A 37-year-old migrainous woman was referred for dysarthria during a conversation. Dysarthria was with sudden onset, lasted less than 1 min and was accompanied by a migraine headache identical to the patient’s usual ones.

She was admitted many hours after the symptom’s resolution. General and neurological examinations were unremakable.

Brain MRI undertaken a few hours later showed no acute cerebral infarction on the diffusion-weighted imaging (DWI) sequence. However an intraluminal thrombus within a distal branch of the middle cerebral artery (MCA) was seen on the T2*-weighted gradient echo images as well as hyperintense vessel related to slow flow beyond the occlusion’s site. Intravenous thrombolysis was not indicated because the patient was asymptomatic and out the therapeutic time window. Control MRI 24 h later confirmed the presence of a small infarct affecting the distal portion of the MCA territory but the thrombus was no longer visible (Fig. 1).

Fig. 1
figure 1

Initial DWI sequence showed no acute cerebral infarct (A). T2*-weighted gradient echo image disclosed a distal MCA occlusion located within the M4 segment (B) as well as hyperintense vessels beyond the site of occlusion (C). On the 24-h control MRI, axial DWI sequence demonstrated a small hyperintense lesion affecting the distal part of the MCA territory (D) associated with severe apparent diffusion coefficient decrease (not shown) corresponding to an acute brain infarction. The corresponding thrombus of the affected artery was no longer seen (not shown)

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No arterial dissection was seen on cervical MRI. After a complete etiologic workup, no identifiable cause other than a patent foramen ovale with large interatrial shunt was found. Thus, she benefited from a transcatheter closure without stroke recurrence during the follow-up.

Though the sensitivity of DWI sequence for acute ischemia is high, false negative can occur particularly in cases of transient ischemic attack, brainstem strokes or lacunar syndromes [1]. According to some studies, up to one third of patients with nondisabling stroke do not have an acute lesion on DWI sequence [2]. Several explanations have been suggested. Concerning brainstem or lacunar strokes, technical concerns such as artifacts and slice gap thickness may be an explanation. In other cases, DWI negativity could be attributable to DWI reversal due to early recanalization, for example after thrombolytic treatment [3]. A small proportion of patients could also have reduced perfusion not severe enough to produce DWI abnormality at the time of MRI [1, 2]. Hyperintense vessels on FLAIR imaging are often observed distal to arterial occlusion, more frequently when the occlusion is proximal. They represent slow retrograde flow through collaterals distal to the occlusion’s site, as in our patient [4].

Despite a negative DWI sequence, stroke diagnosis is easy to suspect in the setting of persistent neurological symptoms in a patient with vascular risk factors especially when a proximal arterial occlusion is seen on MR angiogram. Conversely our patient was younger, without vascular risk factors and with a very transient deficit. Moreover the intraluminal thrombus and hyperintense vessels were difficult to see because of their very distal location. Only the realization of a follow-up MRI led to the diagnosis of stroke.

To conclude, physicians should not hesitate to perform an early control MRI when the diagnosis of cerebral infarction is suspected despite a normal initial DWI sequence even in a young patient.