Disorder Of 1st Cranial Nerve

History

Fact

Explanation

Introduction

Olfactory nerve is the first cranial nerve responsible for the sensation of smell. [1] The olfactory epithelium has a 2.5 cm2 of area at the apex of each nostril. The receptors are surrounded by nasal mucous membrane and there are two kinds of receptors: one related to the trigeminal nerve fibers that are sensitive to irritating substances and temperature and the other type, olfactory nerve cells, which form the receptor for olfaction. Olfactory nerve cell is bipolar first-order neuron. [2] The olfactory neuron has a life span of only 30 to 40 days. New neurons are originated from the olfactory epithelium, basal pole of the neuron gives rise to a single unmyelinated axon. Axons are arranged into the bundles, sheathed in Schwann cells, and pass through the cribiform plate and then synapse in the olfactory bulb. [3] Smelling disorders can be either sinonasal or non sinonasal. Sinonasal disorders can be inflammatory or non inflammatory and non sinonasal disorders can be congenital, postviral, posttraumatic and toxic. [6]

Complete loss of the sense (anosmia)

Regarding perception of the smell, receptor neurons carry information to central processing centers in the medial temporal cortex and amygdale, which relay to association centeres like frontal and limbic structures. [1] Odors have to penetrate the mucus overlying the sensory epithelium and go to the receptors by partition and diffusion coefficients in the olfactory mucus. [2] An odorant traverses the mucus in the range of a few dozen milliseconds, and forms a complex with the receptor in about the same time span. Olfactory epithelium of the nose is stimulated by volatile substances. [6] Complete loss of the sense (anosmia) occurs when there is damage to the receptors or neurons early in the afferent pathway. [1]

Perceptual deficits (e.g. inability to identify or match odors)

These deficits are due to the problems in further afferents or relays. Activation of simple circuits may be responsible for the reflexive motor responses, such as sniffing. [1]

Hyperosmia, dysosmia

Hyperosmia is increased olfactory acuity, and dysosmia is an abnormal sense of smell. [2]

Nasal congestion, rhinorrhoea

Rhinitis and sinusitis may be a barrier for stimulus to reach the olfactory nerve endings and can disable the receptors. [4] It causes transient ansomnia, but may be persistent in certain viral upper respiratory infections. [1]

Abnormalities in taste

Components that comprise the sensation of taste include the food's smell, taste, texture, and temperature. Loss of smell may be a factor for decreased taste sensation. [2]

Some disorders are shown to have ansomia as an associated features. Eg:-
Schizophrenia where the olfactory identification deficits present with preserved acuity,patients with post traumatic stress disorder and patients with alzheimer’s disease have increased olfactory identification deficits etc. [1] Pathological changes in the olfactory system is one of the earliest changes in Alzheimer's disease. Olfactory hallucinations (unpleasant odors such as burned rubber) is seen in epilepsy, alcohol withdrawal states, and some psychiatric conditions. [2]

Foster-Kennedy syndrome is associated with a sphenoidal ridge tumor. Clinical manifestation is in the form of three features: unilateral anosmia with ipsilateral optic nerve atrophy and contralateral papilledema. [1]

History of head injury/recent surgical procedures involving the nose

Head injury is a frequent cause of ansomia. [4] Subarachnoid hemorrhage can cause anosmia. Head injury can cause olfactory dysfunction by various ways such as over extension, distortion and tearing of the olfactory nerves, and contusions of the olfactory bulbs and orbital frontal regions of the brain. [5] Due to the contra coup effect of blows to the occiput that may produce more anosmia than blows to the forehead. [2] Surgical procedures involving the nose may also can cause ansomia.

Fever

Meningitis or abscess associated with osteomyelitis of the frontal or ethmoid bones may cause anosmia. [2]

Onset

Congenital absence of smell is seen in albinos. [2]

Smoking

Cigarette smoke is a toxin that causes dose-related reduction in the olfactory sensitivity. It recovers gradually after cessation of smoking. [1]

Examination

Fact

Explanation

Testing the olfactory nerve

Ask the patient to place an index finger over one nostril to block it and close the eyes. Then the patient is asked to sniff repetitively and to tell the odor. Non irritating stimuli are used to check the olfactory function. [2]

Complete loss of the sense: anosmia/inability to identify odors

As mentioned in the history, damage to the receptors or neurons early in the afferent pathway may cause complete loss of the sense (anosmia) and in further afferents or relays may cause perceptual deficits. [1]

Rhinitis and sinusitis may be a barrier for stimulus to reach the olfactory nerve endings and can disable the receptors. [1,3] Intranasal polyps or carcinoma may cause local processes that involve both the nasal and olfactory mucosa causing hyposomia. [2]

Tremor, shuffling gait, bradykinesia

Anosmia is a common feature of Parkinson’s disease, and may precede the usual motor manifestations. [1]

Behavioural changes, urinary incontinence, emotional disturbances

As the primary olfactory structures adhere to the ventral frontal lobes and connect to limbic circuits, which involve ventromedial frontal cortex, any disorder that involves the frontal lobe may manifest as ansomia. [1] The frontal lobes support higher-level cognitive processes, including executive skills and working memory. Therefore behavioural problems and emotional changes may be the other associated manifestations of the frontal lobe dysfunction. [5]

