Clinical History
A 59-year-old man was admitted to the emergency room with sudden onset of unconsciousness. Neurological examination showed altered mental status, vertical gaze palsy and VII cranial nerve impairment. A not specified genetic bleeding diathesis was reported from his wife. CT and MRI were consequently performed.
Imaging Findings
The CT, performed 3 hours after symptoms onset, didn’t show any alterations. Therefore the patient underwent MRI, which displayed two areas of restricted diffusion in both medial-thalamic regions [Fig. 1b], while no areas of hyperintensity were detected on T2-FLAIR sequences [Fig. 1a]. TOF MRA furthermore showed an absent P1-segment and fetal origin of the omolateral right posterior cerebral-artery [Fig. 1c]. Although the mismatch between FLAIR and DWI signal was interpreted as cytotoxic oedema, intravenous thrombolytic treatment was not suitable for this patient for his history of bleeding diathesis, and he was treated with cardioaspirin.
The patient underwent echocardiogram in the next days, which showed a patent foramen-ovale.
The MRI performed 10 days later demonstrated focal areas of T2-FLAIR hyperintensity in both the medial thalamic-region and cranial-midbrain, and absence of restricted signal in DWI. These areas, moreover, showed enhancement following contrast-media administration [Fig. 4].
The pattern of distribution of ischaemic areas was suspicious for occlusion of the artery of Percheron (AOP).
Discussion
The bilateral infarction of the thalamic regions with or without involvement of the cranial midbrain is a condition related with the occlusion of the Percheron artery [1]. This artery represents an anatomic variant in which a single trunk originating from the P1-segment replaces the paramedian thalamic arteries of both sides, and it could be associated to the agenesis of the contralateral P1 segment or to the absence of bilateral PcoA. The cranial midbrain involvement is due to possible commune arising of paramedian and superior mesencephalic arteries from P1 [1, 2, 3, 6]. Literature describes 4 distinct patterns of AOP infarction: bilateral paramedian thalamic with rostral midbrain (43%), bilateral paramedian thalamic without midbrain (38%), bilateral paramedian and anterior thalamic with midbrain (14%), and bilateral paramedian and anterior thalamic without midbrain [1]. The prevalence of the AOP is unknown and likely underdiagnosed [1].
Normal cardiovascular risk-factors represent the principal aetiology of the vessel occlusion. Bilateral paramedian thalamic strokes are typically characterized by a triad of altered mental-status, vertical gaze palsy and memory-impairment [5].
CT is the first step examination in the emergency-setting; nevertheless often it shows subtle findings before 12-24 hours. Thus, MRI is performed when clinical evidence is suspicious for ischaemic disease. MRI allows us to recognize an acute ischaemic lesion as restricted diffusion on DWI, sign of the presence of cytotoxic-oedema. Conversely, T2 FLAIR sequence doesn’t detect hyperintensity areas in hyper-acute phase but highlights the vasogenic oedema in the evolution of the ischaemic lesion [4].
MRI is more sensitive than CT to evaluate the evolution of the stroke and to rule out haemorrhagic complications. Subacute ischaemic lesions appear as areas of hyperintensity on T2-FLAIR sequences because of the vasogenic oedema (lasting until 4 weeks). Axial FLAIR images through the midbrain, as reported in the literature, can show a V-shaped hyperintense signal intensity along the pial surface of the midbrain at the interpeduncular-fossa (the V sign) [1].
Moreover, we can appreciate disappearance of restricted diffusion after 1 week, and enhancement of the lesion following c.m. administration due to the impairment of the blood-brain barrier [2].
In case of bilateral medial thalamic infarcts, occlusion of the artery of Percheron should be considered as the main diagnosis. Angiography may not be indicated, because lack of visualization of the artery does not exclude its presence (because it is occluded) [2].
The DWI/T2-FLAIR mismatch within 3/4.5 hours since the symptoms onset allows to start the intravenous thrombolysis, improving the prognosis.
Differential Diagnosis List
Bithalamic and cranial-midbrain infarction from occlusion of the Percheron artery.
Tip of the basilar artery thrombosis
Deep venous brain thrombosis
Wernicke encephalopathy
Primitive thalamic bilateral glioma
Final Diagnosis
Bithalamic and cranial-midbrain infarction from occlusion of the Percheron artery.
