Myokymia with neonatal epilepsy
Overview
KCNQ2 and KCNQ3 are two homologous K+ channel subunits that can combine to form heterotetrameric channels with properties of neuronal M channels. Loss-of-function mutations in either subunit can lead to benign familial neonatal convulsions (BFNC), a generalized, idiopathic epilepsy of the newborn. We now describe a syndrome in which BFNC is followed later in life by myokymia, involuntary contractions of skeletal muscles. All affected members of the myokymia/BFNC family carried a mutation (R207W) that neutralized a charged amino acid in the S4 voltage-sensor segment of KCNQ2. This substitution led to a shift of voltage-dependent activation of KCNQ2 and a dramatic slowing of activation upon depolarization. Myokymia is thought to result from hyperexcitability of the lower motoneuron, and indeed both KCNQ2 and KCNQ3 mRNAs were detected in the anterior horn of the spinal cord where the cells of the lower motoneurons arise. We propose that a difference in firing patterns between motoneurons and central neurons, combined with the drastically slowed voltage activation of the R207W mutant, explains why this particular KCNQ2 mutant causes myokymia in addition to BFNC. Myokymia is characterized by spontaneous involuntary contraction of muscle fiber groups that can be observed as vermiform movement of the overlying skin. Electromyography typically shows continuous motor unit activity with spontaneous oligo- and multiplet-discharges of high intraburst frequency (myokymic discharges). Localized myokymic activity can have its cause in circumscribed disturbances of peripheral nerves or the central nervous system, e.g., in snake poisoning, hypoxia, radiation therapy, pontine tumors, or multiple sclerosis. Generalized myokymia, with or without associated muscle stiffness and delayed relaxation is a feature of Isaacs' syndrome (including acquired neuromyotonia) (1), episodic ataxia type 1 (2), and Morvan's fibrillary chorea (3). In some patients the spontaneous muscle movement occurs without apparent underlying cause, and the frequently positive family history (≈30%) indicates that genetic factors are among the possible causes of idiopathic generalized myokymia (IGM) (4). IGM affects men and women equally, and the disease often comes to their attention because of stiffness, cramps, weakness, and muscle twitching. The clinical features can include generalized myokymia, hyporeflexia, grip myotonia, and calf hypertrophy. The symptoms often improve with carbamazepine or phenytoin treatment
Symptoms
There is evidence that one of the main pathophysiological mechanisms of peripheral nerve hyperexcitability in myokymia is a suppression of outward potassium currents. Anti-voltage-gated potassium channel antibodies are found in autoimmune Isaacs' syndrome, a disorder characterized by neuromyotonia, increased cramping, and excessive sweating (5, 6). Sera from patients with Isaacs' syndrome suppressed voltage-gated outward potassium currents in PC-12 cells (7) and induced repetitive firing of action potentials in posterior root ganglion cells (6). A similar suppression of voltage-gated K+ currents was observed with serum from a patient with Guillain-Barré syndrome, a disease often showing transient myokymic discharges during early stages (8, 9). Antibodies to voltage-gated K+ channels are also detected in patients with Morvan's fibrillary chorea (3). Beside the autoimmune pathophysiology in Isaacs' syndrome, myokymic discharges can also be caused by inherited defects of K+ channels. Point mutations of the Shaker-related voltage-gated K+ channel gene Kv1.1 (KCNA1) result in episodic ataxia type 1, an autosomal dominant neurological disorder characterized by continuous myokymia, episodic attacks of cerebellar ataxia, and, sometimes, partial epilepsy (10). Some mutations lead to myokymia and epilepsy without ataxic episodes, or myokymia only, indicating differences in the sensitivity of the central and peripheral nervous system toward KCNA1-caused increased neuronal excitability (11)
Causes
KCNQ2 and KCNQ3 are two homologous K(+) channel subunits that can combine to form heterotetrameric channels with properties of neuronal M channels. Loss-of-function mutations in either subunit can lead to benign familial neonatal convulsions (BFNC), a generalized, idiopathic epilepsy of the newborn. We now describe a syndrome in which BFNC is followed later in life by myokymia, involuntary contractions of skeletal muscles. All affected members of the myokymia/BFNC family carried a mutation (R207W) that neutralized a charged amino acid in the S4 voltage-sensor segment of KCNQ2. This substitution led to a shift of voltage-dependent activation of KCNQ2 and a dramatic slowing of activation upon depolarization. Myokymia is thought to result from hyperexcitability of the lower motoneuron, and indeed both KCNQ2 and KCNQ3 mRNAs were detected in the anterior horn of the spinal cord where the cells of the lower motoneurons arise. We propose that a difference in firing patterns between motoneurons and central neurons, combined with the drastically slowed voltage activation of the R207W mutant, explains why this particular KCNQ2 mutant causes myokymia in addition to BFNC