Botulinum toxin paralysis of the orbicularis oculi muscle. Types and time course of alterations in muscle structure, physiology and lid kinematics

Effects of Repeated Eyelid Injections with Botulinum Toxin A on Innervation of Treated Muscles in Patients with Blepharospasm

PURPOSE

To assess changes in innervation and muscle morphology after repeated botulinum toxin A injections in subjects with benign essential blepharospasm.

METHODS

Surgical waste specimens were processed for histologic examination of nerve fibers, neuromuscular junctions, fiber size, and central nucleation and compared to age matched controls and to two subjects with blepharospasm that had not received botulinum toxin A injections.

RESULTS

There was a significant increase in amount of nerve fibers and numbers of neuromuscular junctions in the orbicularis oculi muscles from subjects with blepharospasm treated repetitively with botulinum toxin A. In addition there was a significant decrease in mean muscle fiber cross-sectional area and an increase in central nucleation. The specimens from the subjects with only blepharospasm had the same density of nerves but had intermediate levels of neuromuscular junctions.

CONCLUSIONS

These data suggest that repeated injections of botulinum toxin A has an effect on nerve and neuromuscular junction numbers, which are partly mirrored in orbicularis oculi muscle from subjects with blepharospasm only. These studies suggest the potential for modulating these changes in order to extend the duration of effectiveness of botulinum toxin.

Botulinum Toxin Induced Atrophy: An Uncharted Territory

Botulinum neurotoxins (BoNTs) produce local chemo-denervation by cleaving soluble N-ethylmaleimide-sensitive factor activating protein receptor (SNARE) proteins. Botulinum neurotoxins are therapeutically indicated in several neurological disorders and have been in use for three decades. The long-term efficacy, safety, and side effects of BoNTs have been well documented in the literature. However, the development of muscle atrophy following chronic exposure to BoNTs has not received sufficient attention. Muscle atrophy is not only cosmetically distressing, but also has an impact on future injections. An extensive literature search was conducted on atrophy and mechanisms of atrophy. Five hundred and four relevant articles in the English language were reviewed. This review revealed the surprising lack of documentation of atrophy within the literature. In addition, as demonstrated in this review, the mechanisms and the clinical factors that may lead to atrophy have also been poorly studied. However, even with this limited information it is possible to indicate factors that could modify the clinical approach to botulinum toxin injections. This review highlights the need for further study of atrophy following BoNT injections.

Animal models for investigating benign essential blepharospasm

The focal dystonia benign essential blepharospasm (BEB) affects as many as 40,000 individuals in the United States. This dystonia is characterized by trigeminal hyperexcitability, photophobia, and most disabling of the symptoms, involuntary spasms of lid closure that can produce functional blindness. Like many focal dystonias, BEB appears to develop from the interaction between a predisposing condition and an environmental trigger. The primary treatment for blepharospasm is to weaken the eyelid-closing orbicularis oculi muscle to reduce lid spasms. There are several animal models of blepharospasm that recreate the spasms of lid closure in order to investigate pharmacological treatments to prevent spasms of lid closure. One animal model attempts to mimic the predisposing condition and environmental trigger that give rise to BEB. This model indicates that abnormal interactions among trigeminal blink circuits, basal ganglia, and the cerebellum are the neural basis for BEB.

Conditioned eyelid movement is not a blink

Based on kinematic properties and distinct substrates, there are different classes of eyelid movement described as eyeblinks. We investigate whether the eyelid movements made in response to a conditioned stimulus (CS) are a category of eyelid movements distinct from blinks. Human subjects received 60 trials of classical eyelid conditioning with a tone as the CS and electrical stimulation of the supraorbital branch of the trigeminal nerve as the unconditioned stimulus (UCS). Before and after training, reflex blinks were elicited with the UCS. The kinematics of conditioned responses (CRs) differed significantly from those of reflex blinks. The slope of the amplitude-maximum velocity function was steeper for reflex blinks than for CRs, and reflex blink duration was significantly shorter than CR duration. Unlike reflex blinks, for which maximum velocity was independent of blink duration, the maximum velocity of CRs depended on CR duration. These quantitative and qualitative differences indicated that CRs were a unique class of eyelid movements distinct from blinks and eyelid movements with vertical saccadic gaze shifts.

