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.

TABLE 34-1 Classification of Myoclonus

CLINICAL FEATURES

Myoclonic jerks are usually detectable by visual observation without much difficulty. However, when the jerks are small, palpation of the corresponding muscles helps in identifying the myoclonus. Because most myoclonic jerks are associated with co-contraction of agonist and antagonist muscles, it is useful to palpate, in the case of hand myoclonus, the wrist flexors and extensors simultaneously.

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.

Figure 34-1 Electroencephalographic (EEG)-electromyographic (EMG) polygraph recorded from a patient with progressive myoclonus epilepsy. EMG discharges associated with cortical myoclonus are abrupt and very brief in duration, involving one or more muscles of the right (Rt) upper extremity almost synchronously. There are frequent sharp spikes or multiple spikes on EEG, some of which are associated with the myoclonus. EEG electrode placement is in approximate accordance with the International 10-20 System as indicated by a prime symbol (′). ECR, extensor carpi radialis muscle; 1stDI, first dorsal interosseous muscle.

Figure 34-2 Electromyographic (EMG) polygraph obtained from a patient with clinical diagnosis of corticobasal ganglionic degeneration. Repetitive discharges are seen in the right (RT) biceps and triceps muscles almost synchronously at an approximate rate of 7 to 8 Hz. br., brachii; Lt, left.

Cortical myoclonus is induced or enhanced when the patient attempts to move or actually moves the corresponding part of the body or other parts of the body (action myoclonus). Furthermore, it is often stimulus-sensitive; for example, jerks are elicited by tendon tap during neurological examination. In this case, it appears as if the deep tendon reflex is exaggerated, but the cortical reflexmyoclonus occurs slightly later than the expected time for the monosynaptic spinal reflex. In addition, it can be differentiated from the enhanced deep tendon reflex in that cortical reflex myoclonus spreads to other parts of the corresponding extremity (e.g., from distal to proximal muscles) and even to the contralateral extremity. In patients with spastic paraparesis, the deep tendon reflex in response to the patellar tendon tap may be recognized also in the contralateral leg; a typical example is the crossed adductor reflex. This phenomenon, however, is the result of simultaneous mechanical activation of the proprioceptive input to the contralateral spinal segment by the knee tap, which causes a visible reflex in the contralateral leg as a result of the hyperactive state of the contralateral anterior horn cells.

Furthermore, cortical myoclonus is elicited or enhanced when the corresponding limb is passively moved or when its posture is changed. These maneuvers are thought to induce a kind of proprioceptive reflex myoclonus. Cortical reflex myoclonus is sometimes elicited by flash stimulus and is noticed when the pupillary light reflex is tested (photic cortical reflex myoclonus). Typical examples have been reported in the advanced stage of Creutzfeldt-Jakob disease (CJD).

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).

Epilepsia partialis continua manifests as continuous, repetitive focal muscle jerks at the rate of 1 to 6 Hz, localized unilaterally to one finger, several fingers, or a foot.

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.

Reticular reflex myoclonus is a rare form of reflex myoclonus, characterized by shocklike jerks first appearing in the sternocleidomastoid and/or trapezius muscles and then spreading rostrally to the masseter, orbicularis oris, and orbicularis oculi muscles in this order, as well as caudally from proximal to distal muscles of extremities. Diagnosis of this condition requires demonstration of the characteristic spread of jerks by polygraphic recording of surface EMG from different muscles.

Startle syndrome is a group of diseases characterized by exaggerated startle responses to sudden unexpected acoustic or tactile stimuli. Physiologically, this condition is considered to be an exaggerated form of physiological startle reaction. Familial startle disease, or hyperexplexia, is mostly an autosomal dominant disorder characterized by muscular rigidity in the neonatal period and the exaggerated startle responses. This condition has drawn special attention in relation to the discovery of heterogeneous mutation of genes encoding inhibitory glycine receptors.

Segmental spinal myoclonus is seen as brisk contraction of muscles innervated by a certain spinal segment. It is often quasi-rhythmic or periodic and may be stimulus-sensitive. Propriospinal myoclonus involves mainly trunk muscles. Each jerk starts at a certain segmental level, most commonly at the thoracic segments, and spreads rostrally as well caudally at a slow speed of approximately 5-10 m/second. Within each individual patient, the jerk starts always from the same segment, although it may shift one or two segments during the course of illness. The term propriospinal is derived from the propriospinal tract, which connects successive spinal segments. The causative lesion in spinal myoclonus, either segmental or propriospinal, is often difficult to be identified even by extensive laboratory investigation.

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).

Figure 34-3 Electroencephalographic (EEG)-electromyographic (EMG) polygraph recorded from a patient with Creutzfeldt-Jakob disease. The rectified EMG discharge associated with jerks involving the left upper limb is relatively long in duration and varies from one to the other jerk in its waveform. The myoclonus is associated with periodic synchronous discharge on the EEG, but their time relationship is not fixed.