A condition associated with plasma cells proliferation that contain IgG-4 receptors has been reported to involve the pachymeninges. The process has a similar appearance to rheumatoid granulomatous infiltration and it is responsive to corticosteroids. In addition to evident thickening and gadolinium enhancement of the dura on imaging studies, the characteristic features are an elevation of the IgG fraction in the serum and in CSF and infiltration of meningeal tissue by uniform plasma cells that exhibit IgG-4 markers (Plasma cells are not prominent in the CSF in this condition; they are found in other states such as Listeria meningitis and neurosyphilis.). This process has been known to occur in other organs such as the salivary glands, lung, and kidney; it tends to occur in men in their fifth or sixth decade according to the case series of pathologic samples reported by Lindstrom and colleagues.
The subdural space and dura can be involved by extension of a pathologic process from the arachnoid, especially in infants and children, in whom subdural hygromas regularly follow meningitis. The fibrous connective tissue of which the dura is composed may also undergo pronounced thickening in the course of a mucopolysaccharidosis, especially in cases where fibroblasts are implicated. The basal pia-arachnoid may be involved, leading to obstructive hydrocephalus. Older medical writings often made reference to syphilitic cranial pachymeningitis, which later proved to be the thickened membranes of subdural hematomas. The neovascular response and fibrosis of the dura and meninges that result from low CSF pressures and give the same appearance as pachymeningitis on enhanced CT scans and MRI were discussed earlier in the chapter.
Superficial Siderosis of the Meninges
Superficial siderosis is the consequence of repeated contamination of the meninges by blood (McDougal and Adams; Fishman, 1993). An oozing vascular malformation, previous subrachnoid hemorrhage or tumor has been the usual cause in our experience, although there have been instances in which the source of the blood could not be found; in these a past history of head trauma is common. The red blood corpuscles are phagocytosed, with the formation of hemosiderin, and gradually both iron pigment and ferritin are released into the CSF. As a result, the surface of the cerebellum, spinal cord, hippocampi, and olfactory bulbs are stained orange-brown. Iron pigments and ferritin, which are toxic, gradually diffuse through the pia into superficial parts of the cerebellum, eighth cranial nerve, and spinal cord, destroying nerve cells and exciting a glial reaction. In microscopic sections stained for iron, the histiocyte–microglial cells contain iron and ferritin, and particles of iron can be seen studding nerve and glial cells for a distance of several millimeters beneath the pia.
The clinical syndrome of siderosis of the meninges consists essentially of a progressive ataxia and nerve deafness; sometimes a spastic paraparesis is added and, rarely, mental impairment, probably from hydrocephalus. The hemosiderin and iron-stained meninges are readily visualized by MRI, as iron is strongly paramagnetic. All the iron-impregnated tissues are hypointense in T2-weighted images. Koeppen and associates attributed the vulnerability of the acoustic nerves to their extended meningeal exposure before acquiring a fibroblastic perineurium and epineurium. There is no treatment other than finding the source of the meningeal blood and preventing further hemorrhage and treating hydrocephalus if it is present. Kumar and also Fishman (1993) have provided a useful review of the problem of superficial siderosis. Curiously, the systemic disease hemochromatosis, does not affect the brain or meninges.