Director of the Multiple Sclerosis Program
Professor of Clinical Neurosciences
University of California San Diego
We spend so much time discussing alterations in sight and tactile sensations (e.g. pins and needles, burning sensations, electrical sensations, itching) in MS that we often forget that the other senses may be affected as well. I thought I would briefly discuss these for you today.
Acute hearing loss during the course of MS is relatively rare, although I suspect it occurs far more frequently than the reported 10 % incidence would suggest. Hearing loss at the onset of MS is even less frequent (1%) and often suggests another disorder masquerading as MS. Hearing loss may occur in isolation but is more readily associated with the multiple sclerosis when accompanied by other brainstem symptoms and signs such as vertigo, weakness or sensory disturbance on one side of the body, facial weakness or sensory disturbance sometimes opposite the affected side of the body, or incoordination of a limb. Unilateral hearing loss is more suggestive of an MS related cause, but MS may also rarely cause bilateral hearing loss. Many patients report symptoms such as tinnitus or fullness in the ear that may fall short of “hearing loss”, but still negatively affects auditory function and quality.
Hearing loss is almost always caused by inflammatory demyelination in the part of the brain called the brainstem where the auditory nerve (also called the eighth cranial nerve) enters the central nervous system, or in the auditory pathways within the brainstem. It is said that high frequency hearing loss is more common with involvement of auditory nerve entry zone and tract, whereas low frequency hearing loss is more common with involvement of the auditory brainstem pathways. When hearing loss occurs acutely in MS, it often recovers along with other symptoms of a relapse after treatment with high dose steroids. However, like other symptoms there are some people who recover very little and are left nearly deaf. Tests can be done to determine if the hearing loss is caused by a problem in the cochlea (the inner ear and therefore not caused by MS) or the brainstem (more likely related to MS) but these tests are rarely necessary in a person known to have MS who experiences hearing loss.
Altered smell and taste are reported more frequently in people MS (up to 2/3’s of people with MS may experience diminished smell perception and discrimination). Altered smell may even be a presenting symptom of MS, presenting either as diminished ability to smell (hypoguesia) or unpleasant abnormal smells (odors misinterpreted as smelling like gasoline, for instance). Rarely, patients will present with both altered smell and taste suggesting another problem such as a viral infection or a consequence of smoking. Altered taste can take many forms including inability to taste only sweet flavors, only sour or bitter flavors or both. The abnormal taste may affect only one or both sides and when it affects both sides the type of taste affected can differ on the two sides (i.e. sweet taste affected on one side and sour bitter tastes affected on the other side). In most situations people do not notice loss of taste when it affects only one side. Loss of or distorted smell is noticed with greater ease than loss of taste and often affects a person’s perception of smell as well.
The proportion of people with MS who experience a decrease in smell perception and discrimination increases with the duration of disease and is related to amount of demyelination (white spots on MRI) in the inferior frontal lobes and temporal lobes of the brain. Not infrequently the diminished sense of smell is associated with problems controlling emotions or behavior, staying on task, planning activities or thoughts and seeing them through to the end.
Both altered taste and smell have a large differential diagnosis. Common reversible causes of diminished smell include the common cold and other respiratory viruses, allergic rhinitis, chronic sinusitis, malnutrition and smoking. Other causes readily excluded by history, examination or imaging include head trauma, frontal lobe or suprasellar tumors, normal aging, Alzheimer’s disease and Parkinson’s disease. Diminished taste may also occur with common viral infections and with smoking but is particularly common with certain medications; for instance, most MS patients have noticed the bitter taste that occurs during the administration of high dose steroids to treat a relapse. Other correctable causes include vitamin B12 deficiency, zinc deficiency, and dry mouth either from medications or a disorder of salivary glands such as Sjogren’s syndrome.
So the next time someone tries to tell you that MS does not affect hearing, smell or taste pull out this blog and educate them. But never assume a problem affecting these senses is related to your MS without a thorough evaluation by your MS Specialist.