Table 1: Olfactory dysfunction in relation to three major cause
	Head trauma	Upper respiratory infection	Nasal / sinus disease
Probable cause	Shearing of olfactory filaments at the cribriform plate	Viral destruction of olfactory epithelium	Polyps, secondary edema due to local inflammatory processes
Epithelial findings	Degeneration	Metaplasia, defective development of ORNs	Mostly normal
Occurance of olfactory disturbances	5 %	1 %	Frequent
Approximate age	20-50 years	Older than 60 years	20-60 years
Rapid onset of olfactory dysfunction	+++	+++	+
Degree of olfactory loss	+++	++	++
Frequent occurance of parosmia	++	+++	+
Recovery possible	+ Improvement mostly in hyposmic patients, mostly within the first years following trauma	++ Improvement possible, even over a period of several years	+++ Responsive to surgical therapy or treatment with corticosteroids

In both, Alzheimer's and Parkinson's disease, loss of olfactory sensitivity is thought to be among the earliest signs of the disease ^49-51. Considering the relatively high prevalence of both disorders ^{52, 53}, it can be estimated that these neurodegenerative processes significantly contribute to the presence of olfactory dysfunction in the general population

Other frequent causes include congenital anosmia or exposure to toxic substances, each of which has been reported to occur in less than 5% of the cases ^{10, 11, 47}. A wide variety of still other etiologies include, for example, psychiatric causes (e.g., schizophrenia, depression), epilepsy, sarcoidosis, lupus erythematodes, multiple chemical sensitivities, pregnancy, diabetes, hypothyroidism, renal failure, liver disease, olfactory meningeomas, and neoplasms of the brain (for review see ¹⁷). Finally, iatrogenic causes of olfactory disorders include rhinoplasty, neurosurgery, radiation or drug therapy. In an additional 20% of the patients, a cause for the chemosensory disturbance cannot be identified (idiopathic).

It seems reasonable to assume that acute loss of olfactory function following trauma or viral infection is perceived as more severe compared to the gradual loss of olfactory function, e.g., in sino-nasal disease. In fact, signs of higher degree of depression or global psychological distress, respectively, have been reported in patients with a more recent and sudden onset of anosmia ⁵⁴. However, observations have been published indicating that this correlation is weak and difficult to obtain ^{11, 55}. In fact, in patients with post-traumatic olfactory loss it is a characteristic that this deficit is only noted weeks or even months following the actual incident ⁵⁶.

Further difficulties in this area of research become apparent when considering that most, if not all studies, are biased by investigations in subjects who present themselves to specialized centers, or subjects who identify themselves as compromised in terms of olfactory function. Epidemiological research in this area seems to be particularly difficult considering that, within the general population, difficulties to adequately judge olfactory function/dysfunction are frequently found ⁵⁷, and, consequently, many patients would not seek professional advice, even if olfactory function was seriously compromised.

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