Benign Essential Blepharospasm is a Disorder of Neuroplasticity: Lessons From Animal Models

Trigeminal nerve blocks attenuate afferent input to the puff-induced blink reflex in chronic orbital pain assessed by high-speed video

PURPOSE

To assess the trigeminal blink reflex in chronic orbital pain and its modification by orbital anesthetic injections using a novel blink reflexometer.

METHODS

The EyeStat (Generation 3, Blinktbi, Inc. Charleston, SC) is a device that triggers and analyzes the carbon dioxide puff-evoked trigeminal blink response. In this prospective study, CO2 puffs were delivered to each eye of 6 patients with unilateral, chronic orbital pain at baseline and 30 minutes after unilateral orbital anesthetic injections consisting of lidocaine and bupivacaine. Ten healthy subjects underwent the testing once without injections. Main outcome measures were comparison of the average of the right and left eyelid responses to stimuli given to the right or left eye (to assess afferent response in the right vs left eye) and comparison of the right eyelid to left eyelid responses to both the right and left stimuli (to assess efferent response in the right vs left eye).

RESULTS

In 10 healthy patients, direct blink response of the stimulated eyelid exceeded the consensual response of the opposite eyelid (latency p = .001, excursion p = .04, time to close p = .03). Among the 6 pain patients, eyelid kinematics for afferent trigeminal input were similar between the affected eye and the fellow eye. However, after injection, the afferent latency of the affected eye was significantly prolonged relative to the unaffected eye (p = .006). Efferent blink responses were similar between the two eyelids at all time points.

CONCLUSION

Peribulbar anesthetic injections modulate mechanical afferent input to the trigeminal blink reflex.

The pathogenesis of blepharospasm

Blepharospasm is a focal dystonia characterized by involuntary tetanic contractions of the orbicularis oculi muscle, which can lead to functional blindness and loss of independent living ability in severe cases. It usually occurs in adults, with a higher incidence rate in women than in men. The etiology and pathogenesis of this disease have not been elucidated to date, but it is traditionally believed to be related to the basal ganglia. Studies have also shown that this is related to the decreased activity of inhibitory neurons in the cerebral cortex caused by environmental factors and genetic predisposition. Increasingly, studies have focused on the imbalance in the regulation of neurotransmitters, including dopamine, serotonin, and acetylcholine, in blepharospasm. The onset of the disease is insidious, and the misdiagnosis rate is high based on history and clinical manifestations. This article reviews the etiology, epidemiological features, and pathogenesis of blepharospasm, to improve understanding of the disease by neurologists and ophthalmologists.

Differential expression of striatal proteins in a mouse model of DOPA-responsive dystonia reveals shared mechanisms among dystonic disorders

Dystonia is characterized by involuntary muscle contractions that cause debilitating twisting movements and postures. Although dysfunction of the basal ganglia, a brain region that mediates movement, is implicated in many forms of dystonia, the underlying mechanisms are unclear. The inherited metabolic disorder DOPA-responsive dystonia is considered a prototype for understanding basal ganglia dysfunction in dystonia because it is caused by mutations in genes necessary for the synthesis of the neurotransmitter dopamine, which mediates the activity of the basal ganglia. Therefore, to reveal abnormal striatal cellular processes and pathways implicated in dystonia, we used an unbiased proteomic approach in a knockin mouse model of DOPA-responsive dystonia, a model in which the striatum is known to play a central role in the expression of dystonia. Fifty-seven of the 1805 proteins identified were differentially regulated in DOPA-responsive dystonia mice compared to control mice. Most differentially regulated proteins were associated with gene ontology terms that implicated either mitochondrial or synaptic dysfunction whereby proteins associated with mitochondrial function were generally over-represented and proteins associated with synaptic function were largely under-represented. Remarkably, nearly 20% of the differentially regulated striatal proteins identified in our screen are associated with pathogenic variants that cause inherited disorders with dystonia as a sign in humans suggesting shared mechanisms across many different forms of dystonia.

Macaque monkey trigeminal blink reflex circuits targeting orbicularis oculi motoneurons

The trigeminal blink reflex plays an important role in protecting the corneal surface from damage and preserving visual function in an unpredictable environment. The closing phase of the human reflex, produced by activation of the orbicularis oculi (ObOc) muscles, consists of an initial, small, ipsilateral R₁ component, followed by a larger, bilateral R₂ component. We investigated the circuitry that underlies this reflex in macaque (Macaca fascicularis and Macaca mulatta) monkeys by the use of single and dual tracer methods. Injection of retrograde tracer into the facial nucleus labeled neurons in the principal trigeminal nucleus, and in the spinal nucleus pars oralis and interpolaris, bilaterally, and in pars caudalis, ipsilaterally. Injection of anterograde tracer into the principal trigeminal nucleus labeled axons that directly terminated on ObOc motoneurons, with an ipsilateral predominance. Injection of anterograde tracer into pars caudalis of the spinal trigeminal nucleus labeled axons that directly terminated on ipsilateral ObOc motoneurons. The observed pattern of labeling indicates that the reticular formation ventromedial to the principal and spinal nuclei also contributes extensive bilateral input to ObOc motoneurons. Thus, much of the trigeminal sensory complex is in a position to supply a monosynaptic drive for lid closure, and the adjacent reticular formation can supply a disynaptic drive. These findings indicate that the assignment of the R₁ and R₂ components of the blink reflex to different parts of the trigeminal sensory complex cannot be exclusively based on subdivision connectional relationships with facial motoneurons. The characteristics of the R₂ component may be due, instead, to other circuit properties.

