Cranial nerve (1): The Olfactory Nerve.pptx

- Cranial nerves are nerves that emerge directly from the brain and brain stem.
- There are twelve pairs of cranial nerves
- The olfactory nerve (CN I) is the first and shortest cranial nerve. It is a special visceral afferent nerve, which transmits information relating to smell.
- Embryologically, the olfactory nerve is derived from the olfactory placode (a thickening of the ectoderm layer), which also give rise to the glial cells which support the nerve.
- The olfactory placode eventually invaginates and forms the olfactory pit, which further develops into the nasal cavity and the olfactory epithelium, where olfactory receptor neurons reside.

1. Olfactory Nerve Mostafa Meshref Assistant lecturer of Neurology, Faculty of Medicine, Al-Azhar University, Cairo, Egypt. 1

2. Agenda - Introduction - Anatomy and Physiology - Clinical examination - Disorders of olfactory function 2

3. Introduction - Cranial nerves are nerves that emerge directly from the brain and brain stem. - There are twelve pairs of cranial nerves - The olfactory nerve (CN I) is the first and shortest cranial nerve. It is a special visceral afferent nerve, which transmits information relating to smell. - Embryologically, the olfactory nerve is derived from the   olfactory placode   (a thickening of the ectoderm layer), which also give rise to the glial cells which support the nerve. - The olfactory placode eventually invaginates and forms the olfactory pit, which further develops into the nasal cavity and the olfactory epithelium, where olfactory receptor neurons reside. 3

4. Anatomy and physiology 4

5. Anatomy 5 (1) The first-order neurons of the olfactory system Are bipolar sensory cells that lie in the olfactory epithelium , which occupies a small area on the superior nasal concha, upper nasal septum, and roof of the nose. - Their peripheral ramifications are ciliated processes that penetrate the mucous membrane of the upper nasal cavity. - Tiny knobs on the cilia are the sites of chemosensory signal transduction . - Odorant binding to receptors causes ion fluxes, excitation, and the activation of messenger systems. Specific odorants stimulate specific receptor cells, and specific cells respond to particular odorants.

6. Anatomy 6 In the olfactory epithelium , bipolar neurons receive signals from olfactory receptor neurons, which detect odor molecules, and transmit these signals to the olfactory bulb in the brain. Bipolar neurons have multiple functions including sensory reception, signal processing, and transmission of signals - Dendritic process projects to the surface of the epithelium, where they project a number of short cilia, the olfactory hairs, into the mucous membrane. These cilia react to odors in the air and stimulate the olfactory cells. - Central process (also known as the axon) projects in the opposite direction through the basement membrane.

7. - Most nasally inspired air fails to reach the olfactory epithelium because of its location in the nasal attic. Sniffing creates a better airflow pattern for reaching the olfactory endings. - The central processes of the olfactory neurons are unmyelinated axons that form approximately 20 branches on each side. These are the olfactory nerves. - They penetrate the cribriform plate of the ethmoid bone, acquire a sheath of meninges, and synapse in the olfactory bulbs. - Basal cells in the olfactory epithelium can regenerate, an unusual neuronal property. Olfactory receptor cells are continuously replaced by newly formed cells. 7

8. 8 Olfactory Bulb - Once in the cranial cavity, the fibers enter the  olfactory bulb ,   which lies in the olfactory groove within the anterior cranial fossa. - The olfactory bulb is an ovoid structure which contains specialized neurons, called  mitral cells .

9. 9 - Within the olfactory bulbs, axons of incoming fibers synapse on dendrites of mitral and tufted cells in the olfactory glomeruli . - The mitral and tufted cells are the output cells of the olfactory bulb.

10. 10 (2) The Second order neurons of olfactory pathway: - The axons of the second-order neurons, mainly the mitral cells, course posteriorly through the olfactory tracts, which lie in the olfactory grooves, or sulci, beneath the frontal lobes in the floor of the anterior cranial fossa. - N.B: The olfactory bulb is not the olfactory nerve. - The olfactory bulb is an ovoid structure which contains specialized neurons, called mitral cells . - The olfactory nerve fibers synapse with the mitral cells , forming collections known as synaptic glomeruli . From the glomeruli, second order nerves then pass posteriorly into the olfactory tract. - The olfactory nerves are the unmyelinated filaments, wrapped by a special type of glia called olfactory ensheathing glia, that pass through the cribriform plate.

