Olfactory Nerve

Anatomy And Physiology

• The olfactory nerve is the first cranial nerve.
• It is a purely sensory nerve.
• It is responsible for the perception and identification of smell.
• The olfactory epithelium contains bipolar sensory cells located in the superior part of the nasal septum, nasal attic, and superior part of the lateral wall of the nasal cavity.
• The central processes penetrate the cribriform plate of the ethmoid bone to synapse in the olfactory bulb.
• The lateral striae terminate in the primary olfactory cortex in the temporal lobe.

Assessment From History

• Elicit a history of recent upper respiratory tract infection or common cold.
• Ask for a history of nasal trauma or head injury.
• Elicit a history of allergic rhinitis, sinusitis, or nasal polyps.
• Ask about an altered sense of smell or complete loss of smell.
• Inquire about hallucinations of smell.
• Hallucinations of smell occur in temporal lobe epilepsy.

Clinical Examination

Prerequisites

• Ensure that the nasal passages are patent bilaterally.
• Ask the child to close one nostril and blow air through the other onto a glass slide.
• Look for misting on the glass slide to confirm patency.
• Rule out nasal obstruction due to a common cold before testing.

Testing Procedure

• The test is purely subjective and requires child cooperation.
• Ask the child to close their eyes during the test.
• Compress and occlude one nostril to test each side separately.
• Keep the test substance near the open nostril without touching the nasal mucosa.
• Ask the child to take two good sniffs.
• Ask the child if they can perceive any smell first.
• If perceived, ask the child to identify the smell.
• Allow some time for the odor to disperse before repeating the test with another odor.
• Use common, familiar non-irritating substances.
• Appropriate substances include coffee powder, soap, chocolate, peppermint, or clove oil.
• Do not use irritants like ammonia or substances with a pungent smell.
• Irritants will falsely stimulate the trigeminal nerve endings rather than the olfactory nerve.

Interpretation Of Findings

• Perception of smell indicates an intact olfactory pathway.
• Correct identification of smell indicates an intact cortical center for smell.

Anosmia

• It refers to the complete loss of the sense of smell.
• It can be caused by head trauma involving a cribriform plate fracture.
• It can be due to tumors like a subfrontal meningioma, frontal lobe glioma, or pituitary tumor.
• It can occur as a sequela of bacterial meningitis.
• It is a feature of Kallman syndrome, which presents with anosmia and hypogonadism.

Parosmia

• It refers to a perverted, distorted, or unpleasant sense of smell.
• It can occur due to local causes in the nose or head injury.

Hyposmia And Hyperosmia

• These refer to a decreased or increased ability to smell, respectively.

Age-Specific Key Points

Newborns And Infants

• Olfaction is functional even during prenatal life.
• Newborns have highly efficient olfactory abilities.
• They can discriminate the odor of their mother's breast milk from others.
• The baby will turn their face toward the smell of breast milk.
• The baby will show discomfort or turn away if an offending smell is presented.
• Formal olfactory differentiation may not be possible in infants.

Older Children

• Objective testing of olfactory discrimination is reliably testable from five years of age onwards using standard tools.

Example For EXAM

Olfactory Nerve Assessment:

• Pre-requisites: Nasal passages are patent bilaterally. No history of recent upper respiratory tract infection.
• Procedure: Tested each nostril separately with eyes closed using familiar substances (coffee and soap).
• Findings: The child is able to perceive and correctly identify the smell in both nostrils independently.
• Interpretation: First cranial nerve (Olfactory nerve) is intact and functioning normally.