Ultrastructure of thalamic neuronal inclusions in myotonic dystrophy.

Recently, the presence of thalamic neuronal cytoplasmic inclusions in patients with myotonic dystrophy has been reported. At the ultrastructural level, the inclusions were described "containing a fibrillar material within a limiting membrane studded on its outer surface with ribosomes". We have studied the brain of a 48-year-old woman with myotonic dystrophy. Many neuronal inclusion bodies were found within the thalamus, and examined in the electron microscope. Inclusions were found to have an electron density resembling that of nuclear chromatin, were not membrane-bound, and had an internal structure composed of parallel alternating dark and pale lines. We confirm the previously-reported existence of cytoplasmic thalamic neuronal inclusions in myotonic dystrophy, but differ in our observations of their ultrastructural appearance, and note that these inclusions bear no resemblance to previously described inclusion bodies.