CHAPTER 34 MYOCLONUS
The term myoclonus originates from report of a case by Friedreich in 1881 with the title of “paramyoclonus multiplex.” The patient was a 50-year-old man manifesting involuntary small muscle jerks mostly in the resting state. Myoclonus is defined as involuntary shocklike movements associated with sudden contraction of skeletal muscles (positive myoclonus), sudden interruption of the ongoing muscle contraction (negative myoclonus), or a combination of the two.
CLASSIFICATION
Myoclonus can originate from either the central nervous system or the peripheral nervous system, but most of myoclonic jerks occur in association with disorders of the central nervous system.1 It can originate from the motor cortex, brainstem, and spinal cord, and there are some other forms of myoclonus whose source has not been clarified completely (Table 34-1). Cortical myoclonus occurs either spontaneously or through a reflex mechanism in response to external stimulus (cortical reflex myoclonus). Epilepsia partialis continua is a focal, continuous form of cortical myoclonus, usually involving the distal part of the upper or lower limb. Cortical myoclonus is often epileptic in nature and thus is also called epileptic myoclonus. Palatal tremor, reticular reflex myoclonus, and startle syndrome are known to originate from brainstem structures. There are two forms in spinal myoclonus: segmental and propriospinal. Periodic myoclonus and dystonic myoclonus are easily recognized from their unique clinical features, but their underlying mechanisms have not been elucidated precisely.
CLINICAL FEATURES
Cortical myoclonus appears as brisk, shocklike movements involving fingers, hands, arms, facial muscles, and/or legs, and sometimes trunk muscles, independently (Fig. 34-1). When hand intrinsic muscles are involved, it appears as small twitches of each individual finger or a group of fingers. When, in contrast, proximal muscles of an extremity are involved, it appears as big jerks. When the jerks rapidly spread from proximal to distal muscles of an extremity, it appears as if the whole extremity is involved almost simultaneously. Moreover, a jerk of one hand can be followed by another jerk in the other hand by a very short time interval: in fact, as short as 10 milliseconds, corresponding to the transcallosal conduction time. In this case, it appears as if both upper extremities are almost simultaneously involved. Cortical myoclonus appears rhythmic when it repeats in the same muscle groups at a fast rate (7 to 8 Hz), and thus it often resembles tremor (cortical tremor) (Fig. 34-2). Rhythmic cortical myoclonus is commonly seen in corticobasal ganglionic degeneration, familial adult myoclonic epilepsy, postanoxic myoclonus, and Angelman’s syndrome.
Cortical myoclonus sometimes manifests as negative myoclonus, which is caused by sudden interruption of the ongoing muscle contraction (silent period of the electromyogram [EMG]). Most of the negative myoclonus are either immediately preceded or immediately followed by abrupt muscle contraction (positive myoclonus), but on occasion, the isolated form of negative myoclonus is seen. Thus, the pure negative myoclonus can be easily overlooked unless the extremity is examined during sustained muscle contraction: for example, while the wrists are kept in an extended posture. When the trunk muscles are suddenly involved by negative myoclonus, the patient may fall down abruptly (drop attack). On occasion, negative myoclonus is induced by somatosensory or photic stimulus through a transcortical reflex mechanism (cortical reflex negative myoclonus).
Palatal tremor used to be called palatal myoclonus, but the name was changed after the first International Congress of Movement Disorders, held in Washington, D.C., in 1990, because of the lack of shocklike features and its resemblance to tremor, especially when other skeletal muscles are also involved. Essential palatal tremor is characterized by repetitive elevation of the soft palate at a rate of 2 to 3 Hz, often associated with ear click. Familial cases of essential palatal tremor have been reported. The movement may be associated with repetitive, brisk muscle contractions of other cranial muscles, which are approximately synchronous with the palatal movement. Symptomatic palatal tremor consists of rhythmic vertical oscillation of the soft palate and is frequently associated with rhythmic vertical oscillation of eyes (ocular myoclonus). This condition is commonly associated with organic lesions of brainstem or cerebellum and often involves other cranial and extremity muscles. In this condition, the movement of extremities is not very shocklike and may resemble real tremor. This form of palatal tremor may be persistent even during sleep.
Underlying mechanisms have not been disclosed for other kinds of myoclonus, including periodic myoclonus and dystonic myoclonus. There are two representative forms of periodic myoclonus; one seen in CJD and the other seen in subacute sclerosing panencephalitis. Periodic myoclonus seen in CJD is quasi-periodic repetition of shocklike, quasi-synchronous jerks involving extremities and facial muscles at a rate of about 1 Hz. It might shift from one extremity to others and continue during sleep, although the rate and the periodicity might change from time to time. It is often associated with periodic synchronous discharge (PSD) on electroencephalographic (EEG) recording, but there is no fixed time relationship between PSD and periodic myoclonus in this condition (Fig. 34-3).