A rete middle cerebral artery (MCA) anomaly is a rare clinical condition characterized by occlusion of the M1 segment of the MCA associated with a rete pattern of collateralization. ^{Reference Kajita, Yanaka and Hanai1–Reference Yu3} This condition is differentiated from moyamoya disease by isolated MCA trunk occlusion and the absence of bilateral involvement. ^{Reference Cho, Kim, Jang, Hong, Joo and Kim2} There is a debate on whether this anomaly is congenital or acquired. ^{Reference Kajita, Yanaka and Hanai1–Reference Yu3} In addition, whether ischemic events with rete MCA are caused by hypoperfusion or artery-to-artery embolization is unknown. Transcranial Doppler (TCD) can detect microembolic signals (MES), helping establish the stroke mechanism. ^{Reference Azarpazhooh and Chambers4} However, data regarding the presence of MES in the rete MCA are rare. We describe a case of a rete MCA anomaly in which serial magnetic resonance angiography (MRA) showed de novo formation of the anomaly and TCD monitoring showed MES.

A 55-year-old man with a history of hypertension was admitted to our hospital with aphasia. Brain magnetic resonance imaging (MRI) and MRA showed an acute infarction in the left MCA territory and stenosis in the M1 segment of the left MCA (Supplementary Figure 1A), although he had been prescribed aspirin and cilostazol as an outpatient. His stroke was presumed to be attributed to large artery atherosclerosis because he had hypertension and dyslipidemia (low-density lipoprotein cholesterol concentration, 3 mmol/L). He was started on atorvastatin 80 mg. Five years later, he developed recurrent and transient dysarthria. Six years after his first stroke, he was readmitted, and a repeat MRA showed near occlusion of the left M1 segment (Supplementary Figure 1B). The antiplatelet therapy was changed to aspirin plus clopidogrel therapy. A diagnostic TCD revealed focal stenotic signals in the left proximal MCA (Figure 1A), but not in the left distal MCA (Figure 1B). TCD emboli monitoring was performed by insonating both MCAs at a depth of 52 mm (Spencer Technologies, Seattle, WA, USA). After 30 min of TCD monitoring, 42 MES were identified in the left MCA only (Figure 1C). Aspirin was discontinued after he was discharged. Two years later, he developed recurrent and transient dysarthria and right-sided hemiparesis. Cerebral perfusion single-photon emission computed tomographic (SPECT) imaging showed hypoperfusion in the left hemisphere (Supplementary Figure 2). Antiplatelet therapy was changed to aspirin plus dipyridamole therapy and then to triflusal monotherapy. Ten years after his first stroke, he was readmitted due to right-sided hemiparesis and numbness on the right side of his body. Brain MRI showed acute infarcts located in the left ACA–MCA border zone distribution (Figure 2A), and conventional angiography showed a rete-like configuration of the left MCA (Figure 2B). A superficial temporal artery to MCA bypass was performed to augment the flow to the left hemisphere. However, during hospitalization, he experienced aphasia and worsening right-sided hemiplegia. MRI showed new infarcts in the left hemisphere.

Figure 1.

TCD findings. (A) M indicates M-mode Doppler screen; S, spectra. The right side box shows the velocity parameters. TCD shows focal stenosis in the left proximal MCA with a mean flow velocity of 128 cm/second. (B) TCD shows normal flow velocity in the left distal MCA. (C) Channel 1 indicates the right MCA; Channel 2 indicates left MCA (white box on the left side of the screen). TCD monitoring for MES shows MES (arrows) in the left MCA only. MCA = middle cerebral artery; MES = microembolic signal; TCD = transcranial Doppler.

Figure 2.

Brain diffusion-weighted MRI and conventional angiogram performed 10 years after first stroke. (A) Brain diffusion-weighted MRI shows diffusion restriction in the left ACA–MCA border zone distribution. (B) Left internal carotid injection angiogram shows an arterial rete surrounding the occlusion of the M1 segment of the left MCA. ACA = anterior cerebral artery; MCA = middle cerebral artery; MRI = magnetic resonance imaging.

Some researchers have hypothesized that interference with the coalescence of the embryonic mesh into the MCA trunk during embryonic development results in a rete MCA anomaly. ^{Reference Kajita, Yanaka and Hanai1–Reference Yu3} The fact that most reported patients are of East Asian descent supports this hypothesis. ^{Reference Cho, Kim, Jang, Hong, Joo and Kim2} However, other researchers have argued that the rete MCA is an acquired abnormality resulting from the formation of a compensatory collateral network in the setting of occlusion in the M1 segment. ^{Reference Kajita, Yanaka and Hanai1–Reference Yu3} Our case supports this hypothesis, considering the presence of MCA stenosis before the development of an arterial rete. Our patient may have had atherosclerotic MCA stenosis that progressed to occlusion, resulting in the gradual formation of the arterial rete.

Patients with stroke caused by rete MCA have been treated with bypass surgery because hypoperfusion has been considered the cause of stroke. ^{Reference Yu3} In our patient, however, bypass surgery failed to prevent further stroke, although cerebral perfusion SPECT imaging showed hypoperfusion.

MES are brief, high-intensity transient signals that are observed when microemboli in the artery traverse through the ultrasound beam, as evident in our patient. ^{Reference Azarpazhooh and Chambers4} This finding suggests that cerebral embolism distal to the rete MCA anomaly is one of the mechanisms underlying ischemic events. In our case, dual antiplatelet therapy was insufficient to prevent MES on TCD monitoring, resulting in recurrent ischemic events. Further prospective studies are needed to understand the natural history of the rete MCA and to compare various therapies.