Episodic ataxia type 2 and
familial hemiplegic migraine are two rare hereditary disorders that are linked to dysfunctional
ion channels and are characterized clinically by paroxysmal neurological symptoms. Impaired regulation of cerebral excitability is thought to play a role in the occurrence of these paroxysms, but the underlying mechanisms are poorly understood. Normal
ion channels are crucial for coordinating neuronal firing in response to facilitatory input. Thus, we hypothesized that channel dysfunction in
episodic ataxia type 2 and
familial hemiplegic migraine may impair the ability to adjust cerebral excitability after facilitatory events. We tested this hypothesis in patients with
episodic ataxia type 2 (n = 6), patients with
familial hemiplegic migraine (n = 7) and healthy controls (n = 13). All subjects received a high-frequency burst (10 pulses at 20 Hz) of
transcranial magnetic stimulation to transiently increase the excitability of the motor cortex. Acute burst-induced excitability changes were probed at 50, 250, 500 and 1000 ms after the end of the burst. This was done using single-pulse
transcranial magnetic stimulation to assess corticospinal excitability, and paired-pulse
transcranial magnetic stimulation at an interstimulus interval of 2 and 10 ms to assess intracortical inhibition and facilitation, respectively. The time course of burst-induced excitability changes differed between groups. Healthy controls showed a short-lived increase in excitability that was only present 50 ms after the burst. In contrast, patients with
episodic ataxia type 2 showed an abnormally prolonged increase in corticospinal excitability that was still present 250 ms after the
transcranial magnetic stimulation burst. Furthermore, while controls showed a decrease in intracortical facilitation during the 1 s period following the
transcranial magnetic stimulation burst, patients with
episodic ataxia type 2 had increased intracortical facilitation 1000 ms after the burst. Intracortical inhibition was unaltered between groups. Patients with
familial hemiplegic migraine were not significantly different from either controls or patients with
episodic ataxia type 2. Together, these findings indicate that patients with
episodic ataxia type 2 have an excessive increase in motor cortex excitability following a strong facilitatory input. We argue that this deficient control of cortical excitability may set the stage for the emergence of paroxysmal neural dysfunction in this disorder.