Subarachnoid lymphatic-like membrane

Last revised by Rohit Sharma on 30 Mar 2024

The subarachnoid lymphatic-like membrane (SLYM) is a fourth meningeal layer which compartmentalises the subarachnoid space into two functional compartments.

Gross anatomy

The subarachnoid lymphatic-like membrane is only a few cells thin layer, which renders it invisible using current clinical neuroimaging techniques. The SLYM forms a barrier through which cerebrospinal fluid solutes over 3 kDa cannot pass, thus it divides the subarachnoid space into two distinct compartments 1. 

Clinical importance

The membrane is morphologically similar to peripheral mesothelial membranes and it is rich in myeloid cells, thus it likely has a significant role in the response to inflammation. It has also been proposed that physical injury to this thin tissue layer could alter CSF flow, and could explain the prolonged neuroinflammation and the increased risk of developing conditions such as Alzheimer disease after closed head injury 1.

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