Olfactory Pathways

OLFACTORY NERVE DISORDERS

Anosmia is not always apparent to the patient, and due to the close association of flavor perception and olfaction, may be reported as altered taste rather than loss of smell. Bilateral anosmia is more common and usually of benign nature, whereas unilateral anosmia should raise suspicion for a more serious disorder, such as an olfactory groove meningioma or frontal basal tumor. The most common cause of anosmia is nasal and paranasal sinus infection with inflammation and is referred to as transport or conductive olfactory disorders. Post-traumatic olfactory dysfunction is the cause for 20% of patients with anosmia and is the result of olfactory nerve shearing as it passes through the cribriform plate. In more substantial damage, the olfactory nerve is torn by fractures involving the cribriform plate, with cerebrospinal fluid rhinorrhea and possible meningeal infection. Post-traumatic anosmia or hyposmia may be either unilateral or bilateral. Tumors of the olfactory groove affect the olfactory bulb and tract. The most common are olfactory groove meningiomas, which are usually histologically benign tumors causing mostly unilateral, and occasionally bilateral, gradual olfactory dysfunction. Other tumors include sphenoid and frontal osteomas, pituitary tumors, and nasopharyngeal carcinomas. Unless specifically tested, a presentation of anosmia is unusual because of generally unilateral involvement and slow tumor growth with slow decline in olfactory function. Once such tumors are large enough (>4 cm in diameter), they cause pressure on the frontal lobes and the optic tracts, with symptoms of headaches, visual disturbances, personality changes, and memory impairment. Very large olfactory groove tumors on rare occasion cause ipsilateral optic atrophy by exerting direct pressure on the optic nerve with contralateral papilledema from increased intracranial pressure. The finding of ipsilateral optic atrophy, contralateral papilledema, and ipsilateral anosmia is known as the Foster-Kennedy syndrome. Esthesioneuroblastomas arise from the upper nasal cavity and manifest with nasal obstruction and epistaxis. Rarely, they involve the orbit and cause diplopia, visual loss, proptosis, and periorbital swelling. Anosmia is an early sign of neurodegenerative processes, particularly Parkinson disease, Alzheimer disease, and Lewy body dementia. It frequently precedes other neurologic signs, such as motor findings or cognitive changes. Olfactory discrimination is affected by many medications thought to disrupt the physiologic turnover of receptor cell and includes opiates, anticonvulsants, and various immunosuppressive agents. Congenital or hereditary anosmia is rare. Kallmann syndrome consists of congenital hypoplasia or absence of the olfactory bulbs and hypogonatropic hypogonadism.

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