Dysosmia

Dysosmia (disordered smell perception) presents as either a distortion in the perceived quality of an odor ( parosmia, cacosmia) or as the presence of a strange odor in the absence of actual odor stimulation (phantosmia or olfactory hallucinations). Most dysosmias reflect dynamic elements associated with degeneration (or, more rarely, regeneration) of the olfactory epithelium and remit over time. However, it is common for patients with anosmia to report that prior to the onset of anosmia they experienced a period of weeks or months when dysosmia was present. Extremely debilitating chronic dysosmias have been reported that have required surgical intervention, such as ablation of portions of the olfactory epithelium y or removal of the olfactory bulbs.y Most such cases present unilaterally. More commonly, dysosmias are part of the sequelae of events that occur after the olfactory nerve fibers have been partially damaged by upper respiratory infections, head trauma, nasal sinus disease, or other disorders. In the majority of these cases, marked smell loss does not accompany the dysosmic condition, implying that it requires a relatively intact sensory system for expression.

TABLE 7-2 -- SELECTED ETIOLOGIES ASSOCIATED WITH DISORDERS O

f CRANIAL NERVE I

Etiological Category

Selected Specific Etiologies

Chapter

STRUCTURAL DISORDERS

Developmental

Kallmann's syndrome

28

Degenerative and compressive

Schizophrenia

Usher's syndrome

29

HEREDITARY AND DEGENERATIVE DISORDERS

Amino/organic acidopathies, mitochondrial enzyme defects and other metabolic errors

Trimethylaminuna

31

Chromosomal abnormalities and neurocutaneous disorders

Down'.s syndrome

32

Degenerative dementias

Alzheimer's disease

33

Pick's disease

Movement disorders

Idiopathic Parkinson's disease

34

Huntington's disease

Degenerative motor, sensory, and autonomic disorders

Amyotrophic lateral sclerosis

36

ACQUIRED METABOLIC AND NUTRITIONAL DISORDERS

Endogenous metabolic disorders

Diabetes

38

Hypothyroidism

Kidney disease

Liver disease

Exogenous acquired metabolic disorders of the nervous system: Toxins and illicit drugs

Toxic ehemical exposure

39

Nutritional deficiencies and syndromes associated with alcoholism

Wernicke-Korsakoff syndrome

40

Cirrhosis

INFECTIOUS DISORDERS

Viral infections

Upper respiratory viral infection

41

Acute viral hepatitis

Acute poliomyelitis (CNs XI, XII)

Nonviral infections

Bacterial meningitis

43

Neurosyphilis (CN XII)

Pneumonia

HIV and AIDS

Human immunodeficiency virus (HIV)

44

NEUROVASCULAR DISORDERS

Stroke

45

NEOPLastIC DISORDERS

Primary neurological tumors

Foster Kennedy syndrome

46

Metastatic neoplasms and paraneoplastic syndromes

Metastatic carcinoma

47

DEMYELINATING DISORDERS

Demyelinating disorders of the central nervous system

Multiple sclerosis

48

AUTOIMMUNE AND INFLAMMATORY DISORDERS

Allergies

50

TRAUMATIC DISORDERS

Head trauma

51

EPILEPSY

Temporal lobe epilepsy

52

DRUG-INDUCED AND IATROGENIC NEUROLOGICAL DISORDERS

Rhinitis medicamentosa

55

Nasal surgery

Medications (neuroleptics)

In rare instances, dysosmias reflect aura-like processes that are suggestive of central brain tumors or lesions, particularly lesions of the temporal lobe. In some cases, aura-like dysosmias can be chronic or occur regularly without producing any evidence or any clear sign of seizure activity (although, as noted earlier in this chapter, inpatient EEGs may be needed to detect infrequent partial seizure episodes). A few psychiatric syndromes, like olfactory reference syndrome, are also associated with dysosmic episodes, as are metabolic disturbances such as trimethylaminuria.

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