Fletcher CD, Gustafson P, Rydholm A, et al: Clinicopathologic re-evaluation of 100 malignant fibrous histiocytomas: prognostic relevance of subclassification, J Clin Oncol 19:3045–3050, 2001. [Seminal article reassessing the diagnosis of malignant fibrous histiocytoma (MFH) allowing more specific classification.] http://ebooksmedicine.net studentconsult.com
Miscellaneous Neuromuscular Junction Schwannomas and Neurofibromatosis Patterns of Peripheral Nerve Injury
Patterns of Skeletal Muscle Injury and Neurofibromatosis Type 1
Inherited Disorders of Skeletal Muscle
Acquired Disorders of Skeletal Muscle
The peripheral nerves and skeletal muscles permit purposeful movement and provide the brain with sensory information about our surroundings. Both the anatomic distribution of lesions and their associated signs and symptoms are helpful in classifying neuromuscular diseases. The following discussion of neuromuscular disorders is organized along anatomic lines and traces the neuromuscular system from proximal to distal by discussing disorders of peripheral nerves, neuromuscular junctions, and skeletal muscle.
The two major functional elements of peripheral nerves are axonal processes and their myelin sheaths, which are made by Schwann cells. Axonal diameter and myelin thickness are correlated with each other and with conduction velocity. These characteristics are used to distinguish different types of axons, which mediate distinct sensory inputs and motor function. Light touch, for example, is transmitted by thickly myelinated large-diameter axons with fast conduction velocities, whereas temperature sensation is transmitted by slow, unmyelinated thin axons. In the case of myelinated axons, one Schwann cell makes and maintains exactly one myelin segment, or internode, along a single axon (
Fig. 22.1A ). Adjacent internodes are separated by the nodes of Ranvier along which saltatory conduction occurs. Any given nerve contains axons of different sizes and axons serving different functions. These are arranged into fascicles ensheathed by a layer of perineurial cells. Perineurial cells form a barrier between endoneurium on the inside of the fascicle and epineurium on the outside.
Peripheral neuropathies are often subclassified as axonal or demyelinating, even though many diseases exhibit mixed features. Axonal neuropathies are caused by insults that directly injure the axon. The entire distal portion of an affected axon degenerates. Axonal degeneration is associated with secondary myelin loss (
Fig. 22.1B). The term
Wallerian degeneration is borrowed from experimental studies involving the transection of nerves to describe analogous changes observed in axonal neuropathies. Regeneration takes place through axonal regrowth and subsequent remyelination of the distal axon (
Fig. 22.1C ). The morphologic hallmark of axonal neuropathies is a decrease in the density of axons, which in electrophysiologic studies correlates with a decrease in the signal strength or amplitude of nerve impulses.
Demyelinating neuropathies are characterized by damage to Schwann cells or myelin with relative axonal sparing, resulting in abnormally slow nerve conduction velocities but preserved amplitude. Demyelination typically occurs discontinuously, affecting individual inter-nodes along the length of an axon in a random distribution. This process is termed segmental demyelination (
Fig. 22.1B ). Morphologically, demyelinating neuropathies show a relatively normal density of axons and features of segmental demyelination and repair. This is recognized by the presence of axons with abnormally thin myelin sheaths and short internodes (
Fig. 22.1C ). The discontinuous nature http://ebooksmedicine.net
Fig. 22.1 Patternsofperipheralnervedamage.(A)Innormalmotorunits,typeIandtypeIImyofibers(see