11. Olfactory Tract - The olfactory tracts are mainly formed by axons myelinated by oligodendrocytes and are thus part of the central nervous system (CNS). - The bulbs and tracts are part of the rhinencephalon. - Olfactory information is processed in primitive areas of the brain. Olfaction is the only sensation not directly processed in the thalamus. - The olfactory tract travels posteriorly on the inferior surface of the frontal lobe. As the tract reaches the anterior perforated substance (an area at the level of the optic chiasm) it divides into medial and lateral olfactory stria. - Some olfactory stria fibers decussate in the anterior commissure to join the fibers from the opposite side; some go to the olfactory trigone and tuberculum olfactorium within the anterior perforated substance. 11

12. - Medial olfactory stria – carries the axons across the medial plane of the anterior commissure, where they meet the olfactory bulb of the opposite side. It terminates on the medial surface of the cerebral hemisphere in: The para olfactory area Subcallosal gyrus Inferior part of the cingulate gyrus 12

13. - Lateral olfactory stria – carries the axons to the primary olfactory cortex, located within the uncus of temporal lobe. It courses obliquely along the APS and beneath the temporal lobe to terminate in: The uncus Anterior hippocampal gyrus Piriform cortex Entorhinal cortex Amygdaloid nucleus - The primary olfactory cortex sends nerve fibers to many other areas of the brain, notably the piriform cortex, the amygdala, olfactory tubercle and the secondary olfactory cortex. These areas are involved in the memory and appreciation of olfactory sensations. 13

14. Some connections and projection fibers - The Para hippocampal gyrus sends impulses to the hippocampus. - The hippocampi and amygdaloid nuclei on the two sides are intimately related through the anterior commissure. - These nuclei send projection fibers to: The anterior hypothalamic nuclei Mammillary bodies Tuber cinereum, Habenular nucleus. - Communications with the superior and inferior salivatory nuclei are important in reflex salivation. 14

15. Olfaction Olfaction is a phylogenetically ancient sensation. In lower mammals in whom olfaction is extremely important, the olfactory cortex constitutes a large part of the cerebral hemispheres. In higher primates and man , the area of the uncus and anterior hippocampal gyrus is likely the primary olfactory cortex . The olfactory nerve is a sensory nerve with but one function—smell. Only volatile substances soluble in lipids or water are perceived as odors. In true anosmia, there is loss of ability to perceive or recognize not only scents but also flavors, for much of what is interpreted as taste involves smell. 15

16. - Flavor is a synthesis of sensations derived from 3 components : ✔ The odor (olfactory nerve), ✔ Taste (taste buds, 7th nerve) and ✔ Sensory end-organs (5th nerve). - A patient with olfactory impairment may complain of loss of taste rather than of smell. - Patients with unilateral anosmia may be unaware of any impairment. 16

17. Histology of the Sensory Function - The sensory function of the olfactory nerve is achieved via the olfactory mucosa . - It is located in the roof of the nasal cavity and is composed of pseudostratified columnar epithelium which contains a number of cells: (1) Basal cells – form the new stem cells from which the new olfactory cells can develop. (2) Sustentacular cells – tall cells for structural support. These are analogous to the glial cells located in the CNS. (3) Olfactory receptor cells – bipolar neurons which consist of two processes: - Dendritic process projects to the surface of the epithelium, where they project a number of short cilia, the olfactory hairs, into the mucous membrane. These cilia react to odors in the air and stimulate the olfactory cells. - Central process (also known as the axon) projects in the opposite direction through the basement membrane. (4) Bowman’s glands present in the mucosa, which secrete mucus. 17

