There are two key types of tumours that can afflict the peripheral nervous system, and they are known as schwannomas and neurofibromas. The types of cells that are affected by these growths are fibroblasts, Schwann cells and perineurial cells (cells around the nerve fibres). A feature which is present in every human being is that when the nerves leave the brain and travel to the rest of the body, the type of cell which covers the nerve changes. Within the brain, the nerves are covered by oligodendrocytes, but as the nerves descend, the nerves are covered by Schwann cells. This transition happens fairly close to the dura, so the tumours are capable of spreading to the meninges.
Schwannomas, as the name suggests, are tumours of the Schwann cells that cover points along the axon, creating a broadband connection to the rest of the body, if you will. For more details regarding Schwann cells and how they work with nerves, see Structure of a Neuron. When Schwann cells grow out of control, they can lead to compression of the spine or to the brain. A condition where this can be seen is acoustic neuroma: where a patient may exhibit hearing loss and tinnitus (ringing in the ears) because a tumour has developed within the cranial vault and against the eighth cranial nerve. When seen with the naked eye, they are grey, firm masses. When the tumours develop within the dura, sensory nerves are preferentially involved, but outside it, the tumours often are located at large nerve bundles where motor and sensory nerves mix.
Neurofibromas are associated with the condition neurofibromatosis, particularly type 1. The most common types neurofibroma happens in the skin (known as cutaneous neurofibroma) and in the peripheral nerves ( known as solitary neurofibroma). They are nodular in appearance and occasionally there is extra pigmentation in the local area. They grow within the dermis and in the fat under the skin, and the major concern regarding these growths is the cosmetic changes rather than conversion to the malignant variety: these tumours are generally benign in nature. The other major type of neurofibroma is the the plexiform neurofibroma: they most commonly arise within the nerve trunks, although they may grow anywhere along the nerves.
How does the type of tumour affect the surgical outcome?
In this case, the type of tumour significantly influences the surgical outcome: schwannomas are fairly easy to remove, whereas plexiform neurofibromas are relatively difficult to remove. This is because the schwannomas are easy to separate from the nerve, but because the plexiform neurofibromas are greatly inter-weaved within the nerve trunks, they are pretty much impossible to remove.
Malignant Peripheral Nerve Sheath Tumours
Unlike the other two types of tumours, malignant peripheral nerve sheath tumours are both malignant and invasive in nature: they may metastasize and even when resected, they may grow back again. You may think that these tumours are the malignant form of schwannomas, but they are not: instead they arise in one of two circumstances: either from radiation therapy or from transformation of plexiform neurofibromas. Necrosis and anaplastic cells are very common with these tumours, and under the microscope, they resemble Schwann cells, albeit they are poorly defined.