Definition: Anosmia is complete loss of sense of small, while hyposmia is diminished sense of smell.
Causes of Anosmia and Hyposmia
Nasal:
I. Obstruction: the small particles do not reach the olfactory mucosa e.g. Deviated septum, nasal polyps and nasal synechiae.
II. Misdirection of air current (disruption of nasal aerodynamics or turbulence):
- Destruction of alae nasi e.g. granulomas and tumors.
- Aggressive removal of turbinates.
Mucous membrane changes:
- In allergic rhinitis there is excess secretions which wash out the small particles.
- In atrophic rhinitis, the mucus is too thick to dissolve the smell particles.
Interference with nerve endings:
- Degeneration of nerve endings as in atrophic rhinitis.
- Post infleunza neuritis of olfactory nerve endings.
- Diabetic neuritis
- Lead poisoning.
Intracranial:
- Congenital absence of olfactory bulbs and nerves.(rare)
- Fracture base of skull involving the anterior cranial fossa.
- Basal meningitis.
- Tumors of anterior cranial fossa e.g.: meningioma and neuroblastoma leading to compression of olfactory pathway.
Psychogenic:
Malingering: The patient will deny smell of any odor even irritant odors which stimulate trigeminal nerve (ammonia).
Tests of smell:
- Subjective: Measuring the minimum perceptible odor, Identification test.
- Objective: Olfactory evoked response.
Cacosmia:
Definition: Bad odor perceived by patient and sometimes his contacts.
Causes:
- Neglected foreign body in the nose.
- Atrophic rhinitis.
- Maxillary sinusitis especially of dental origin.
Parosmia (Hallucination of smell):
Abnormal sense of smell which is perceived by patient alone, this disorder is found in many psychiatric disorders as schizophrenia and depression.
Hyperosmia:
Increased sense of small, it is met in some psychiatric diseases or irritative lesions of olfactory bulb. It may also lead to migraine (migraine related hyposmia).