Multifocal intracerebral schwannosis
Schwannosis is recognized as a rarely occurring process that is considered to be a benign proliferation of Schwann cells with simultaneous partial myelination of central nervous system axons. It has been mainly observed in the spinal cord (4, 9, 10). Schwannosis was only reported once in the pontine parenchyma (6). The case present here is the first example of a multifocal manifestation of schwannosis in the brain stem, i.e. in the nucleus ruber and in the pons. Schwannosis seems to occur in response to various chronic (i.e. traumatic, neoplastic, degenerative, or compression producing) stimuli, possibly representing an attempted, though anomalous repair by inwardly migrated Schwann cells. Within this context, it is interesting that all three lesions were present close to small stage III infarctions.
Various hypotheses have been put forward with respect to the ontogenesis of schwannosis and the origin of the involved Schwann cells in the development of these lesions. O'brien et al. (7) proposed the existence of a transitional zone between the central and peripheral nervous tissue compartments within the dorsal spinal nerve rootlets with a sharp discontinuity of different tissue types at its interface. While axons peripheral to the transitional zone are myelinated by Schwann cells and endoneurium mainly comprises the supporting tissue, axons central to the transitional zone are engulfed by oligodendrocytes and astrocytes which constitute the main cellular component of the surrounding neuropil. It has been further suggested that the transitional zone can become penetrable, thus allowing the migration of Schwann cells from the peripheral to the central nervous system via the dorsal root ganglia under particular conditions, especially after trauma of the spinal cord (7, 8), or even through penetrating blood vessels (8). It has been postulated in one study that the presence of Schwann cells in the CNS may delay regeneration following spinal cord injury (2) whereas two other studies have shown a supportive role for Schwann cells in the axonal elongation across transaction lesions in the spinal cords of adult rats (1, 3). An alternative hypothesis, put forward by Rubinstein (9), favors an ectopic derivation of the Schwann cells situated in the dorsal gray horn and its vicinity (zona terminalis of Lissauer) and in all cases described by him were found adjacent to schwannoma of the dorsal nerve root. According to this hypothesis these cells have been displaced centrally during ontogenesis and can potentially develop under specific stimuli to intramedullary schwannosis. In support of this statement is the frequent association of these lesions with other cellular ectopias, in particular those of ependymal cells (9). An alternative interpretation was given by Hori who proposed a regenerative reaction of peripheral nervous system elements in response to a local neoplastic lesion (e.g. ependymoma) that through its mass effect could result in focal spinal cord destruction or compression (4). Rubinstein (9) also described "angioneuromatosis" or "schwannosis of the spinal blood vessels" as a special form of schwannosis that involves the perivascular sheaths of the spinal blood vessels. This special type of schwannosis has been regarded as evidence of collateral regeneration from intra- or extramedullary neurites since they may also contain aberrant axons (9). These lesions could also lead to the development of microscopic neuromatous nodules with expansion of the perivascular space (9) or even become neoplastic, thus evolving into small true central schwannomas (5, 9).
Contributed by Dimitrios N. Kanakis MD, PhD, Thomas Kamphausen MD, Johannes Van de Nes MD, PhD