Acute and long-term effects of botulinum neurotoxin on the function and structure of developing extraocular muscles

Strabismus is a misalignment of the visual axes, due to an imbalance in extraocular muscle (EOM) function. Botulinum neurotoxin (BoNT) treatment can correct the misalignment with permanent therapeutic effects in infants, possibly because the toxin causes structural alterations in developing EOM. To determine whether BoNT indeed permanently weakens developing EOMs, we examined the chicken oculomotor system. Following injections of BoNT in hatchling chicks, we quantified physiological parameters (contractile force measurements) and morphological parameters (myofiber morphometry, innervation, quantitative transmission electron microscopy of mitochondria/fiber types). Treatment of developing EOM with BoNT caused acute reductions of muscle strength and mitochondrial densities, but minimal changes in muscle fiber diameter and neuromuscular junction structures. Contrary to expectations, contractile force was fully recovered by 3-4 months after treatment. Thus, permanent therapeutic effects of BoNT most likely do not cause permanent changes at the level of the peripheral effector organ, but rather involve central (CNS) adaptive responses.

Histology of nerves and muscles in adductor spasmodic dysphonia

To elucidate the etiology and pathophysiology of spasmodic dysphonia, we examined the adductor branch of the recurrent laryngeal nerve and the lateral cricoarytenoid muscle from 9 consecutive patients with this disorder who were previously treated with botulinum toxin. Histologic examination revealed average muscle fiber diameters ranging from 21 to 57 microm. Botulinum toxin treatment-related muscle atrophy was observed up to 5 months after injection. Endomysial fibrosis was present in all samples. Histochemical analysis in 8 patients revealed type 2 fiber predominance in 7 patients and fiber type grouping in 2. Type-specific muscle fiber size changes were not present. Nerve samples were examined in plastic sections. In 8 patients the nerves contained homogeneous, large-diameter myelinated nerve fibers and sparse small fibers. One patient had a relatively increased proportion of small myelinated nerve fibers. Overall, the nerve fiber diameter was slightly larger in patients than in controls. These findings may implicate the central nervous system in the pathophysiology of adductor spasmodic dysphonia.

Serial SFEMG studies of orbicularis oculi muscle after the first administration of botulinum toxin

Serial single fiber electromyography (SFEMG) examinations of orbicularis oculi muscle in patients with blepharospasm or hemifacial spasm treated with botulinum toxin injections were performed. The aim of the study was to evaluate the impairment of neuromuscular transmission, to follow reinnervation after botulinum toxin administration and to find out whether there was a relationship between SFEMG parameters and clinical symptoms. Examinations were performed before injection, during early and late remission of symptoms, and after recurrence of the involuntary movement. Severe impairment of neuromuscular transmission, as revealed by increased jitter and increased presence of abnormal potential pairs and pairs with blocking, was found in early remission, but fiber density remained unchanged when compared with pretreatment values. In late remission, increased fiber density was registered for the first time. The recurrence of involuntary movements was related to the further increase of fiber density and tendency to normalization of jitter parameters. The study therefore suggests that formation of new neuromuscular junctions and their functional maturation is responsible for muscle recovery after botulinum toxin administration.

Muscle fiber-type changes induced by botulinum toxin injection in the rat larynx

This study examined muscle fiber-type alterations after single or multiple botulinum toxin (BT) injections to better understand possible morphologic changes induced by therapeutic BT injections in patients with spasmodic dysphonia. Muscle fiber staining was accomplished in rat intrinsic laryngeal muscles with antibodies to specific myosin heavy chains. Results indicated that the typical baseline distributions of type II muscle fibers (ie, types IIa, IIb, IIx, and IIL) were altered by BT injection, while no change was observed in type I fibers. Embryonic fibers were observed only along the needle insertion site at 7 days post BT injection. Although inferences from these animal data to human neuromuscular function must be made with caution, our findings provide insight into the possible cellular and molecular changes characterizing BT-injected muscles.

