Objectives-To describe the neuropathological features of clinical syndromes associated with tomacula or focal myelin swellings in sural nerve biospies and to discuss possible common aetiopathological pathways leading to their formation in this group of neuropathies. Methods-Fifty two patients with sural nerve biopsies reported to show tomacula or focal myelin swellings were reviewed, light and electron microscopy were performed, and tomacula were analysed on teased fibre studies. Molecular genetic studies were performed on those patients who were available for genetic testing. Results-Thirty seven patients were diagnosed with hereditary neuropathy with liability to pressure palsies (HNPP), four with hereditary motor and sensory neuropathy type I (HMSN I) or CharcotMarie-Tooth disease type 1 (CMT1), four with HMSN with myelin outfolding (CMT4B), three with IgM paraproteinemic neuropathy, three with chronic inflammatory demyelinating polyneuropathy (CIDP), and one with HMSN III (CMT3). Conclusions-Most of these syndromes were shown to be related to genetic or immunological defects of myelin components such as peripheral myelin protein 22 (PMP22), myelin protein zero (P0), or myelin associated glycoprotein (MAG). These proteins share the HNK-1 epitope which has been implicated in cell adhesion processes. Impaired myelin maintenance may therefore contribute to the formation of tomacula and subsequent demyelination. (J Neurol Neurosurg Psychiatry 2000;68:483-488)

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