to the brain.
Taste- Gustatory sense is broken down to the perception of four different tastes: salt, sweet, sour and bitter. Taste is sensed in the taste buds and has evolved in human evolution. There are 2000-4000 taste buds with most of them residing on the tongue. Sensory cells on the buds are in the form of capsules. The capsules have pore like areas and have hair cells in them. The hair cells bind chemicals needed for tasting. The hair cells work in conjunction and give the sensation of taste. Taste and the concentration or intensity is a combination of multiple cells and once all the information is combined a a sensation that is taste if fully appreciated.
Smell- According to the National Institute of Health, humans may be able to smell over one trillion different scents. The olfactory cleft, found on the roof of the nasal cavity, is adjacent to the olfactory bulb and olfactory fossa. This is the smelling part of the brain. Nerve endings in the olfactory cleft transmit signals to the brain. Dogs are considered great smell detectors but recent studies suggest humans are just as great. Humans have over 400 receptors dedicated to smell. While not as many as other mammals, the human brain makes up for the difference.
Aging reduces the sense of smell as does other disease states. 75% of adults over the age of 80 have olfactory dysfunction. Also decreased smell is a symptom of depression and schizophrenia.
Alteration /Loss of sense of smell
Nearly 1 in 4 or 23% over the age of 40 report some alteration in their sense of smell.
Rates increase in older populations and are highest over 80 at 32%.
- Sinus symptoms
- Heavy drinking
- Previous head injury
- Chronic dry mouth
1 in 8 Americans over age 40 have smell dysfunction (12.4% of the population or 13.3 million people).
3% of Americans have Anosmia (no smell at all).
Prevalence of any type of smell impairment
- 4% in Americans ages 40-49
- 11% in Americans ages 50-59
- 13% in Americans ages 60-69
- 25% in Americans ages 70-79
- 39% I Americans ages 80 and above
- Higher in males
- Higher in ethnic minorities
- Higher in lower educated
- Higher in lower socio-economic groups
In a recent paper published in JAMA Otolaryngology-Head and Neck Surgery in November 2021, estimates that up to 1.6 million people in the United States lost their sense of smell for at least 6 months due to COVID-19 infection.
After 6 months the chance of recovery is less than 20% and 5% of all cases will result in permanent loss.
The precise cause of the sensory loss of smell due to Covid is not known. Scientists believe it stems from damage to the olfactory epithelium. The epithelium protects the olfactory neurons which transmit the smell.
In a study of participants from UK and the US who were infected with Covid, 68% reported a loss of smell or taste as a symptom. (69,841 patients)
After comparing the genetic differences between those who lost their sense of smell and those who did not, a recent study found a region of the genome associated with this split that is located near two genes, UGT2A1 and UGT2A2. Both of these genes are expressed within tissue inside the nose and are involved in smell and play a role in metabolizing odors.
A neurobiologist at Harvard has found that cells that support sensory neurons in the nose called sustentacular cells, are probably what Covid is infecting. Covid is attacking these sustentacular cells and leaving the olfactory neurons vulnerable and deprived of nutrients.
A research team in Italy showed that smell and taste loss occur at the same time due to increased blood levels of the inflammation signaling of interleukin-6
Note- The author of this article is not stating that loss of smell due to Covid can be cured by Platelet Rich Plasma.
Treatment of loss of smell
- No treatment
- Intranasal steroids
- Platelet Rich Plasma
- The Use of Platelet Rich Plasma in the Treatment of Olfactory Dysfunction: A Pilot Study Larynoscope Investig Otolaryngol 2020 Apr; 5(2) 187-193 doi: 10.1002/lio2.357
Background- Olfactory dysfunction is a prevalent problem with a significant impact on quality of life and increased mortality. Limited therapies exist. Platelet Rich Plasma is an autologous biologic product with anti-inflammatory and neuroprotective effects. This study evaluated the role of PRP on olfactory neuro-regeneration in patients with hyposmia
Discussion- Patients had olfactory loss greater than 6 months and failed treatment with intra-nasal steroids. The patients received a single injection of PRP into the mucosa of the olfactory cleft and testing was done initially, 1 and 3 months post.
Conclusion- PRP appears safe for the use in the treatment of olfactory loss and the data suggest possible efficacy especially for those with moderate yet persistent loss.
- The Effectiveness of Platelet-Rich-Plasma in an Anosmia Induced Mice Model Laryngoscope 2018 May;128(5): E157-E162 PMID 29243256
Objective- In this study, we aimed to functionally and morphologically demonstrate the effectiveness of platelet rich plasma on anosmia in a mouse model
Conclusion- We showed that PRP administration has a curative effect on olfactory functions in an anosmia induced mice model
- Anosmia Treatment by Platelet Rich Plasma Injection Int Tinnitus J 2017 Apr 19;20(2):102-105 PMID 28452719
Objectives- Platelet Rich Plasma has been described as a promising but unproven therapy
Conclusion- Based on our results, Platelet Rich Plasma administration to the olfactory region could be a promising, last chance therapy for complete anosmia
Efficacy of Platelet Rich Plasma as an Adjuvant Therapy to Endoscopic Sinus Surgery in Anosmia Patients with Sino-nasal Polyposis
Med J Islam Repub Iran 2021;35:156 Nov 24 doi: 10.47176/mjiri.35.156
Background: Treatment of patients with anosmia ia a major therapeutic challenge. The present study evaluated the efficacy of PRP in the treatment of anosmia in patients with sino-nasal polyposis after surgery
Conclusion- According to the current study, PRP injection showed no short- term effect on the recovery of olfactory function in patients with sino-nasal polyposis. However, PRP should be considered in cases where anosmia is caused by non-sinusoidal pathologies and olfactory epithelial degeneration is presented
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Dr. Robert McGrath