this is an interesting case of a child who , following a minor head injury a few weeks earlier , was initially found to have a thin chronic subdural haematoma on plain computed tomography ( ct ) imaging . following burr hole surgery for evacuation of the subdural haematoma his post - operative scan revealed an underlying arachnoid cyst .
it is established that intra - cystic bleeding can ensue following minor head injury in patients with this benign developmental malformation .
often the haemorrhage is obvious on initial imaging but this is not always the case .
a 13-year - old boy presented with a 1-month history of worsening global headache with associated nausea and vomiting .
the symptoms developed following an injury sustained by falling and hitting his head on a chair .
he had no significant past medical history , met all his developmental milestones and had no significant family history . on examination he had normal higher mental and cranial nerve function and no focal motor deficits .
a plain ct head [ figure 1 ] revealed a thin isodense subdural collection overlying the left cerebral convexity with 4 mm of midline shift .
sulcal effacement and distortion of the frontal horn of the left lateral ventricle could also be seen .
minor head trauma often precedes complications causing rupture of small bridging vessels between the dura and outer membrane of the cyst .
, intra- and extra - cystic bleeding had occurred simultaneously causing an isodense arachnoid cyst with an accompanying subdural collection .
the child was managed surgically first with a left - sided burr hole evacuation of the chronic subdural hematoma .
a second ct head scan was , therefore , performed [ figure 2 ] , revealing the left middle cranial fossa arachnoid cyst causing the residual midline shift .
this was also managed surgically with a temporal craniotomy and marsupialization of the high - pressure cyst [ figures 3 and 4 ] .
axial computed tomography brain : postburr hole drainage : left temporal arachnoid cyst revealed photograph : craniotomy to reveal membrane of arachnoid cyst photograph : evacuation of high - pressure cystic hematoma the majority of arachnoid cysts are incidental findings on brain imaging and do not require treatment
treatment options vary and include microscopic or endoscopic cystocisternostomy , cyst wall resection , and shunt insertion . in this case , the cyst was located in the cerebral convexity and had displaced deeper brain structures ; therefore , craniotomy for cyst wall resection / marsupialization was indicated .
there is increasing evidence to suggest that fenestrations made endoscopically are as safe and effective as open surgical techniques .
. magnetic resonance imaging should show the cyst as a well - defined nonenhancing mass that is isointense to cerebrospinal fluid and should , therefore , be considered in any child presenting with a subdural collection .
 we present an unusual case of a 13-year - old child who following minor head injury presented with what appeared to be a thin chronic subdural hematoma on plain computed tomography imaging . the child was found to have an underlying arachnoid cyst . 
 intra- and extra - cystic bleeding had occurred simultaneously causing an isodense cyst with an accompanying subdural collection . this radiographically occult pathology should be excluded using magnetic resonance imaging in any child presenting with a subdural collection . 
Introduction
Clinical Background
Discussion
Conclusion
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