Increased acute and chronic mitotic activity in rat laryngeal muscles after botulinum toxin injection

OBJECTIVES

To characterize the acute and chronic cellular effects of botulinum toxin (BT) injection into rat laryngeal muscles. A complete characterization of these effects is important because patients with focal dystonias of the head and neck are commonly treated with BT injection. Further, potential muscular changes in the larynx must be carefully delineated owing to the critical phonatory and airway protective functions of these muscles.

STUDY DESIGN

The acute and chronic cellular effects of BT injection were studied using 5'-bromo 2'-deoxyuridine (BrdU) following single and repeated BT injection into rat laryngeal muscles. BrdU is incorporated into mitotically active nuclei such that changes in cell proliferative behavior following BT injection can be monitored.

RESULTS

Increased mitotic activity was detected in the tissue samples studied following BT injection. Differences in the times of the peak distribution of BrdU-labeled cells in each laryngeal muscle were observed. This may be related to the diffusion effects of BT. Prolonged muscle fiber changes, including splitting, were also observed as the result of repeated BT injection.

CONCLUSIONS

The results of this study suggest that BT may induce a proliferative response in muscle tissue.

The long-term use of botulinum toxin for adult strabismus

PURPOSE

To characterize patients choosing repeated botulinum toxin injection as a treatment for their strabismus, and assess their demand for it over time (up to 8 years).

METHOD

Patients who had undergone at least eight injections were identified and their clinical records analyzed for diagnosis, demographic details, and demand for toxin injections with time. To establish any predictive variables, the details for these cases were compared with those of matched controls who had undergone fewer injections. Lastly, a questionnaire was mailed to research patients' views as to the indications and preferences for regular toxin injection as a method of treatment.

RESULTS

Ninety-five patients were identified (34 men, 61 women; median age 37 years), of whom 35 had consecutive exotropia and 16 had secondary exotropia. Other diagnoses represented were residual and primary deviations, restrictive exotropias, and oscillopsias. A trend of fewer injections over the attendance period was seen, and the only complication observed was upper lid ptosis in 1% of injections. Cases exhibited similar demographic composition to matched controls, but were more likely to have consecutive exotropia or secondary esotropia as a diagnosis. Univariate analysis showed evidence (p < 0.001) of an association between the number of previous operations and the odds of being a case. No evidence was found that cases and controls lived at differing distances from our hospital. The questionnaire found that 71% of patients stated appearance as the prime reason for seeking treatment, and 37% stated simplicity of toxin therapy as their reason for reattendance. Twenty-six percent of the patients were disillusioned with the results of previous surgery and preferred toxin therapy as a means of controlling their symptoms.

CONCLUSION

Botulinum toxin injection is an appropriate long-term treatment for some strabismus patients who choose not to undergo further surgery. A trend toward fewer injections with time was observed, and no adverse effects were associated with long-term treatment.

Effects of botulinum neurotoxin type A on abducens motoneurons in the cat: alterations of the discharge pattern

The discharge characteristics that abducens motoneurons exhibit after paralysis of the lateral rectus muscle with botulinum neurotoxin type A were studied in the alert cat. Antidromically identified motoneurons were recorded during both spontaneous and vestibularly induced eye movements. A single injection of 0.3 ng/kg produced a complete paralysis of the lateral rectus muscle lasting for about 12-15 days, whereas after 3 ng/kg the paralysis was still complete at the longest time checked, three months. Motoneurons recorded under the effect of the low dose showed differences in their sensitivities to both eye position and velocity according to the direction of the previous and ongoing movements, respectively. These directional differences could be explained by post-saccadic adaptation of the non-injected eye in the appropriate direction for reducing ocular misalignment. Thus, backward and forward post-saccadic drifts accompanied on- and off-directed saccades, respectively. The magnitude of the drift was similar to the magnitude of changes in eye position sensitivity. The discharge of the high-dose-treated motoneurons could be described in a three-stage sequence. During the initial 10-12 days, motoneuronal discharge resembled the effects of axotomy, particularly in the loss of tonic signals and the presence of exponential-like decay of firing after saccades. In this stage, the conduction velocity of abducens motoneurons was reduced by 21.4%. The second stage was characterized by an overall reduction in firing rate towards a tonic firing at 15-70 spikes/s. Motoneurons remained almost unmodulated for all types of eye movement and thus eye position and velocity sensitivities were significantly reduced. Tonic firing ceased only when the animal became drowsy, but was restored by alerting stimuli. In addition, the inhibition of firing for off-directed saccades was more affected than the burst excitation during on-directed saccades, since in many cells pauses were almost negligible. These alterations could not be explained by adaptational changes in the movement of the non-injected eye. Finally, after 60 days the initial stages of recovery were observed. The present results indicate that the high dose of botulinum neurotoxin produces effects on the motoneuron not attributable to the functional disconnection alone, but to a direct effect of the neurotoxin in the motoneuron and/or its synaptic inputs.

