Teaching Case

A 47-year-old man with no cardiac or neurological problems in the past, was admitted to the emergency department with altered, fluctuating consciousness (GCS 10/15), right mydriasis and strabismus of the right eye. His vital parameters remained normal throughout the investigation. Cardiovascular, pulmonary and gastro-intestinal investigation showed no abnormalities.

Initial CT, approximately 2 hours after admission showed no haemorrhage or any signs of recent ischaemia.

An MRI was performed 20 minutes later. The diffusion-weighted sequences showed diffusion restriction bilateral in the paramedian thalamic region, more pronounced on the left side (Fig. 1), and an ovalar hyperintensity paramedian in the mesencephalon, including the pial surface of the mesencephalon adjacent to the interpeduncular fossa (V-sign [1], Fig. 2).

MR-angio showed a patent basilar artery, a normal appearance of the left P1 segment of the posterior cerebral artery (PCA), but an irregular aspect and abrupt ending of the right P1 (Fig. 3). This finding together with the bilateral ischaemic lesions in the thalami suggests the presence of a Percheron type 2b artery, occluded due to thrombo-embolism.
A control MR-angio 6 days after symptom onset demonstrated the presence of the artery of Percheron, after regaining arterial flow (Fig. 4).

The blood supply of the thalamus runs through multiple small branches originating from the posterior cerebral artery and posterior communicating artery. The thalamus is mainly subdivided in 4 territories, based on neuro-anatomical and neuropathologic data [2]: anterior, paramedian, inferolateral and posterior. These territories are supplied by the polar (originating from posterior communicans artery), paramedian, thalamogeniculate and posterior choroidal arteries, respectively.
The paramedian arteries originate from the P1 segment of the PCA. A great number of variations exist in the origin of the paramedian arteries [3, 4]. Percheron described 4 distinct variants of the paramedian arteries: type 1 is the most common where multiple branches originate bilateral on the P1 segment of the PCA, in type 2a multiple small branches originate on the P1 segment of one of both PCA, type 2b is clinically the most important where one branch originates on the P1 segment of one of both PCA and in type 3 multiple branches originate from a bridging artery (arcade) between both PCA. Type 2b is also known as the Percheron artery (Fig.5).

The incidence of ischaemic infarction due to occlusion of the Percheron artery is estimated at 0.1-0.3% of all ischaemic infarctions [5]. The clinical diagnosis remains difficult because the symptoms are diverse and not specific.
The paramedian arteries supply the paramedian thalamic region as aforementioned, but most often their branches also supply the mesencephalon in some degree. This is why in most cases (43%) of an ischaemic infarction of the Percheron artery territory parts of the mesencephalon are affected [1].

A triad of symptoms almost always present in bilateral thalamic stroke includes altered mental state, vertical gaze palsy and memory impairment. If the mesencephalon is affected, a mesencephalothalamic or thalamopeduncular syndrome occurs in which other occulomotor disturbances, hemiplegia, cerebellar ataxia and movement disorders can appear [2, 5, 6].

The only abnormality always present on imaging studies of Percheron infarction is a hypodense appearance of both paramedian regions of the thalamus on CT and diffusion restriction in these areas on diffusion weighted MRI [1, 4, 6].
The Percheron artery most often is not visualised on MRA or CTA.
A typical V-shaped hyperintensity on DWI and FLAIR can be present along the pial surface of the midbrain adjacent to the interpeduncular fossa, called the V-sign by Lazzaro [1]. This V-sign was present in 67% of the patients with involvement of the mesencephalon.