Introduction

Primary blepharospasm is a common adult-onset focal dystonia, characterized by involuntary contractions of the periocular muscles that result in forceful eye closure and impaired opening and closing of the eyes (Marsden, 1976; Berardelli et al., 1985; Defazio et al., 2007). The severity of blepharospasm can vary from repeated frequent blinking, causing only minor discomfort, to persistent forceful closure of the eyelids leading to functional blindness (Fig. 8.1). Blepharospasm can be caused by tonic or phasic contractions of the orbicularis oculi muscles and may also be associated with levator palpebrae muscle inhibition (apraxia of eyelid opening) or involuntary movements in the lower face and masticatory muscles (Meige’s syndrome). In most cases, blepharospasm is considered primary and is only occasionally secondary to structural brain lesions or drug therapy (Jankovic, 2006).

Pathophysiology

Neurophysiological recordings of the blink reflex have given important insights into the pathophysiology of blepharospasm. In patients with blepharospasm, the recovery cycle of the R2 component of the blink reflex is enhanced, presumably owing to a lack of brainstem interneuronal inhibition (Berardelli et al., 1985, 1998). Blepharospasm is also associated with an abnormal responsiveness of the blink reflex to sensory stimuli. Studies using magnetic brain stimulation also suggest a loss of inhibition and increased plasticity in the central nervous system of patients with blepharospasm (Defazio et al., 2007; Quartarone et al., 2008).

Clinical features and pathophysiology

Primary blepharospasm is a common adult-onset focal dystonia, characterized by involuntary contractions of the periocular muscles resulting in forceful eye closure, and impairing normal opening and closing of the eyes (Marsden,1976; Berardelli et al., 1985). The severity of blepharospasm can vary from repeated frequent blinking, causing only minor discomfort, to persistent forceful closure of the eyelids leading to functional blindness (Figure 7.1). Blepharospasm can be caused by tonic or phasic contractions of the orbicularis oculi muscles and may also be associated with levator palpebrae muscle inhibition (apraxia of eyelid opening) or involuntary movements in the lower face or jaw muscles (Meige's syndrome). In most cases blepharospasm is considered primary and is only occasionally secondary to structural brain lesions or drug induced (Jankovic, 2006).

Neurophysiological recordings of the blink reflex have given important insight into the pathophysiology of blepharospasm. In patients with blepharospasm, the recovery cycle of the R2 component of the blink reflex is enhanced, presumably owing to a lack of brain stem interneuronal inhibition (Berardelli et al., 1985, 1998). Blepharospasm is also associated with an abnormal responsiveness of the blink reflex to sensory stimuli. Recent studies with the technique of magnetic brain stimulation also suggest a loss of inhibition and increased plasticity in the central nervous system of patients with blepharospasm.

Anatomy of the periocular muscles

Knowledge of the anatomy of the upper facial muscles is essential for treating patients with blepharospasm.

Introduction

Movement disorders are a disease group related to functional or structural abnormalities of the basal ganglia.

The term “basal ganglia” refers to the following structures: the striatum (caudate and putamen), globus pallidus (internal and external segments, GPi and GPe), subthalamic nucleus, and substantia nigra (SN) (SN pars compacta, SNpc, and SN pars reticulata, SNpr). The striatum receives connections from specific cortical areas and from the SNpc; the basal ganglia output nuclei, the GPi and SNpr, exert an inhibitory effect on the thalamus. Diminished phasic activity in the GPi/SNpr disinhibits the thalamus thus facilitating cortical motor areas, whereas increased phasic activity in the GPi/SNpr causes the opposite effect. The GPi–SNpr inhibition of the thalamus is modulated through two parallel pathways. According to the classical model of basal ganglia functioning (Albin et al., 1989; Alexander & Crutcher, 1990; Alexander et al., 1990; De Long, 1990; Parent & Hazarati, 1995; Wichmann & De Long, 1996) the first is an inhibitory “direct” pathway that originates in the striatum and projects directly onto the GPi/SNr; the second is an “indirect” inhibitory pathway that crosses the GPe and subthalamic nucleus to project indirectly onto the GPi/SNpr. Activation of the direct pathway tends to disinhibit the thalamus. Activation of the indirect pathway disinhibits the subthalamic nucleus thereby increasing GPi/SNpr excitation thus resulting in increased thalamic inhibition: the two parallel circuits have an opposing action on the GPi/SNr and hence on the thalamus.

Magnetoencephalography (MEG): physiological background

Magnetoencephalography (MEG) is a non-invasive technique able to spatially identify the synchronous firing neurons in restricted cortical areas, either for spontaneous cerebral activity or in response to an external stimulus. MEG is unaffected by scalp and skull, and preferentially reflects the tangential component of dipoles in the depth of gyri and sulci.

The neuromagnetic technique consists of measurement of the magnetic field over the scalp, as generated by the bioelectrical currents in the brain. In order to achieve the sensitivity needed to measure these very weak magnetic fields (about 10 as compared to the earth magnetic field), the use of new superconducting magnetometers (superconducting quantum interferences devices or SQUID) and of devoted shielding is mandatory. Under the symmetry conditions, well approximated in the case of the head, it can be shown that the component of the magnetic field perpendicular to the skull is mostly sensitive to the tangential component of the primary current source, and negligibly to the field generated by the volume currents and to the distortions, smearing effects and filtering of frequency components, mainly in the faster rhythms caused by the intervening tissues (Romani & Rossini, 1988; Okada et al., 1999). This represents an advantage in respect to the purely electric measurement of neural activity, especially for the postsynaptic component at the level of the cortical mantle. The magnetic field is simultaneously measured over many scalp sites, with a rapid sampling in the time domain, and from these data the isofield contour maps are calculated and studied.