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19. CLINICAL EXAMINATION - The patients with disturbances of smell sensation: ✔ Miss out on some of life's pleasures; ✔ They may also miss olfactory danger signals, such as spoiled food, smoke, and leaking gas. - As with hearing, olfactory deficits are sometimes divided into:  Conductive deficits , due to processes interfering with the ability of odorants to contact the olfactory epithelium, such as nasal polyps;  Sensorineural or neurogenic deficits , due to dysfunction of the receptors or their central connections. 19

20. - Unilateral loss of smell is more significant than bilateral, which may be caused by many conditions, primarily conductive - Important historical points to address in a patient with a smell or taste disturbance include past head injury; smoking; recent upper respiratory tract infection (URI); systemic illness; nutrition; and exposure to toxins, medications, or illicit drugs. 20

21. ➔ Ensure that the nasal passages are open . ➔ Smell is tested using non-irritating stimuli . ➔ Avoid substances such as ammonia that may stimulate the trigeminal nerve instead of the olfactory nerve, causing a response that can be confused with olfaction. ➔ The nasal passages are richly innervated by free nerve endings from the trigeminal system, which respond to many substances. ➔ Some patients with impaired taste and smell enjoy spicy food because of its stimulation of the trigeminal system. ➔ Examine each nostril separately while occluding the other. ➔ With the patient's eyes closed and one nostril occluded , bring the test substance near the open one. ➔ Ask the patient to sniff and indicate whether she smells something and if so, to identify it. ➔ Repeat for the other nostril and compare the two sides. ➔ The side that might be abnormal should be examined first . 21 Before evaluating smell:

22. - Many substances can be used to test smell (e.g., wintergreen, cloves, coffee, peppermint, mouthwash, toothpaste, alcohol, soap and cinnamon). - Volatile substances reach easily to olfactory epithelium. - Commercially available quantitative smell and taste tests include the University of Pennsylvania smell identification test (UPSIT) and the Connecticut chemosensory test. The UPSIT requires no trained personnel and may be self administered. Its forced-choice design helps identify malingering. 22

23. - The perception of odor is more important than accurate identification: ► Perceiving the presence of an odor indicates continuity of the olfactory pathways ► Identification of the odor indicates intact cortical function as well. - Since there is bilateral innervation, a lesion central to the decussation of the olfactory pathways never causes loss of smell, and a lesion of the olfactory cortex does not produce anosmia. - The appreciation of the presence of a smell, even without recognition, excludes anosmia. 23

24. DISORDERS OF OLFACTORY FUNCTION (terms and Definitions) 24 Term Definition Anosmia No sense of smell Hyposmia A decrease in the sense of smell Hyperosmia An overly acute sense of smell Dysosmia Impairment or defect in the sense of smell Parosmia Perversion or distortion of smell ► The distortion may be scented, foul or strange ► Parosmia may be of local cause (partial and uncommon) or due to central cause (common). Phantosmia Perception of an odor that is not real Presbyosmia Decrease in the sense of smell due to aging Cacosmia Inappropriately disagreeable odors (a hallucination of an unpleasant odor) Coprosmia Cacosmia with a fecal scent Olfactory agnosia Inability to identify or interpret detected odors

25. Some notes: - Disorders other than hyposmia or anosmia occasionally occur. ★ Hyperosmia is usually functional, but it can occur with certain types of substance abuse and in migraine. ★ Parosmia and cacosmia are often due to psychiatric disease but occasionally follow head trauma and may accompany conductive dysosmia . ★ Olfactory hallucinations are most often due to psychosis, but they can result from a lesion of the central olfactory system, usually neoplastic or vascular, or as a manifestation of seizure. ★ So-called uncinate fits are complex partial or temporal lobe seizures preceded by an olfactory or gustatory aura, usually disagreeable. ★ Such attacks are typically due to a seizure focus involving medial temporal lobe structures. ★ There is never objective loss of smell interictally. 25

