The olfactory nerve is responsible for smell detection. It contains bipolar sensory neurons whose receptors are located in the nasal cavity. The nerve fibers penetrate the cribriform plate and synapse in the olfactory bulb. Secondary fibers from the bulb project to areas involved in smell perception and integration like the piriform cortex and amygdala. Common causes of smell disorders include upper respiratory infections, head injuries, nasal/sinus diseases, and neurodegenerative diseases. Lesions in different areas can cause unilateral or bilateral smell loss or distortions.

1. Olfactory NerveOlfactory Nerve
Dr Arun OommenDr Arun Oommen

2. • Anatomy and physiology

3. The olfactory nerve is a sensory nerve
with only one function- smell
First order neurons of olfactory
system are bipolar sensory cells
The olfactory receptors are located in
the superior posterior nasal septum and
lateral wall of the nasal cavity

4. Olfactory receptors have the unique
property to regenerate
Specific odorants stimulate specific
receptor cells and specific cells respond
to particular odorants
Around 20 central processes are given
off from these ciliated cells (filaments
of the 1st
nerve)

6. These filaments (olfactory nerve)
penetrate the cribriform plate of
ethmoid to enter the olfactory bulb.
They acquire a sheath of myelin
.
 In the bulb the olfactory afferent
fiber synapse with the dendrites of the
2nd
order neurons called the mitral and
tufted cells
At the point of synapse conglomerate of
fibres called the olfactory glomeruli are
formed

8. • The axons of the mitral and tufted cells
leave the bulb and course posteriorly as
the olfactory tract in the olfactory
sulcus on the orbital surface of the
frontal lobe
• The olfactory tract divide into medial
and lateral olfactory stria on either side
of the anterior perforating substance

10.  Some of these fibres decussate in the anterior
commissure and join fibres from the opposite olfactory
pathway. Some go to the olfactory trigone and
tuberculum olfactorium (In the APS)

11. Some of the medial olfactory stria
terminate in paraolfactory area, inf part
of cingulate gyrus, subcallosal gyrus

12. Other fibres esp the lateral stria
supply the ipsilateral piriform lobe of
the temporal cortex (primary
olfactory cortex) and terminate in the
• uncus,
• anterior hippocampal gyrus,
• pyriform cortex,
• entorhinal cortex,
• amygdaloid nucleus,

18. • The parahippocampal gyrus sent impulse
to the hippocampus
• The amygdaloid and hippocampal nuclei
(connected on each side thru the ant
commissure) sent projecting fibres to
the ant hypothalamic nuclei, mamillary
body ,tuber cinerum and habenular
nucleus

19. • This in turn project to the thalamus,
cingulate gyrus,striatum and
mesencephalic reticular formation]
• Olfaction is the only sensation not
directly processed in the thalamus
• Connection with the superior and
inferior salivatory nucleus is important
in reflex salivation

23. Clinical examination

24. 2 types of deficits
 Conductive deficits
 Sensorineural/Neurogenic

25. • Proper history
Past head injury
Smoking
Recent UTI
• Systemic illness
• Toxins medications and illicit drugs

26. Pre requisites
 Ensure nasal cavity is open
 Avoid irritating substances
 Test 1 nostril at a time

27. Substances used
 Cloves, Coffee ,Cinnamon
 Commercially available
substance like UPSIT (University of
Pennsylvania smell identification test)

28.  Unilateral loss of smell is more
significant than bilateral
 Perception of odor is more
important than accurate
identification
 Perceiving the presence of an odor
indicate continuity of the olfactory
pathway
Key points

29. 
 Identification of odor indicate intact
cortical function
 Since there is bilateral innervations,
lesion central to decussation does not
cause loss of smell and lesion in olfactory
cortex does not produce anosmia
 The appreciation of presence of smell
even without recognition excludes anosmia

30. Dissorders of olfactory function andDissorders of olfactory function and
localisationlocalisation

31. Terminologies
• Anosmia -Decreased sense of smell
• Hyperosmia -Increased sense of smell
• Dysosmia -Defective sense of smell
• Parosmia -Pervertion of smell
• Phantosmia -Perception of smell that
is no real
• Presbyosmia -Decresed smell due to
aging
• Cacosmia -Inappropriately
disagreeable odor
• Coprosmia -Faecal scent
• Olfactory agnosia - Inability to identify
detected odors

32.  Conditions causing disturbed olfactionConditions causing disturbed olfaction

33.  Congenital-Cleft palate, Downs syn, Turners, Kallmans , Familial
dysautonomia
 Endocrine/metabolic -adrenal insufficiency ,Diabetes, Hypothyroidism
 Iatrogenic-Ethmoidectomy, Hypertelorism, orbitofrontal lobectomy,
Radiotherapy, Rhinoplasty, temporal lobectomy, Repair of ACA anuerysm
 Infections-HIV, Herpes simplex, UTI
 Liver diseases -Cirrhosis, hepatitis
 Local processes -Hansens disease ,Polyps, Rhinitis, Adenoids, tumors
 Neurogenic-Alzheimers disease, Head trauma, Huntingtons disease
Migraines, meningiomas,(Foster kennedy syndrome) ,parkinsonism,
temporal lobe disease
 Psychiatric-Schizophrenia, Hypochondriasis
 Uremia
 Miscellaneous-Cystic fibrosis ,sarcoidosis ,Occupational exposure
,Refsums disease

34. Most common causes of anosmia
• Upper resp tract infection
• Head injury (15-30%)
1. Local injury to olfactory nerves at
cribriform plate due to coup or contrecoup
forces
2.Temporal/orbito frontal injury
• Nasal and sinus disease
• Idiopathic

35. • Lesions involving the orbital surface of
brain may cause unilateral anosmia
• In meningiomas of olfactory groove or
cribriform plate areas unilateral anosmias
occur followed by bilateral anosmias
• Parosmias and cacosmias are often due
to Psychiatric diseases or may follow head
injuries

36. • Olfactory hallucinations are often due to
Psychosis but can result from neoplastic
or vascular lesions of the central olfactory
system or following seizures
• In seizure focus involving medial temporal
lobe structures (uncinate or complex
partial seizures) often preceded by
disagreeable olfactory aura

37. • Following temporal lobectomy olfactory
discrimination is confined to ipsilateral
nostrils.
• Following right fronto orbital lobectomy
impairment seen in both nostrils
• In olfactory epileptic auras tumors are the
most common cause of seizures and the
amygdyla is the most likely symptomatic
zone

38. Foster kennedy
syndrome
 Seen in olfactory groove or
sphenoidal ridge meningiomas
or frontal lobe ICSOL
 3 signs-Ipsilateral anosmia
-Ipsilateral optic atrophy
-Contralateral papilledema

39. Pseudo Foster Kennedy
syndrome
 Seen when increased IC pressure of
any cause occur in patients who
have previous unilateral optic
atrophy
 Most commonly seen due to
sequential anterior ischaemic optic
neuropathy or optic neuritis (optic
disc oedema on one side associated
with optic disc atrophy on other
side)

40. Kallmann’s syndrome
 X linked dissorder
 Familial syndrome of permanent
anosmia with hypogonadotropic
hypogonadism.
 Hypoplasia or aplasia of olfactory
bulbs and tract
 Can be associated with cerebellar
ataxia and mirror movements of hands