MODEL RELEASED. Hypotonia test. A 6-week-old baby girl is held horizontally by the trunk in a test for hypotonia (floppy infant syndrome) during a routine health check. Hypotonia is abnormally low muscle tone. In babies it causes floppiness of the limbs and lack of movement. When held horizontally and face downwards the hypotonic baby's limbs hang limply. Hypotonia may be caused by Down's syndrome, cerebral palsy, muscular dystrophy, hypothyroidism or Werdnig-Hoffman disease. Premature babies also tend to be hypotonic but usually develop normal muscle tone as they mature.