Anosmia (Loss of Sense of Smell) Causes and Treatment

What is Anosmia?

Anosmia Definition

Anosmia is the medical term for a complete loss of the sense of smell (olfaction). It can be temporary or permanent and while taste may not be affected, it does impact on the perception of flavors.

If the sense of smell is reduced (partial, not complete loss) , then this is known as hyposmia.

Other olfactory phenomenon include :

  • Parosmia which is the sensation of smell despite the absence of appropriate stimuli.
  • Dysosmia which is when the sense of smell is distorted.
  • Cacosmia which is the sensation of a bad smell or foul odor.

Olfactory Anatomy and Physiology

The olfactory epithelium is z small layer on the roof of he nasal activity that is responsible for the sense of smell. The olfactory receptor cells which make up the olfactory epithelium are neurons (nerve cells) that  transmits impulses to the brain where it is perceived as an odor.

There are two other types of olfactory cells apart from the receptor cells. Firstly, the supporting cells as the name suggests supports the neighboring receptor cells and helps ‘trap’ chemicals with their microvilli. Secondly, the basal cells which are immature or stem cells that can differentiate into olfactory receptor cells or supporting cells. It is important to note that the olfactory neuroepithelium is one of the only neural tissue in the body that can regenerate.

Another important component to take note of is the Bowman’s gland which secretes fluids that bathe the olfactory epithelium. This allows airborne chemicals to dissolve in the serous fluid and stimulate the receptor cells.

Olfactory receptor fibers bundle to join at the olfactory bulb which in turn leads to the olfactory nerve (cranial nerve CNI). This enters the brain at the olfactory center (part of the pyriform cortex) with nerve fibers to the thalamus, amygdala and enterorhinal area. Simply this means that apart from the perception of smell, the other areas may play a part in memories associated with odors which could trigger certain emotions and behavior.

Causes Anosmia

The causes can be divided according to disturbances in olfactory physiology :

  1. Any obstruction of air intake through the nose or in making contact with the olfactory epithelium.
  2. Damage of the olfactory epithelium.
  3. Damage of the pathways that transmits the sensory impulses from the olfactory receptor cells to the brain.

Nasal Obstruction

  • Any cause of nasal congestion will restrict air flow through the nose.
  • Nasal polyps.
  • Nasal bone deformities, deviated septum.
  • Foreign body in the nasal passages which may occur in children.
  • Tumors.

Patients with a tracheostomy will also experience anosmia since air intake is not through the nasal passages.

Damage of the Olfactory Epithelium

  • Chronic rhinitis – allergic (like hay fever) or non-allergic.
  • Sinusitis – acute or chronic
  • Viral infections of the upper respiratory tract like the common cold, seasonal flu (influenza).
  • Tumors may also cause damage of the olfactory epithelium.
  • Smoking – tobacco, marijuana, narcotic drugs.
  • Environmental or occupational hazard from constant smoke and toxic gas inhalation.
  • Chemical toxins including chlorine, sulfuric acid, formaldehyde, cadmium, manganese, lead and other hazardous substances. Excessive or prolonged use of nasal decongestants can also damage the epithelium.
  • Drug side effects including psychotropic drugs, hormones and corticosteroids. Radiation therapy may also result in damage of the olfactory epithelium.
  • Age related changes.

Damage of the Nerve Pathway(s)

While the olfactory epithelium, including the receptor cells can regenerate to some extent from differentiation of the basal cells, other neural tissue cannot if there is cell death. However not all cases of nerve damage resulting in anosmia is permanent as sometimes there is temporary irritation along the nerve pathway which will ease upon removing the causative factor.

Some of the causes which affects the transmission of sensory impulses to the brain and/or processing of these impulses includes :

  • Age related changes.
  • Chemical toxins like lead and certain insecticides.
  • Head injury.
  • Infection of the central nervous system like meningitis, encephalitis.
  • Brain or nerve tumors.
  • Intracranial surgery.
  • Any disease resulting in nerve or nerve sheath degeneration, nerve atrophy, inflammation or death or neurotransmitter dysfunction can result in anosmia like Parkinson’s disease, diabetic neuropathy, Alzheimer’s, stroke (CVA) and other related conditions.

If the anosmia is of a sudden onset and/or presents along with other neurological symptoms, immediate medical attention is required, especially if there is a history of head trauma.

Treatment of Anosmia

Treatment should be directed at the cause once it is identified. This may require further investigative tests like a CT scan or MRI of the head. The most common causes of anosmia include sinusitis, rhinitis, upper respiratory tract infections, drug side effects, smoking and age related changes, particularly in Alzheimer’s patients.

  • Rhinitis may require antihistamines and corticosteroids and nasal decongestants should be limited due to possible side effects.
  • Surgery may be necessary to correct bone deformities or remove obstructions. This includes a sinus drainage.
  • Chronic drugs should be changed to verify whether the sense of smell returns.
  • Smoker’s should be made aware that the condition will persist as long as they continue smoking.

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