a 53-year - old man presented with transient acute onset of left - sided numbness and speech disturbance one week prior to admission . transient ischemic attack or acute cerebral infarction were possible diagnoses , so mri and mr angiography were performed , showing an occlusion of his right mca at the proximal m1 segment without any acute ischemic lesion or infracted area on mri , including diffusion - weighted images . the next day , a conventional angiogram was taken and revealed a tapered occlusion at proximal m1 segment of right mca , collateral pathways through perforating arteries , rich pial collateralization from distal anterior cerebral artery , and a shift of the watershed zone ( fig . these findings were compatible with the long - standing stage of the occlusion of mca rather than the acute occlusion . he also has an incidental saccular aneurysm at left posterior communicating artery ( p - com ) origin . the brain single photon emission computed tomography ( spect ) showed mildly decreased perfusion in right middle cerebral artery territory and the vascular reserve was also decreased mildly after acetazolamide injection ( fig . an echocardiogram and myocardial spect excluded cardiac embolus as the etiology of the occlusion . although the patient had no past medical history , untreated hypertension and diabetic mellitus were found during evaluation . in this setting of presumably chronic occlusion the patient was initially treated with antiplatelet and circulating drug and carefully observed for two weeks . after two months , the left p - com aneurysm was managed by the microsurgical aneurysmal neck clipping and there were no developing or new neurologic symptoms after surgery . immediate postsurgical computed tomography angiogram ( cta ) showed persistent right mca occlusion and complete aneurysm clipping ( fig . the brain cta of 21 months after neck clipping developed recanalization of the previously occluded right mca . a subsequent conventional angiogram confirmed nearly complete patency of the right mca with focal mild stenosis , normal blood flow through mca , and the normalization of the shift of watershed zone ( fig . our patient experienced late spontaneous recanalization and restoration of blood flow by an unknown mechanism within 21 months , without any neurologic deficit . spontaneous recanalization of the occluded mca has been a common finding in acute ischemic stroke . several studies demonstrated that most recanalization may occur in the acute phase of stroke , within approximately 48 hours after onset [ 2 , 5 ] . a recent meta - analysis of thrombolytic therapy attempted to quantify spontaneous recanalization in ischemic stroke . spontaneous recanalization occurred in 21.4% of patients within 24 hours and in 52.7% of patients by a week . although the analysis of these data had limitations , including the variety of times examined , but the suggestion is that the natural history of cerebral embolus is dissolution and spontaneous recanalization over a period of time . like these studies , spontaneous recanalization of mca , in which occlusion is apparent in the acute phase have occasionally occurred in the subacute phase of stroke . but late recanalization in the chronic phase has not been reported to the best of our knowledge . unlike recanalization of occluded artery in acute stroke , the spontaneous recanalization of a long - standing occlusion of extracranial artery has been only anecdotally reported [ 7 - 9 ] . the mechanism by which recanalization of chronically occluded carotid arteries occurs is still little known . in 1999 , suggested the possibility of occlusions resulting from ulcerated plaque thrombosis presenting long - term recanalization by thrombolysis . colon et al . , in 1999 , published a series of four cases of spontaneous recanalization of the internal carotid artery , which through imaging examinations and intraoperative finding , proved to be a hypertrophy of vasa vasorum , causing reperfusion of the distal to the occlusion . another possible mechanism is that , in the case of myointimal hyperplasia or atherosclerotic disease , the neovascularization is induced , which in the long term can allow perfusion distal to vessel occlusion . persistence of some embryonic vessels can also account for the complete non - occlusion of the whole internal carotid artery segment , allowing action of varied mechanisms for vessel recanalization . nowadays some groups have been following patients with carotid occlusions , with the aim of better evaluating the natural history of such lesions . verlato et al . , in 2000 , published a cohort study including 41 patients with carotid occlusion . the mean follow up periods were 44.5 months . in one case with asymptomatic carotid occlusion of 41 patients , spontaneous recanalization was identified at three years after the diagnosis , remaining without symptoms for the whole period . the mechanism of late recanalization of chronic occlusions of mca could be one as remarked above or not . unlike the extracranial carotid artery , the intradural cerebral artery has a little chance to the hypertrophy of vasa vasorum or the revascularization within a plaque due to the nature of intracranial artery , including the lack of vasa vasorum and the smaller caliber . in our case , we cautiously speculate that the recanalization process could be that the repeated embolic formation and accumulation were progressed to complete obstruction for some period of time in focal thrombotic area . after antiplatelet therapy , the thrombolysis occurred gradually instead of repeated embolic accumulation . finally , spontaneous recanalization occurred at this late stage . the angiographic changes in our patient illustrate that the chronic mca occlusion were spontaneously recanalized by an unknown mechanism within 21 months , with a good clinical outcome . the incidence , mechanism , and ideal management of this extraordinary finding remain unclear . through this rare case , further investigation and studies into the underlying mechanism of spontaneous recanalization should be performed .
early spontaneous recanalization of the middle cerebral artery in acute ischemic phase artery is not uncommon , whereas the late spontaneous recanalization of chronic occluded artery is a very rare phenomenon and exact incidence and the timing of this event have not been quantified . 
 we present a case in which late spontaneous recanalization of long - lasting middle cerebral artery occlusion occurred in the absence of surgical , endovascular and thrombolytic treatments .