5.1. Cranial Nerve I
Anatomy
- General features:
- Olfactory nerve penetrates the cribriform plate of the ethmoid bone to synapse on the olfactory bulb
- Axons of mitral and tufted cells form the olfactory tract
- Olfactory tract divides into a median and lateral olfactory stria
- Lateral striae project to ipsilateral piriform lobe of temporal cortex (primary olfactory cortex)
- Projections to amygdala, septal nuclei and the hypothalamus
- Anosmia:
- Representation of smell in the piriform cortex is bilateral therefore no loss from cortical lesions
- Unilateral loss of smell is caused by lesion of the olfactory bulb, tract or striae
- Parasomnia: perversion of smell
- Cacosmia: unpleasant odors
Differential Diagnosis of Anosmia
- Trauma:
- Fractures of cribriform plate of the ethmoid bone
- Closed head injury without fracture (tear of olfactory filaments):
- Impairment of recognition
- Preserved olfactory detection
- Lesion of the orbitofrontal or temporal lobe
- Basilar skull fracture
- Disturbance of complex olfactory function
- Detection preserved > discrimination ability:
- Korsakoff's syndrome
- Thalamic lesion
- Prefrontal lesion
- Alzheimer's disease
- Parkinson's disease
- Huntington's disease
- Neurodegenerative disease
- Tumor:
- Olfactory groove meningioma
- Esthesioblastoma (tumor of the olfactory epithelium)
- Glioma of the frontal lobe
- Osteoma of sphenoid or frontal bone
- Pituitary adenoma with frontal suprasellar extension
- Carcinomatosis of the meninges
Meningioma of the Olfactory Groove (Foster–Kennedy Syndrome)
- General Features:
- Mucocele of the frontal sinus
- Subfrontal subdural hematoma
- Ipsilateral anosmia
- Ipsilateral optic atrophy
- Contralateral papilledema
- Differential Diagnosis:
- Mucocele
- SDH (subdural hematoma)
- Esthesioblastoma:
- Tumor of the olfactory epithelium
- Frontal lobe behavior syndrome
- CSF leak through the nostril
- Seizure presentation
Pseudo Foster–Kennedy Syndromes
- Papilledema of the spared eye in a patient with prior optic atrophy in contralateral eye
Syndromic Anosmia
- Walker–Warburg Syndrome
- Kallmann's (ovarian dysgenesis)
- Hecker's Type I and II (post viral); Type I taste and smell lost; Type II posterior pharyngeal taste preserved
- Machado-Joseph (SCA Type III)
- Foster–Kennedy Syndrome
Miscellaneous
- Abscess of the frontal lobe
- Mucocele of the frontal sinus
- Post viral infection (Hecker's variant)
- Siderosis (Fe++ deposition on the nerve from recurrent cerebral bleeding; AVMs and cavernous hemangioma)
Systemic Disease
- B12 deficiency
- Paraneoplastic disease
Parosmia and Cacosmia
- Head injury
- Psychiatric disease
Olfactory Hallucination
- Primary olfactory cortical injury (piriform lobe of temporal cortex)
- Direct interruption of the olfactory pathway
- Unilateral paroxysmal olfactory hallucination (paroxysmal dysosmia), possible irritation of the olfactory bulb or nerve
- Complex partial seizures
- Hyperosmia occurs with migraine and hyperemesis gravidarum
- Hyposmia and parosmia (dysosmia)
- Associated with hypogeusia (diminished and perverted taste)
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