Olfactory Dysfunction Overview:

Olfactory dysfunction (sense of smell) is not an uncommon symptom in patients with multiple sclerosis (MS) affecting it is estimated some 20-40% of patients to some extent.  Most people impacted by olfactory dysfunction do not have a complete loss of ability to smell scents or fragrances but some studies suggest the condition affects more people than had been estimated in the past.

Since sense of taste is directly associated with sense of smell olfactory dysfunction may not be noticed directly in scents but instead appears in changes in tastes of foods.

The complete loss or inability to smell scents is known as anosima and partial loss is known as hyposmia.  Since taste may be affected as well the names associated are shown below for reference purposes:

  • Anosmia – Inability to detect odors
  • Hyposmia – Decreased ability to detect odors
  • Dysosmia – Distorted identification of smell
    • Parosmia – Altered perception of smell in the presence of an odor, usually unpleasant
    • Phantosmia – Perception of smell without an odor present
    • Agnosia – Inability to classify or contrast odors, although able to detect odors
  • Ageusia – Inability to taste
  • Hypogeusia – Decreased ability to taste
  • Dysgeusia – Distorted ability to taste

Olfactory dysfunction can be a serious impediment. It limits your ability to recognize danger signals, such as the smell of smoke. It can affect your ability to taste, making food less appealing and causing you to eat too much or too little. It can influence you to eat too much salt or sugar in an effort to make food taste better, and this can increase your risk of heart attack, diabetes, and stroke.

Olfactory Dysfunction Diagnosis:

Quantitative measurement of smell and taste dysfunctions is most important when chemosensory dysfunction is the primary symptom. The major goal of sensory testing is to assess the degree of chemosensory dysfunction.

A neurologist may be able to locate plaques (lesions) in the brain the are directly related to areas responsible for olfactory sensations.

Clinical testing can be time consuming and difficult to perform precisely, but some commercially available tests attempt to simplify and standardize these efforts.

Tests of olfactory function that evaluate threshold of odor detection and odor identification have been developed that can provide a reliable measure of olfactory ability. These tests include butanol threshold test, the University of Pennsylvania Smell Identification Test (UPSIT) (Sensonics, Inc. www.sensonics.com), and the Sniffin’ Sticks test (Burghart Messtechnik GmbH www.burghart-mt.de). Another test, the olfactory-evoked response, has been used in research centers along with odor identification tests to evaluate aberrant olfaction with relation to neurologic disease.

Butanol threshold test

The butanol threshold test involves a forced-choice test using an aqueous concentration of butyl alcohol in one sniff bottle and water in the other. The patient is asked to identify the bottle containing the odorant, with each nostril tested separately.

After each incorrect response, the concentration of butanol is increased by a factor of 3 until the patient either achieves 5 correct responses or fails to correctly identify the bottle with 4% butanol.

The detection threshold is recorded as the concentration at which the patient correctly identifies the butanol on 5 consecutive trials. The scoring relates the patient’s threshold to a normal subject population

Olfactory Brain
Click To Enlarge

University of Pennsylvania Smell Identification Test

The UPSIT involves 40 microencapsulated odors in a scratch-and-sniff format, with 4 response alternatives accompanying each odor. The patient takes the test alone, with instructions to guess if not able to identify the item.

Anosmic patients tend to score at or near chance (10/40 correct). The scores are compared against sex- and age-related norms, and the results are analyzed. This test has excellent test-retest reliability.

A chart is available relating scores to varying patient populations, including patients with multiple sclerosis, with Korsakoff syndrome, and those feigning anosmia. Those in the latter group tend to score much lower on the test than expected by chance.

Cross-Cultural Smell Identification Test

Olfactory Bulb Circuits
Click To Enlarge

A variant of the UPSIT, which can be given in 5 minutes, was proposed for a quick measure of olfactory function. The 12-item Cross-Cultural Smell Identification Test (CC-SIT) was developed using input on the familiarity of odors in several countries, including China, Colombia, France, Germany, Italy, Japan, Russia, and Sweden.

The odorants chosen include banana, chocolate, cinnamon, gasoline, lemon, onion, paint thinner, pineapple, rose, soap, smoke, and turpentine. Representatives from each country identified these odorants most consistently.

This test is an excellent alternative for measuring olfactory function in the clinical setting, especially when time is limited, since it is rapid and reliable.

The disadvantage of this test is that its brevity limits its sensitivity in detecting subtle changes in olfactory function.

Sniffin’ Sticks

These use a series of reusable penlike odor-dispensing devices and tests odor the threshold through a single staircase method, odor discrimination with forced choice among 3 of 16 different common odorants, and odor identification with multiple forced choice from 4 verbal items. A composite score is calculated from a composite of all 3 scores to provide an overall evaluation of olfactory function.

Olfactory-evoked response (usually reserved for research studies)

To standardize the patient reaction to eye movements, electroencephalogram (EEG) electrodes and an electrooculogram measure olfactory-evoked potentials. A visual tracking task is performed to ensure constant alertness to the task, and headphones playing white noise are worn to mask auditory clues.

Either carbon dioxide (no odor but a trigeminal stimulant) or hydrogen sulfide is delivered via an olfactometer to the nose in a constantly flowing air stream. N1 is the first negative peak measured, and P2 is the second positive trough. Latencies are measured to these 2 values.

In patients with neurologic disease, the UPSIT revealed abnormality more frequently than olfactory-evoked responses.

For clinical olfactory function testing, the authors’ experience is that the self-administered UPSIT test allows for practical use during a busy clinical practice. However, in the absence of the olfactory tests described above, a simple screening test using a common alcohol pad can be used. The envelope is opened at one end and presented to the patient. With the patient’s eyes closed, the pad is then positioned at the level of the umbilicus and slowly brought closer to the nose. The patient is instructed to notify the tester when the alcohol is again detected. The distance of the pad from the nose correlates with the patient’s olfactory ability, with a distance of less than 20 cm indicating hyposmia.

Olfactory Dysfunction Treatment:

Treatment of olfactory dysfunctions and the related categories thereof including sense of taste can be quite problematic to solve.

Depending on the condition different medications or supplements may be called for depending on how the condition is impacting the patient.

It is best to consult your medical health care team in respect to working through olfactory dysfunction as it can be quite complex and require trying a variety of methods to ease symptoms.