Case-only study: A method that analyzes data from a case series. It may be seen as an epidemiologic equivalent of the “thought experiment” used by theoretical physicists. It is used in the case-crossover study, in case-specular designs, and in molecular and genetic epidemiology to assess relationships between environmental exposures and genotypes.4

Case report: Detailed description of a few patients or clinical cases (frequently, just one sick person) with an unusual disease or complication, uncommon combinations or diseases, or an unusual or misleading semiology, cause, or outcome.4

Case series: A collection of patients with common characteristics used to describe some clinical, pathophysiological, or operational aspect of a disease, treatment, or diagnostic procedure. Some are similar to the larger case reports and share the virtues of this design.4

Cerebellar ataxia: Loss of muscle coordination caused by disorders of the cerebellum.1

Chronic fatigue syndrome: Syndrome of persistent incapacitating weakness or fatigue, accompanied by nonspecific somatic symptoms, lasting at least 6 months, and not attributable to any known cause.1

Chronic headache: Pain in various parts of the head, not confined to the area of distribution of any nerve.1

Chronic inflammatory disseminated polyneuropathy (CIDP): An uncommon, acquired, demyelinating sensorimotor polyneuropathy, clinically characterized by insidious onset, slow evolution (either steady progression or stepwise), and chronic course; symmetric weakness is a predominant symptom, often involving proximal leg muscles, accompanied by paresthesias, but not pain; CSF examination shows elevated protein, whereas electrodiagnostic studies reveal evidence of a demyelinating process, primarily conduction slowing rather than block; sometimes responds to prednisone.1

Chronic urticaria: A form of urticaria in which the wheals recur frequently, or persist.1

Cohort study: The analytic epidemiologic study in which subsets of a defined population can be identified who are, have been, or in the future may be exposed or not exposed, or exposed in different degrees, to a factor or factors hypothesized to influence the occurrence of a given disease or other outcomes. The main feature of cohort study is observation of large numbers

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