26. Some causes of olfactory disorders The top causes of anosmia are: ► Upper-respiratory tract infection (URI) ★ Persistent olfactory loss following a URI is the most common etiology, accounting for 15% to 25% of cases. ► Nasal and sinus disease as chronic sinus disease ► Trauma and Idiopathic Some causes of persistent loss of smell (1) Olfactory groove or sphenoidal ridge meningioma: ★ They may cause unilateral anosmia ★ A typical clinical picture with sphenoidal ridge meningioma is unilateral optic atrophy or papilledema and exophthalmos, and ipsilateral anosmia. ★ In meningiomas of the olfactory groove or cribriform plate area, unilateral anosmia occurs early, progressing to bilateral anosmia, often accompanied by optic neuropathy. 26

27. (2) Frontal lobe tumor especially glioma: may damage the olfactory bulbs or tracts (3) Sellar - parasellar tumor (4) Deviated nasal septum, Nasal polyps, Chronic rhinitis, Postviral . ( 5) Neuro-olfactory tumor ( esthesioneuroblastoma ), Intranasal tumors (6) Korsakoff's syndrome , epidermoid carcinoma, Vitamin deficiency (B6, B12, A), Zinc or copper deficiency , Smoking. (7) Craniocerebral trauma: ★ It May result in damage to the olfactory nerves at the cribriform plate due to coup or contrecoup forces. ★ Anosmia complicates 5% to 20% of major head injuries, sometimes in isolation and Sometimes with other sequala such as diabetes insipidus and cerebrospinal fluid (CSF) rhinorrhea. ★ The incidence of anosmia may be as high as 80% in patients with CSF rhinorrhea. 27

28. (8) Neurodegenerative diseases ★ Alzheimer's disease: Dopamine is one of the neurotransmitters in the olfactory bulbs ★ Parkinson's disease: Deficits may involve odor detection, identification, and discrimination.: Lewy bodies appear in the olfactory bulb early in Parkinson’s disease (PD). - Olfactory dysfunction has been recognized as a common finding in patients with PD and may help distinguish PD from atypical Parkinson syndromes. Deficits ★ Normal aging (9) Multiple sclerosis: may cause impaired smell due to involvement of the olfactory pathways. 28

29. (10) Pregnancy, Congenital anosmia, Meningitis, Arhinencephaly , Cadmium toxicity, Chemical burns of the olfactory epithelium (11) Kallmann's syndrome: ★ Hereditary disorder, usually X-linked, which causes hypogonadism and anosmia, due to hypoplasia or aplasia of the olfactory bulbs and tracts. (12) Drugs: ★ Antibiotics, Levodopa, Radiation therapy, Chemotherapeutic agents, Antihistamines ★ Cocaine; chronic intranasal cocaine use may cause anosmia ★ Antichlinergic drugs as they produce dry nose so the odor is not dissolved. (13) Psychiatric conditions (depression, conversion disorder, schizophrenia): ★ Anosmia sometimes occurs in conversion disorder; taste is usually not affected. ★ In hysterical anosmia, irritating substances, such as ammonia, which stimulate the trigeminal endings, are detected no better than subtle aromas. 29

30. ► Foster Kennedy syndrome consists of:  Anosmia Unilateral ipsilateral optic atrophy and Contralateral papilledema,  Classically due to a large tumor involving the orbitofrontal region, such as an olfactory groove meningioma.  The anosmia and optic atrophy are due to direct compression;  The contralateral papilledema occurs late when ICP increases. ► Pseudo-Foster Kennedy syndrome.  Ophthalmologic picture, without the anosmia,  It is more often due to anterior optic nerve ischemia, or mass causing asymmetric compression of both optic nerves may cause a similar picture. 30

31. ► Miscellaneous neurologic conditions causing anosmia: Hydrocephalus, disease of the anterior cerebral artery near its origin, Basilar meningitis, frontal lobe abscess, Refsum's disease Temporal lobectomies that include piriform cortex may cause deficits in smell identification. Huntington’s disease, Korsakoff’s syndrome, Wilson’s disease, Corticobasal degeneration, spinocerebellar ataxias, narcolepsy Creutzfeldt-Jakob disease may cause anosmia, and prion protein immunoreactivity has been detected by olfactory biopsy to confirm the diagnosis. 31

32. For more information and feedback please, contact me at: [email protected] mostafameshref1988 @gmail.com 32