Loss of sense of smell, known as anosmia, and taste, known as ageusia, can stem from three main causes: obstruction of the nose, damage to the nose lining, or damage to the olfactory nerve or parts of the brain that deal with smell and taste. Because sense of smell is required for sense of taste, when patients become anosmic they often lose their sense of taste as well.
Olfactory Nerve or Brain Damage
Degenerative diseases, head injury and surgery all can lead to olfactory pathway damage, leading to anosmia. Alzheimer's disease, a progressive neurodegenerative disease characterized by memory loss and confusion, can lead to destruction of the olfactory bulb and olfactory pathways. Anosmia may be one of the first symptoms of Alzheimer's. Huntington's disease and Parkinson's disease as well as diabetes and multiple sclerosis, can also cause anosmia. Schizophrenia, a mental illness characterized by hallucinations and delusions, also can cause anosmia. Head injury can also cause anosmia. Brain surgery, brain radiation, brain tumors, or brain infections can also cause anosmia.
Olfactory Epithelium Damage
Snorting illegal drugs, such as cocaine and amphetamine, can lead to anosmia, as can repeated exposure to intranasal decongestants or even oral use of estrogen. Some viral upper respiratory infections can cause anosmia, as can chronic sinusitis and atrophic rhinitis. Tumors can also damage the olfactory epithelium, although this is rare. Finally, nasal exposure to toxins, such as cadmium or manganese, can cause anosmia.
Obstruction of the nose can lead to loss of smell. Two common causes of nasal obstruction are allergic rhinitis and nasal polyps. Allergic rhinitis, an allergic reaction to airborne pollen, dander, mites, and other irritants, leads to runny nose, sneezing and lack of smell. Nasal polyps, growths of inflamed tissue inside the nose, can lead to symptoms of a cold and loss of smell.