Decompressive craniectomy (DC) is still a matter of debate, with a numerous complications as expansion of haemorrhagic contusions, external cerebral herniation, subdural hygromas, post-traumatic hydrocephalus (HC). The often overlooked “syndrome of the trephined” (ST) as a delayed complication of DC also known as sinking skin flap sy initially described in 1939.ST is characterised by the neurological changes associated with alteration of the pressure/volume relationship between intracranial pressure (ICP), volume of cerebrospinal fluid (CSF), blood, and brain tissue in patients with large bone defects. This review aims at elucidating the mechanisms responsible for the development of ST, and providing useful tips and red-flag signs for healthcare professionals involved with care of post DC patients. Symptoms identified on time could help to develop appropriate treatment strategies for this suddenly deteriorating, but possible reversible condition. Although the treatment strategy is straightforward, calling for a prompt cranioplasty, the correction of HC through CSF diversion devices might require a lengthy optimisation period. Continuous changes in the setting of the shunting systems or spinal tap might lead to dangerous swinging of the midline structures causing further neurological deterioration. Thus, finding the right balance in terms of clinical management often represents a significant challenge.