Screening Gene Mutations in Chinese Patients With Benign Essential Blepharospasm

Objective: This study aimed to screen gene mutations in Chinese patients with benign essential blepharospasm (BEB) to understand its etiology.

Methods: Twenty BEB patients diagnosed by clinical manifestations between April 2015 and October 2015 were enrolled. All the cases were investigated by questionnaires about general conditions, social behavioral factors, environmental factors, psychological factors, genetic factors, and previous diseases. In each patient, a total of 151 genes related to movement disorders were analyzed by second-generation sequencing.

Results: Two patients had a family history of BEB, and they had SYNE1 and Cdkn1A-interacting zinc finger protein 1 (CIZ1) mutation, respectively. We found the SYNE1 mutation in seven patients, the CIZ1 mutation in two patients, the CACNA1A mutation in two patients, the LRRK2 mutation in two patients, and the FUS mutation in two patients. The C10orf2, TPP1, SLC1A3, PNKD, EIF4G1, SETX, PRRT2, SPTBN2, and TTBK2 mutations were found in only one patient, respectively, while not any mutation in the 151 genes were found in two patients. Some patients had mutations in two genes.

Conclusion: Genetic factors, especially SYNE1 and CIZ1 mutations, contribute to the etiology of BEB.

Treatment of Blepharospasm/Hemifacial Spasm

OPINION STATEMENT

The treatment of both hemifacial spasm (HFS) and blepharospasm (BEB) requires making the appropriate clinical diagnosis. Advance imaging and electrophysiologic studies are useful; however, one's clinical suspicion is paramount. The purpose of this review is to summarize current and emerging therapies for both entities. Botulinum toxin (BTX) remains the first-line therapy to treat both conditions. If chemodenervation has failed, surgery may be considered. Due to the risks associated with surgery, the benefits of this option must be carefully weighed. Better surgical outcomes are possible when procedures are performed at tertiary centers with experienced surgeons and advanced imaging techniques. Microvascular decompression is an efficacious method to treat HFS, and myectomy is an option for medication-refractory BEB; the risks of the latter may outweigh any meaningful clinical benefits. Oral agents only provide short-term relief and can cause several unwanted effects; they are reserved for patients who cannot receive BTX and/or surgery. Transcranial magnetic stimulation has gained some traction in the treatment of BEB and may provide safer non-invasive options for refractory patients in the future.

A Dynamic Circuit Hypothesis for the Pathogenesis of Blepharospasm

Blepharospasm (sometimes called “benign essential blepharospasm,” BEB) is one of the most common focal dystonias. It involves involuntary eyelid spasms, eye closure, and increased blinking. Despite the success of botulinum toxin injections and, in some cases, pharmacologic or surgical interventions, BEB treatments are not completely efficacious and only symptomatic. We could develop principled strategies for preventing and reversing the disease if we knew the pathogenesis of primary BEB. The objective of this study was to develop a conceptual framework and dynamic circuit hypothesis for the pathogenesis of BEB. The framework extends our overarching theory for the multifactorial pathogenesis of focal dystonias (Peterson et al., 2010) to incorporate a two-hit rodent model specifically of BEB (Schicatano et al., 1997). We incorporate in the framework three features critical to cranial motor control: (1) the joint influence of motor cortical regions and direct descending projections from one of the basal ganglia output nuclei, the substantia nigra pars reticulata, on brainstem motor nuclei, (2) nested loops composed of the trigeminal blink reflex arc and the long sensorimotor loop from trigeminal nucleus through thalamus to somatosensory cortex back through basal ganglia to the same brainstem nuclei modulating the reflex arc, and (3) abnormalities in the basal ganglia dopamine system that provide a sensorimotor learning substrate which, when combined with patterns of increased blinking, leads to abnormal sensorimotor mappings manifest as BEB. The framework explains experimental data on the trigeminal reflex blink excitability (TRBE) from Schicatano et al. and makes predictions that can be tested in new experimental animal models based on emerging genetics in dystonia, including the recently characterized striatal-specific D1R dopamine transduction alterations caused by the GNAL mutation. More broadly, the model will provide a guide for future efforts to mechanistically link multiple factors in the pathogenesis of BEB and facilitate simulations of how exogenous manipulations of the pathogenic factors could ultimately be used to prevent and reverse the disorder.