Case of the Week

Background:

• Rete anomaly of the M1 segment of the MCA refers to an M1 that lacks a single prominent vessel and instead exists as a multitude of small vessels in a plexiform configuration in the proximal M1. These vessels distally coalesce to form anatomically normal vasculature, at either the distal M1 or M2 level.
• It has an incidence of 0.11–0.67% in patients who undergo angiographic imaging (CTA, MRA, conventional angiography).
• This is thought to occur due to some insult that interrupts a fusion of embryologic-type MCA.

Clinical Presentation:

• May be asymptomatic and identified incidentally, as in this Case of the Week.
• If symptomatic, patients tend to present with aneurysmal or nonaneurysmal intracranial hemorrhage. Less frequently, patients present with ischemic stroke or transient ischemic attack.

Key Diagnostic Features:

• CTA, MRA, or conventional radiography show a discontinuity of the M1 segment whereby the M1 is replaced by a network of "twig-like" vessels. Distal to this network of vessels, the distal M1 or proximal M2s reconstitute and have normal appearance. The ICA is not affected and is normal in appearance.

Differential Diagnoses:

• Moyamoya pattern of vessels: Unilateral or bilateral stenosis of distal ICA, with resulting formation of extensive collateral network of small vessels in the proximal MCA territory, with possible stenosis of other vessels in the circle of Willis. A key imaging feature of the moyamoya pattern is the involvement of the distal ICA, which differentiates it from a rete M1 anomaly where the ICA is normal in appearance.
• Steno-occlusive disease: Severe steno-occlusive disease in the proximal M1 can over time cause formation of collateral circulation that bridges the stenosed segment and could produce an appearance similar to that of a rete M1 anomaly. If such appearance of M1 is seen without any evidence of steno-occlusive disease elsewhere in the brain, then rete M1 anomaly should be the more favored diagnosis.
• Arteriovenous malformation: One or more feeding arteries entering a tightly packed network of vessels (a nidus), which in turn drains through one or more veins. Draining veins opacify in the arterial phase on conventional angiography. A nidus of an AVM may look similar to the rete M1 anomaly. Absence of abnormal feeding vessels and draining veins on angiography would differentiate the two.

Treatment:

• Typically conservative
• If associated with an aneurysm, then treated with coiling or clipping