Anosmia and hyposmia medical appendix

ANOSMIA AND HYPOSMIA

MEDICAL APPENDIX

DEFINITIONS

1. Anosmia refers to complete loss of sense of smell (olfaction); hyposmia refers to

reduction in sense of smell.  Either may be temporary or permanent. Other olfactory

disturbances include distortions of normal smells (parosmia or dysosmia) and a heightened

sense of smell to some or all odourants (hyperosmia).

PATHOPHYSIOLOGY

2. Taste and smell rely on chemical substances to stimulate their receptors and together form

the chemosensory system. Their combination produces the sensation of flavour and

dysfunction in one is often perceived as abnormality in the other.

3. There are two well-characterised nasal chemosensory systems: the free nerve endings of

the trigeminal nerve and the sensory receptors of the olfactory nerve. A third, vomeronasal,

organ exists but is of doubtful function in humans.

4. The free trigeminal nerve endings in the walls of the nasal passages respond non-

selectively to a wide variety of volatile chemical substances, including high concentrations

of most odourants. The olfactory receptors respond to chemical stimuli at lower

concentrations and with far greater selectivity than trigeminal endings.  In total anosmia,

the capacity to distinguish between odours is lost, but the response to nasal irritation is

usually preserved.

5. Olfactory function can be disrupted in three ways, the second and third of which are

termed neurogenic:

5.1. By nasal obstruction preventing volatile substances from reaching the receptors -

transport olfactory loss.

5.2. By impairment of receptor or cranial nerve function - sensory olfactory loss.

5.3. By pathological processes affecting pathways from the olfactory bulb (the termination

of the first cranial nerve) to the olfactory cortex and other parts of the brain.

6. Olfactory impairment is not always permanent. The receptor neurones have a lifespan

limited to about 30 days, with continual replacement.  This is an important factor in

recovery from anosmia of certain causes, which may take months. Recovery of olfaction

occurs in 2/3 of cases with cranial nerve palsy, sometimes as long as five years later.

CLINICAL MANIFESTATIONS

7. Qualitative changes in smell may be complained of in association with quantitative

impairment and both may be accompanied by a perceived disturbance of taste.

8. Subjective tests for the sense of smell tend to be time-consuming and often imprecise.

They rely on measuring minimal perceptible odour, identification, or adaptation. Even

objective tests, using physiological measurements or evoked responses, may or may not

be positive. In clinical practice, simple tests of identification of and discrimination between

familiar substances. Appreciation of an odour, despite the inability to name it, excludes

anosmia.

9. Unilateral non-obstructive loss of smell is not usually noticed by the patient but, if detected,

may indicate a focal neurological lesion.

10. Cases of malingering can sometimes be exposed by comparing responses to odourants

which differ in their propensity to stimulate trigeminal nerve endings. These are invariably

stimulated by, for example, ammonia.

AETIOLOGY

11. Causes of anosmia and hyposmia can be classified thus:

11.1. Lesions of the Nose

11.1.1. Deviated nasal septum is a rare cause of disturbance of smell.  Simple

anatomical defects do not usually result in an abnormality of smell.

11.1.2. Nasal polyps

11.1.3. Allergic and vasomotor rhinitis are common causes of abnormalities of

smell, but only rarely is the associated loss of smell total.  In conditions such

as hay fever and the common cold, the loss of smell is temporary.

11.1.4. Infective rhinitis may damage considerable areas of olfactory mucosa if it

becomes chronic and the affected areas do not regenerate.

11.1.5. Tumours, including papilloma, adenoma, squamous cell carcinoma,

esthesioneuroepithelioma and idiopathic midline granuloma.

11.1.6. Toxic fumes may cause loss of smell, as may heavy smoking.

11.2. Lesions of the olfactory nerves.

11.2.1. Injury, either through a direct blow or from an occipital blow with shearing of

nerve fibres, usually causes immediate and complete loss of smell.

11.2.2. Viral infections such as influenza can cause great damage to the olfactory

nerve fibres, replacing all the neuronal tissue with fibrous tissue.

11.2.3. Meningitis, sarcoid and neurosyphilis may damage the olfactory tract.

11.2.4. Anoxia.

11.2.5. Degenerative conditions include Parkinson's disease, Alzheimer's

disease, motor neurone disease and multiple sclerosis (olfactory

dysfunction occurs in about 40% of patients with MS)

11.3. Intracranial Lesions.

11.3.1. Trauma tends to give complete loss.

11.3.2. Intracranial tumours can affect the sense of smell in two ways, either by

pressure on the olfactory nerve fibres or bulb, or by interference with the

intracerebral pathways.  Osteomas or meningiomas of the anterior fossa

tend to diminish the sense of smell and at first this is unilateral.  Frontal lobe

tumours may do the same.

11.3.3. Obstructive hydrocephalus.

11.3.4. Epilepsy.

11.4. Systemic diseases in which the sense of smell may be impaired include diabetes

mellitus, Paget's disease of bone, polyarteritis, cystic fibrosis and cirrhosis.

11.5. Iatrogenic causes include rhinoplasty, intracranial surgery, radiation therapy and

certain drugs

11.6. Psychogenic Disorders.

Psychiatric disorders such as psychoses (including Korsakoff's), depression and

confusional states can be accompanied by hallucination of smell (phantosmia),

and there is a specific olfactory reference syndrome.  Hysteria and malingering

have a place amongst these disorders. Olfactory hallucinations may also occur in

Alzheimer's disease and alcohol withdrawal.

11.7. Anosmia is occasionally congenital, sometimes hereditary.

CONCLUSION

12. Anosmia and hyposmia are disturbances of the sense of smell which may be temporary

or permanent. There are many possible causes, most of which are listed above.  Many

cases of anosmia and hyposmia resolve naturally, so no treatment is necessary.  In others,

treatment is that of the underlying cause, but the impairment is permanent in some cases.

REFERENCES

Finelli P.F. & Mair R.G.  Disturbances of Taste and smell, in (eds) Bradley et al, Neurology in

Clinical Practice, 3

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Moore-Gillen V L.  Abnormalities of smell. In: Eds. Mackay I S, Bull T R. Scott-Brown's

Otolaryngology. Volume 4. Rhinology. 6

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 Ed. 1997. Oxford. Butterworth-Heinemann. p. 4/5/1-

4/5/9.

Kennard C. in (ed) Donaghy M., Brain's Diseases of the Nervous System. 11

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Oxford University Press. p.312-3.

Leopold D.  Disorders of olfactory perception: diagnosis and treatment.  In Chem. Senses 2002

Sep;27(7):611-5.

October 2002