In medicine, a forme fruste (French, “crude, or unfinished, form”; pl., formes frustes) is an atypical or attenuated manifestation of a disease or syndrome, with the implications of incompleteness, partial presence or aborted state. The context is usually one of a well defined clinical or pathological entity, which the case at hand almost — but not quite — fits.
An opposite term in medicine, forme pleine — seldom used by English-speaking physicians — means the complete, or full-blown, form of a disease.
According to gastroenterologist William Haubrich:
A patient may exhibit sudden, intense, epigastric pain and a rigid abdomen. He is thought to have a perforated peptic ulcer. But at operation, only a penetrating ulcer is found, sealed off by adhesion to the omentum or anterior abdominal wall. Such a patient is said to have a forme fruste of acute free perforation as a complication of his peptic ulcer disease.
The Latin phrase frustra esse means "to be mistaken" or "to be confused". As a technical term in French, the cognate fruste has been used in two related ways. First, as an antiquarian’s term it refers to a coin, medal or ancient stone on which figures and characters can no longer be recognized due to wear. Secondly, it was employed in natural history to denote mollusk shells whose striations, grooves or tips were worn down. By extension, this sense could be applied to sculpture, pottery, or other objects of great antiquity.
It was in this sense of “indistinctness due to wear or through long use” that the great French internist Armand Trousseau (1801–67) first employed the term in connection with an obscured form of Graves' disease, which he described as a “…maladie dite fruste par l’absence du goitre et de l’exophthalmie” (“…disease said to be crude [i.e., indistinct] for its absence of goiter and exophthalmia”)
The sense of the term in medicine has slightly evolved to mean a “not fully developed form of an illness”, rather than simply an obscure form. Sigmund Freud (1856–1939) often used the term forme fruste in connection with incomplete or obscured cases of neuroses and psychoses and thus the literature of psychoanalysis is replete with it. (An equivalent term in German is minimalvariante, but Freud used the French version.)
List of "forme fruste" medical syndromes
- "Forme fruste keratoconus", as opposed to "frank" keratoconus
- "Zona fruste", early name for zoster sine herpete (shingles without the rash)
- "Forme fruste mitral regurgitation", mitral regurg due to fibroelastic deficiency with myxomatous changes (as opposed to fully formed degenerative changes seen in Barlow's disease).
- "Forme fruste cleft lip", also known as microform. This is the stated cause of the scar on actor Joaquin Phoenix's upper lip
- "'Formes fruste of Churg-Strauss syndrome" have also been described as eosinophilic vasculitis and/or eosinophilic granulomas in isolated organs without evidence of systemic disease
- "'Formes fruste type of Calcaneonavicular Coalition", the mildest type of CN coalition prior to Fibrous, Cartilaginous, or osseous types.
- "Forme fruste Choledochal Cyst", variant of Choledocal cyst described in children in which there is Anomalous Pancreatico Biliary Ductal Junction (APBDJ) with minimal dilatation of Bile duct (6-10mm) in children and associated biliary and/ or pancreatic symptoms. Treatment of choice is bile duct excision. Reason for surgery is significant risk of malignancy and symptomatic relief of symptoms.
- "Forme fruste Rett Syndrome", variant of Rett Syndrome which has a later age of onset compared with the classical form, with regression occurring between 1 to 3 years of age; hand use is sometimes preserved with minimal stereotypic movements
- unilateral papillary cystadenoma is supposed to be forme fruste of VHL disease(mostofi and price 1973) 
- Haubrich, William S. (1997), Medical Meanings: A Glossary of Word Origins, Philadelphia, Pennsylvania: American College of Physicians, pg 85.
- Eulenberg, A., (1910), “The Present Status of Graves' Disease (Exophthalmic Goiter. Basedow’s Disease)”; In: Church, Archibald, editor (1910), Diseases of the Nervous System (Series: Modern Clinical Medicine); Translation of German original; New York and London: D. Appleton and Company, pp 961-962.
- Zoghbi, Huda Y. (2007). The Story of Rett Syndrome: From Clinic to Neurobiology. Cell Press.
- Zappella, M; et al. (2001). "Preserved speech variants of the Rett syndrome: molecualr and clinical analysis". American Journal of Medical Genetics 104: 14–22. doi:10.1002/ajmg.10005.