Rhythmic palatal myoclonus and the dentato-olivary pathway.

The anatomical basis of palatal myoclonus and related rhythmic skeletal myoclonus is described. The most constant lesion is a special type of degeneration with hypertrophy of the olivary nucleus of the medulla oblongata, on the side opposite to the myoclonus when it is unilateral. This degeneration is usually secondary to a primary lesion located either in the ipsilateral (to the hypertrophied olive) central tegmental tract or in the contralateral dentate nucleus. To link these data, Trelles (1935, 1943) suggested a dentato-olivary pathway from the dentate nucleus to the contralateral inferior olive via the superior cerebellar peduncle and the central tegmental tract after crossing the midline. The existence of this pathway was demonstrated by Lapresle and Ben Hamida (1965-1971), first by showing a topistic relationship between dentate nucleus and contralateral inferior olive, then by delineating this pathway in the vicinity of the red nucleus at the crossing of the superior cerebellar peduncle and the central tegmental tract. The significance of these lesions with their ensuing symptoms is discussed. It is considered as a transsynaptic degeneration which probably reveals an archaic phenomenon, submerged but not lost through evolution.