These words all refer to involuntary upheavals that disrupt normal processes. Convulsion , most specifically, suggests a violent contraction of muscles, as when the body is struggling to throw off a poison; since these contractions come in a series, the plural is often used in this physiological sense: a case of ptomaine poising that resulted in fatal convulsion . Spasm refers to any involuntary convulsive muscular contraction. When manifested by alternate contractions and relaxations it is a clonic spasm ; when persistent and steady, it is a tonic spasm . Like convulsion , spasm can be used to refer figuratively to any sudden, violent change or burst of feverish activity: a spasm of fear every time she recalled the accident; a series of convulsions due to the abrupt rise in tariffs. Convulsion , both in its figurative and physiological sense, suggests involuntary movement of greater intensity and severity than spasm.

Fi t is more informal than convulsion and suggests a malfunction of psychological or physiological processes rather than the attempt to reject a poison. It is still commonly used to describe an attack of epilepsy: an epileptic fit . But the word may sound outdated when applied to a psychological malfunction: a fit of madness. It is, however, frequently used jocosely to describe being overcome by any extremely intense emotion that has physical manifestations: a fit of anger; a fit of laughter that left us weak; a fit of hiccups.

Seizure , like convulsion , can apply in a number of context. It can be used in conjunction with convulsion and as a more formal substitute for fit . It emphasizes the involuntary nature of the affliction: a seizure of convulsions ; an epileptic seizure . In the latter case, the substitution might once have seemed euphemistic; now, however, it is merely more dispassionate and objective than fit, which when used in this sense evokes the pre-Freudian era when epileptics were considered accursed. Seizure can apply to other intense physical or emotional upheavals: a seizure of coughing; a psychotic seizure . It can also refer to group responses: a nation paralysed by a seizure of terror following the assassination.

Paroxysm may pertain to a physiological situation, referring the one of series: convulsions in which each paroxysm was more prolonged than the one before. More commonly, however, it is used in the plural to refer to an acute emotional attack. In this case, it is like seizure in avoiding the jocose overtones of fit, but is more intense than either, even to the point of hyperbole: paroxysms of anger; paroxysms of grief.