Palatal myoclonus appearing 8 years after removal of a cerebellar low-grade astrocytoma.

Abstract

Sirs: Palatal myoclonus (PM), also called palatal tremor, is a movement disorder characterized by involuntary rhythmic contractions of the soft palate muscles at a frequency of 100–160/min [1, 2]. PM may be essential (EPM) or symptomatic (SPM) [1, 3, 4]. The 2 forms of PM differ clinically and pathophysiologically [3]. In SPM, the movements of the soft palate are usually asymptomatic. Visual observation of the soft palate shows activation of different palatal muscles in the 2 disorders [5]. The levator veli palatini muscle is rhythmically active in SPM, whereas contractions of the tensor veli palatini are observed in EPM [5]. In SPM, movements of the soft palate may be associated with synchronous movements of the eyes, the face, the tongue, limb muscles, and the diaphragm. No other neurological signs are observed in EPM [1, 3]. SPM is caused by a lesion located in the dentato-rubro-olivary tract (also called Guillain-Mollaret triangle) [1–3, 6]. This anatomical pathway extends from the dentate nucleus via the superior cerebellar peduncle to the contralateral red nucleus. The superior cerebellar peduncles decussate just below the red nuclei. The inferior olivary nucleus receives afferences from the ipsilateral red nucleus through the central tegmental tract. Inferior olivary neurons project to the contralateral cerebellar cortex and dentate nucleus through the inferior cerebellar peduncle [1–4]. Anatomically, SPM is associated with a lesion in the central tegmental tract of the pons, a lesion in the dentate nucleus, the superior cerebellar peduncle, the decussation of the superior cerebellar peduncle or a lesion in the inferior olive [7]. Currently, it is considered that the pathway between the inferior olive and the contralateral dentate nucleus through the inferior cerebellar peduncle is always preserved in SPM. Indeed, no lesion in the inferior cerebellar peduncle has been found to be associated with SPM so far [4]. The causes of SPM are cerebrovascular diseases, brainstem tumors, multiple sclerosis, encephalitis, brainstem trauma and obstructive hydrocephalus [1–3]. PM has also been described in association with progressive supranuclear palsy [7]. SPM developing after removal of a low-grade cerebellar tumor is rare and easily overlooked. In the reported cases, patients developed SPM within 12 months after surgery [4]. We present a patient developing SPM 8 years after surgery for a low-grade cerebellar tumor. SPM occurred despite injury of the inferior cerebellar peduncle. A hypothesis explaining this unusual observation is suggested. The patient is a 55-year-old man who underwent surgery in 1991 for a grade 2 astrocytoma located in the right cerebellar hemisphere. Before and after surgery, neurological examination showed a gazeevoked nystagmus, a scanning speech, limbs dysmetria and kinetic tremor on the right side. The patient was regularly followed clinically and radiologically. The neurological deficits remained stable from 1991 until 1999. Inspection of the soft palate was normal. In 1999, he complained of bilateral earclick, predominating in the right ear. In addition to the previously observed signs, a PM at a frequency of about 100/min, without facial myoclonus, was found. Brain MRI disclosed postoperative sequelae in the right cerebellar hemisphere, involving the right inferior and middle cerebellar peduncles (Fig. 1). No olivary hypertrophy was observed, but the area of the left inferior olive seemed minimally hyperintense and shrunken compared with the opposite side. The left red nucleus was intact on MRI. There was no enhancement after gadolinium injection. Review of a previous MRI did not show hypertrophy of the inferior olive. Administration of clonazepam did not improve the PM. Botulinum toxin injection was not performed. In 2000, the patient still complained of an earclick. Brain MRI was unchanged. In February 2001, neurological examination showed synchronous movements of the face and the shoulder on the right side, in addition to the neurological signs previously described. Remote effects of the palatal tremor on the tonic electromyographic activity of the right shoulder muscles were recorded. The levator veli palatini muscle was rhythmically contracting. One critical question in our description is whether the palatal tremor was a SPM or incidental EPM, because of (1) the long delay between surgery and appearance of palatal tremor and (2) the presence of earclick, which is rarely reported in patients with SPM and is exceptional in patients with a history of posterior fossa tumor [1, 3]. The contractions of the levator veli palatini muscle and the extrapalatal involvement argue strongly in favor of a SPM [1, 3, 5]. Palatal tremor exerted remote effects on the tonic electromyographic on the side of the cerebellar signs, as reLETTER TO THE EDITORS