Chiari I malformation (CI) continues to raise great interest among physicians due to the larger and larger number of newly diagnosed cases. The clinical and radiological picture and the management options of such a chronic disease are well acknowledged as well as those of the associated syringomyelia. Little is known, on the other hand, about abrupt clinical onset following decompensation of CI/syringomyelia complex. This review on the sudden onset of these two conditions shows that this is a very rare phenomenon; only 41 cases are being reported in the last three decades. In all these cases, acute onset was referable to CI/syringomyelia and the clinical course quickly precipitated. Motor deficits (36.5 %), respiratory failure (29 %), cranial nerve palsy (17 %), and cardiac arrest (14.5 %) were the most common findings, thus confirming that abrupt onset may have severe and life-threatening consequences. Indeed, sudden or early mortality accounted for 19.5 % of cases. In spite of that, most of the surviving subjects had an excellent outcome following either surgical or medical/rehabilitation treatment. Physiopathology of abrupt onset is attributed to the acute compression of the brainstem/upper cervical spinal cord by ectopic tonsils and syringobulbia/syringomyelia, frequently precipitated by a minor injury, followed by impairment of medullary baroreceptors and midbrain reticular substance (cardiac arrest, syncope), medullary chemoreceptors and phrenic nerve nuclei (respiratory failure), lower cranial nerve nuclei (cardiac arrest, cranial nerve palsy), and pyramidal tracts (motor deficits). About 87 % of patients of this review were asymptomatic prior to their acute onset. The problem of the management of asymptomatic subjects is still open.