Walking into the kitchen, I stop to ask, “Who got bananas?” My husband replies, “We don’t have any bananas.” But I distinctly smell bananas and now I want one. “I did have some strawberry and banana yogurt earlier today," he says. "I washed out the container and it’s next to the sink." Aha. That’s what I smell.
Sometimes hyperosmia (an increased sense of smell) drives me batty. The smell might be something quite pleasant, like a banana, or something offensive, like my cat going to the bathroom at the other end of the house. Hyperosmia is one of those weird things I’ve learned to live with. It doesn’t affect me all of the time, but occasionally I seem to have the nose of a bloodhound.
An altered sense of smell can be related to multiple sclerosis (MS), neurologic disorders, or other causes. It’s not something that my neurologist routinely questions me about, but it is something you should mention to your doctor if you experience it.
The incidence of impaired sense of smell in people with MS is variable, with estimates ranging from 15 percent to 38.5 percent in different trials. A recent systematic review of the literature found prevalence reports ranging from 20 percent to 45 percent of the MS population. Discrepancies among research results may be due to different testing methods, small numbers of people in the trials, and differences in trial designs.
How can the sense of smell be altered?
There are several ways that one’s sense of smell might be affected. Some people might have a reduced sense of smell (hyposmia) while others experience an increased sense of smell (hyperosmia). Olfactory hallucinations (phantom smells) are fairly common and may be associated with nasal infections, seizures, neurological diseases, or mental health disorders.
Hyposmia is multi-faceted and may include reduced ability to identify odors or discriminate between smells and a higher threshold to perceive an odor. Odor detection threshold is the lowest concentration of a compound that is perceivable by a person’s sense of smell. As we get older, a diminished sensitivity to odors can result in a higher odor threshold.
Olfactory dysfunction and multiple sclerosis
The results from a number of studies of olfactory dysfunction in people with MS can seem contradictory. While one study may find no difference between people with MS and healthy people in their ability to detect smells, another might note a significant difference between groups. But with so much variation in MS, it is understandable that people with different disease characteristics or courses may have differing results.
Olfactory dysfunction was once thought to not affect people with MS, but studies have determined that changes in the sense of smell is associated with demyelination and axonal degeneration of olfactory brain regions, primarily the inferior frontal and temporal lobes. People with more advanced MS are more likely to experience altered smell. A study of 50 people with relapsing-remitting MS (RRMS) or secondary progressive MS (SPMS) reported that 40 percent of patients experienced hyposmia, specifically regarding a reduced ability to detect odors (i.e., higher odor threshold). Researchers also noted that 16 percent of patients had reduced abilities in three areas of smelling: odor discrimination, odor identification, and odor threshold.
A study in Turkey published in August 2016 found no differences between people with MS and healthy individuals in measurements of odor discrimination, odor identification, or a composite score of odor threshold, discrimination, and identification. However, the scores for odor threshold alone were higher in the control group than in the MS group, meaning that the people with MS had slightly more difficulty detecting odors in lower concentrations.
In a 2011 study that used a brief five-minute smell test, more people with MS (11.1 percent) experienced a diminished ability to smell compared to healthy controls (3 percent). However, significantly more people with SPMS (11/16, 68.8 percent) experienced an impaired ability to smell compared with those who had RRMS (4/121, 3.3 percent) or primary progressive MS (2/16, 12.5 percent). Researchers concluded that deficiencies in the ability to smell can be used to distinguish between people with RRMS or SPMS, and may be an early indication of disease progression.
In addition to a reduced ability to smell, some people with MS may experience a complete loss of smell (anosmia) or heightened sense of smell (hyperosmia). Not much is written specifically about hyperosmia and MS, however my MS nurse explained to me that it may be associated with anxiety, which can be a symptom of MS. Just like my ears can become overly sensitive to sounds (hyperacusis), my nose seems to become sensitive to smells. Perhaps I need to step back and take notice when I’m experiencing the nose of a bloodhound to determine whether I’m also experiencing extra stress or feeling anxious. There may be an explanation more complicated than simply blaming it on the MS.
Batur Caglayan HZ, Irkec C, et al. Olfactory functioning in early multiple sclerosis: Sniffin' Sticks Test study. Neuropsychiatr Dis Treat. 2016 Aug 26;12:2143-7. doi: 10.2147/NDT.S116195. eCollection 2016.
Good KP, Tourbier IA, Moberg P, et al. Unilateral olfactory sensitivity in multiple sclerosis. [Published online ahead of print October 22, 2016]. Physiol Behav. doi:10.1016/j.physbeh.2016.10.017.
Jordy SS, Starzewski A Junior, et al. Olfactory alterations in patients with multiple sclerosis. Arq Neuropsiquiatr. 2016 Sep;74(9):697-700. doi: 10.1590/0004-282X20160128.
Lucassen EB, Turel A, Knehans A, et al. Olfactory dysfunction in Multiple Sclerosis: A scoping review of the literature. Mult Scler Relat Disord. 2016 Mar;6:1-9. Doi: 10.1016/j.msard.2015.12.002. Epub 2015 Dec 2.
Pimentel ML. Olfactory dysfunction as a marker of multiple sclerosis progression. Arq Neuropsiquiatr. 2016 Sep;74(9):693-694. doi: 10.1590/0004-282X20160133.
Silva AM, Santos E, et al. Olfactory dysfunction in multiple sclerosis: association with secondary progression. Mult Scler. 2012 May;18(5):616-21. doi: 10.1177/1352458511427156. Epub 2011 Oct 21.