Foster-Kennedy syndrome is associated with a sphenoidal ridge tumor.[4] Clinical manifestation is in the form of three features: unilateral anosmia with ipsilateral optic nerve atrophy and contralateral papilloedema. [1]

Differential Diagnoses

Fact

Explanation

Etiology - Head injury

Head injury is a frequent cause of ansomia. [4] Subarachnoid hemorrhage can cause anosmia. Head injury can cause olfactory dysfunction by various ways such as overextension, distortion and tearing of the olfactory nerves, and contusions of the olfactory bulbs and orbital frontal regions of the brain. [5] Due to the contracoup effect of blows to the occiput that may produce more anosmia than blows to the forehead. [3]

Etiology - Neuropsychiatric disorders

Anosmia is associated with Lewy body dementia, alcohol amnestic disorder, depression, schizophrenia and parkinson’s disease. There can be specific symptoms and signs for these disorders that can be used for differentiation. [2] eg:- Tremor, shuffling gait, bradykinasia in parkinson disease, hallucinations and delusions in Schizophrenia and low mood, lack of energy, reduced pleasure in previously enjoyed activities and physical Symptoms in depression. [6] Ansomia itself can be a guide for the diagnosis in certain situations: Schizophrenia causes olfactory identification deficits with preserved acuity, post traumatic stress disorder patients and Alzheimer’s patients have increased olfactory identification deficits etc. [1] Pathological changes in the olfactory system is one of the earliest changes in Alzheimer's disease. Olfactory hallucinations (unpleasant odors such as burned rubber) is seen in epilepsy, alcohol withdrawal states, and some psychiatric conditions. [3]

Etiology - Rhinitis and sinusitis

Rhinitis and sinusitis may be a barrier for stimulus to reach the olfactory nerve endings and can disable the receptors. [4] It causes transient ansomia, but may be persistent in certain viral upper respiratory infections. [1]

Etiology - Hysteria

Hysteria is a cause for ansomia. Comparing perception for coffee or vanilla with ammonia can be used to differentiation. Olfactory cell receptors are stimulated by vanilla, coffee. . Ammonia is a trigeminal nerve stimulator. If the anosmia is due to an organic cause the ammonia can be detected but the coffee or vanilla odor cannot be detected. [1]

References

Investigations - Screening/Staging

Fact

Explanation

University of Pennsylvania Smell Identification Test

Microencapsulated “scratch and sniff” odorant strips with a total of 40 odorants are there in this booklet. Scratch will release the odor and he or she then indicates the smell that is perceived and it is graded. Person is then given a score. This is important to arrive at an olfactory diagnosis by comparing a patient's scores with the normal score from normative tables,that are adjusted for gender and age. [2] Comparison is valuable as there are some gender disparities like women tend to outperform men and olfactory function declines with age, with a lesser decline in women. [3]

Sniffin' stick

Sniffin' stick test is olfactory function. T&T olfactometer is using detection and recognition thresholds for each of five odorants named β-phenyl ethyl alcohol, methyl cyclopentenolone, iso-valeric acid, γ-undecalactone, and scatole. A recognition threshold of 5.6 to 5.8 is defined as 'anosmia', 1.1 to 5.5 as 'hyposmia', and -2 to 1.0 as 'normosmia'. [4]

Management - General Measures

Fact

Explanation

Patient education

Patient has to be educated about the cause and the nature of the problem. Particularly the benign nature of the anosmia in conditions like rhinitis and sinusitis which may cause only a transient anosmia. [2]

Symptomatic management

Rhinitis and sinusitis may be a barrier for stimulus to reach the olfactory nerve endings and can disable the receptors. [1] Symptomatic management with nasal decongestants, antihistamines may have a benefit.

Pathological changes in the olfactory system is one of the earliest changes in Alzheimer's disease. [3] These patients need behavioural modifications, supportive management, added to the cholinesterase inhibitors and an NMDA-antagonist like pharmacological therapy in the management. [4]
Schizophrenia where the olfactory identification deficits present with preserved acuity, and post traumatic stress disorder patients presents with increased olfactory identification deficits etc [2] need specific attention.

Avoidance of toxic substances

Anosmia is associated with alcohol amnestic disorder and smoking. [2] Avoidance of the causative factor is needed.

Management - Specific Treatments

Some will show light recovery of their olfactory function following systemic or intranasal topical administration of corticosteroids. [1] Systemic application seems to be a more effective therapy. [2]

Surgical management

This may be relevant particularly in cases of anosmia associated with head injury. Impact of head injury may depend on the severity of damage to the olfactory bulbs, the local inflammatory response, and the amount of injury-associated tissue that accumulate between the olfactory bulb and cribriform plate. [1] Olfactory nerve transection can be done either using a thin flexible Teflon blade to cut the nerve fibers between the olfactory bulb and cribriform plate or curved stainless steel blade to cut the nerve fibers. [1] Polyps and sinus disease may also require surgical treatment when it is not responding to medical therapy.