Leptomeningeal collateral circulation

Layers of the skull and brain

The middle cerebral artery and its branches

Leptomeningeal collaterals lie within the leptomeninges, the two deep layers of the meninges called the pia mater and the arachnoid mater.[4] Their diameter has been measured at approximately 300 micrometers,[5] but there is variability between individuals in the size, quantity and location of these vessels, and between either hemisphere within the same subject.[6]

Inter-territorial end to end anastomoses exist between branches of the anterior cerebral artery and middle cerebral artery, the posterior cerebral artery and middle cerebral artery, the anterior cerebral artery and posterior cerebral artery, and the right and left anterior cerebral arteries.[7][8][9][10] Intra-territorial anastamoses connect adjacent arterial branches within the same arterial territory (between two branches of the same middle cerebral artery, for example).[5]

Inter-territorial leptomeningeal anastamoses between the posterior cerebral artery and anterior cerebral artery have been observed between the parieto-occipital branch of the posterior cerebral artery, and the precuneal branch or the posterior pericallosal branch of the anterior cerebral artery.[1]

Inter-territoral leptomeningeal anastamoses between the right and left anterior cerebral arteries have been observed between the right and left pericallosal arteries and the right and left callosal marginal arteries. Anastamoses have also been observed between precuneal branches originating from the middle portion of the pericallosal artery, or from the posterior portion of the callosal marginal branch of one side joining the opposite paracentral branch.[1]

There is anatomical variation in collateral circulation from person to person, and as we age, collateral vessels decrease in diameter and number.[2]

Leptomeningeal collateral vessels allow limited cerebral blood flow and brain tissue perfusion when the brain receives insufficient blood supply through an artery, via a series of anastomotic connections between cerebral arteries.[3]

Johann Otto Leonhard Heubner

The term 'leptomeningeal' derives from the Greek word leptos (λεπτός) meaning thin, in reference to the appearance of the pia mater and arachnoid mater.

Descriptions of leptomeningeal collateral vessels are found in Thomas Willis’ Cerebri Anatome (1664).[15][16] German physician Otto Heubner first demonstrated their presence in his 1874 work Die luetische Erkrankung Der Hirnaterien.[17] He injected the middle cerebral artery, anterior cerebral artery and posterior cerebral artery in turn, in an attempt to establish the territories these arteries supply. Even when other anastomoses from the circle of Willis were blocked off, the whole cerebral arterial tree could be filled.[1] Later study in the 1950s and 60s by H.M. Vander Eecken and R.D. Adams provided a comprehensive review of the anatomy of the leptomeningeal collateral circulation.[6]

The concept of the ischaemic penumbra, where brain tissue shows capacity to recover if perfusion is quickly restored, was defined in 1981 by Astrup et al. Persistent blood flow through leptomeningeal vessels is a key part of this recovery.[18]

Haemodynamic studies of leptomeningeal collaterals have been conducted in primates.[19] Leptomeningeal circulation has been observed in mice and rats during experiments to assess changes associated with disease and ageing in these vessels.[20]