A 55-year-old man with no prior medical history presented to the emergency department with sudden onset of severe headache at the occipital region, which he described as ‘the worst headache he had ever had’. On arrival, the vital signs were all within normal limits and clinical examinations revealed no neurological deficit.
Plain Computed Tomography (CT) was performed on the same day which showed acute subarachnoid haemorrhage (SAH) at the prepontine and perimesencephalic cisterns with no extension to the suprasellar cistern or Sylvian fissures (Fig. 1). In view of the concern of possible aneurysmal bleed, cerebral Digital Subtraction Angiography (DSA) was performed subsequently which revealed no intracranial aneurysm or vascular anomaly.
He was then admitted to the neurosurgical ward for observation and supportive treatment. Cervical and brain Magnetic Resonance Imaging (MRI) was done during the same admission which could not identify any cause to explain the SAH. He was then discharged 5 days later after repeated brain CT that showed resolved SAH (Fig. 2).
On follow-up, the patient was asymptomatic with no neurological deficit. DSA cerebral was repeated 4 months after his discharge which showed no abnormality.
PNSAH is a rare condition with annual incidence of 0.5 per 100,000 persons over 18 years of age. It is a unique subset of SAH which was first reported by van Gijn et al. in the year 1985. They described a series of patients who had SAH confined to the cisterns surrounding the midbrain and pons and normal angiograms.  Since then, many studies had been done attempting to identify the cause for PNSAH - which is likely venous in origin, but the exact aetiology is still unknown to date.
A number of studies proposed a link between abnormal drainage of the basal veins of Rosenthal and PNSAH. [2, 3, 4] Basal veins of Rosenthal are paired paramedian veins which are formed primarily by the union of anterior cerebral veins and deep middle cerebral veins. The veins are formed at the medial surface of the temporal lobe, course posteriorly lateral to the midbrain through the ambient cisterns and drain into the vein of Galen normally. (Fig. 3) However, in some individuals, the basal veins of Rosenthal drain directly into the dural venous sinuses. The direct communication with the venous sinuses may predispose the basal veins to sudden increase in venous pressure, causing engorgement and rupture. [2,4]
Clinical Perspective & Imaging Perspective
Patients usually present with sudden headache, meningism and vomiting which are similar to patients with aneurysmal bleed. CT or MRI, if performed within the first 3 days of presentation, will typically show SAH confined anterior to the midbrain or pons and may sometimes involve the interpeduncular, ambient and quadrigeminal cisterns. Redistribution of the bleed to other cisterns may occur if the imaging is performed late.  Despite its classical SAH pattern, CT angiogram or cerebral DSA is still mandatory in the management of these patients in order to rule out aneurysmal bleed which requires further neurosurgical or endovascular intervention.
Treatment of PNSAH is usually supportive with bed rest and analgesics. In addition to that, antifibrinolytics, antihypertensive drugs are sometimes recommended in certain groups of patients. It usually has a benign course of events and has a very good prognosis as shown in our patient. 
Take Home Message/Teaching Points
PNSAH is a rare condition with very good prognosis and clinical outcomes. It is important for the clinician and radiologist to be aware of this condition and its characteristic imaging findings, so that prompt diagnosis can be made.
Written informed patient consent for publication has been obtained.
 Van Gijn J, van Dongen KJ, Vermeulen M, Hijdra A (1985) Perimesencephalic hemorrhage: a nonaneurysmal and benign form of subarachnoid hemorrhage. Neurology 35:493-497 (PMID: 3982634)
 Yamakawa H, Ohe N, Yano H et al (2008) Venous drainage patterns in perimesencephalic nonaneursymal subarachnoid hemorrhage. Clin Neurol Neurosug 1 10:587-91 (PMID: 18433987)
 Van der Schaaf IC, Velthuis BK, Gouw A et al (2004) Venous drainage in perimesencephaluc hemorrhage. Stroke 35:1614-18 (PMID: 15166390)
 Song JH, Yeon JY, Kim KH et al (2010) Angiographic analysis of venous drainage and a variant basal vein of Rosenthal in spontaneous idiopathic subarachnoid hemorrhage. J Clin Neurosci 17:1386-90 (PMID: 20692171)
 Rinkel GJ, Wijdicks EF, Vermeulen M, Ramos LM, Tanghe HL, Hasan D et al (1991): Nonaneurysmal perimesencephalic subarachnoid hemorrhage: CT and MR patterns that differ from aneurysmal rupture. AJNR Am J Neuroradiol 12:829-834 (PMID: 1950905)
 Rinkel GJ, Wijdicks EF, Vermeulen M, Hageman LM, Tans JT, van Gijn J (1990): Outcome in perimesencephalic (nonaneurysmal) subarachnoid hemorrhage: a follow-up study in 37 patients. Neurology 40:1130-1132 (PMID: 2356015)