Fit, attack, access, accession, paroxysm, spasm, convulsion are comparable when they denote a sudden seizure or spell resulting from an abnormal condition of body or mind.
The last three are too specific in their technical medical senses to be synonyms of the others (except of fit in its narrower significations), but in their extended senses they are frequently closely parallel.
Fit is often used narrowly: sometimes to designate a sudden seizure of a disorder (as epilepsy or apoplexy) characterized by such symptoms as violent muscular contractions or unconsciousness or sometimes to designate a period in which there is a marked increase of a physical disturbance characteristic of a disease. In its wider application, fit still may imply suddenness and violence, but it emphasizes temporariness. Occasionally it suggests nothing more than the unusual and passing character of the condition and is applied to things as well as to persons.
Attack always implies a sudden and often violent onslaught but carries no suggestion of length of duration.
Access and accession, though often interchangeable with attack, distinctively imply the initiation of an attack or fit and often come close in meaning to outbreak or outburst. Occasionally they also connote intensification (as of a mood or state of mind) to the point where control is lost or nearly lost. In their technical medical senses paroxysm, spasm, and convulsion are sudden and usually short attacks especially characteristic of certain diseases.
The distinguishing marks of a paroxysm are sudden occurrence or intensification of a symptom (as coughing) and recurrence of attacks; those of spasm are sudden involuntary muscular contraction, in some cases producing rigidity of the body or constriction of a passage and in others producing alternate contractions and relaxations of the muscles; those of convulsion are of repeated spasms of the latter kind affecting the whole or a large part of the body and producing violent contortions of the muscles and distortion of features. The implications of these technical senses are usually carried over into the extended senses.
Paroxysm commonly occurs in the plural and suggests recurrent, violent attacks.
Spasm, especially when used of emotional disturbances, often implies possession by something that for a moment grips and paralyzes.
When used in the plural, it usually suggests the more or less rapid alternation of contrasting moods or states of mind.
Convulsion implies definite physical effects accompanying the mood or state of mind and closely resembling those symptomatic of disease.