leptomeninges

Leptomeninges refer to the two innermost membranes of the meninges, the pia mater and the arachnoid mater. They envelope the brain and spinal cord and enclose the subarachnoid space, which contains cerebrospinal fluid (CSF). The term is used to distinguish these membranes from the outer dura mater. The leptomeninges develop from neural crest–derived tissues that accompany the CNS during embryogenesis.

Pia mater is a delicate, highly vascular membrane that closely follows the contours of the brain and spinal cord, dipping into the sulci. Arachnoid mater is a space-filling, web-like membrane superficial to the pia and separated from it by the subarachnoid space, which is bridged by arachnoid trabeculae. The subarachnoid space contains CSF and cerebral vessels. Arachnoid granulations protrude into the dural venous sinuses and are a primary route for CSF drainage into the venous system.

The leptomeninges contribute to CSF homeostasis and central nervous system protection. They act as a permeability barrier and support CSF circulation between the ventricles, subarachnoid space, and venous drainage. CSF is produced mainly by the choroid plexus, circulates through the ventricular system and subarachnoid space, and is reabsorbed via arachnoid granulations into the venous circulation.

Clinical relevance includes conditions that affect the leptomeninges. Meningitis involves infection or inflammation of the pia and arachnoid, presenting with fever, neck stiffness, and altered mental status. Leptomeningeal carcinomatosis refers to tumor spread to these membranes. Imaging may show leptomeningeal enhancement with contrast in affected individuals.