A 2-year-old child presented in neurosurgery clinic with a tense swelling on left side of the scalp and with a progressive decline of mental status for the prior 2–3 days. The patient had undergone a decompressive craniectomy one and a half months prior for a severe traumatic brain injury. A CT head scan was performed, and revealed a left fronto-temporo-parietal craniectomy defect with a large overlying fluid attenuation collection in the subgaleal space (CT attenuation values between 6 and 12 HU). There was compression of underlying brain parenchyma. Post traumatic gliotic areas were also identified in the left cerebral hemisphere (Fig. 1a–c). Considering the operative history and the typical imaging findings, a diagnosis of external brain tamponade was made. The patient underwent emergent drainage of the collection, and made a dramatic post operative recovery.

Fig. 1
figure 1

Volume rendered CT image (a) showing the large tense swelling in the scalp overlying the craniectomy site (wavy arrows). Coronal MPR image (b) and axial contrast enhanced CT image (c) showing the subgaleal collection at the craniectomy site (arrows) along with post traumatic gliotic changes in the left cerebral hemisphere (arrowheads)

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Decompressive craniectomy implies removal of a portion of the skull to decompress the intracranial contents. This neurosurgical procedure has an established and important role in neurocritical care. The common indications are traumatic brain injury, refractory malignant intracranial hypertension, subarachnoid haemorrhage and malignant middle cerebral artery infarction [1–3]. The cranioectomy procedure can be bilateral in cases of diffuse brain oedema without midline shift, or unilateral in patients with one sided brain swelling with midline shift [2]. Since the operation involves removal of a large piece of skull, it leads to alteration of the cranial pathophysiology, and can also result in a variety of complications like external brain tamponade, trephine syndrome, extra cranial herniations, cerebral contusions, infections, subgaleal or subdural hygromas [2, 4]. External brain tamponade is one of the uncommon, but serious complications, which is characterized by a tense crainectomy flap, neurological decline, subgaleal fluid collection with mass effect on underlying brain, and neurological improvement after drainage [2, 4]. The underlying cause is tense accumulation of fluid in the subgaleal space due to a ball valve type effect or pressure gradient [2]. The CT scan is the imaging modality of choice for evaluation of the post crainectomy complications because of its high speed, relatively low cost and widespread availability. On the CT scan, external brain tamponade is visualized as a fluid attenuation collection in the subgaleal space with bulging of the skin flap, and compression of underlying brain parenchyma [2]. All these findings were present in our case.