Bruxism after brain injury: successful treatment with botulinum toxin-A

Bruxism, the rhythmic grinding of teeth--usually during sleep--is not an infrequent complication of traumatic brain injury. Its prevalence in the general population is 21%, but its incidence after brain injury is unknown. Untreated, bruxism causes masseter hypertrophy, headache, temporomandibular joint destruction, and total dental wear. We report a case of complete resolution of postanoxic bruxism after treatment with botulinum toxin-A (BTX-A). The patient was a 28-year-old man with no history of bruxism who sustained an anoxic brain injury secondary to cardiac arrest of unknown etiology. On admission to our rehabilitation unit 2 months after the injury, the patient presented with severe bruxism and heavy dental wear. The patient was injected with a total of 200 units of BTX-A to each masseter and temporalis. There was total resolution of bruxism 2 days after injection, with no complications. On follow-up 3 months after injection, the patient remained free of bruxism. We propose that botulinum toxin be considered as a treatment for bruxism secondary to anoxic brain injury. Further studies regarding muscle selection and medication dosage are warranted to elucidate the toxin's efficacy in this condition.

Pharmacologic characterization of botulinum toxin for basic science and medicine

The use of Botulinum neurotoxin (BoNT) is increasing in both clinical and basic science. Clinically, intramuscular injection of nanogram quantities of BoNT is fast becoming the treatment of choice for a spectrum of disorders including movement disorders such as torticollis, blepharospasm, Meige Disease, and hemifacial spasm (Borodic et al., 1991, 1994a; Jankovic and Brin, 1991; Clarke, 1992). Neuroscientists are using BoNTs as tools to develop a better understanding of the mechanisms underlying the neurotransmitter release process. Consequently, our ability to accurately and reliably quantify the biologic activity of botulinum toxin has become more important than ever. The accurate measurement of the pharmacologic activity of BoNTs has become somewhat problematic with the most significant problems occurring with the clinical use of the toxins. The biologic activity of BoNTs has been measured using a variety of techniques including assessment of whole animal responses to in vitro effects on neurotransmitter release. The purpose of this review is to examine the approaches employed to characterize, quantify and investigate the actions of the BoNTs and to provide a guide to aid investigators in determining which of these methods is most appropriate for their particular application or use.

The trigeminally evoked blink reflex. I. Neuronal circuits

In this study, we characterized the pathways that generate the trigeminal blink reflex in the guinea pig. Blinks were evoked by stimulation of the supraorbital branch of the trigeminal nerve and measured by recording electromyographic activity in the lid-closing orbicularis oculi muscle (OOemg) and, in one case, lid position. Blinks evoked by stimulation of the supraorbital nerve consisted of two bursts of muscle activity ipsilateral to the side of stimulation. The first, R1, had a latency of 6.9 ms and the second, R2, had a latency of 17.25 ms. Increasing stimulus intensity to 3 times threshold for evoking an ipsilateral blink elicited an R1 and R2 response contralaterally, with latencies of 9.2 ms and 19.25 ms, respectively. We investigated the causes for this bipartite response that is seen in the guinea pig, as well as other mammals including humans. The two-component response could arise from different populations of afferents, or from different central circuits, or a combination of these two causes. Multiunit recording in the trigeminal ganglion and simultaneous measurement of the OOemg showed that activation of A beta afferents alone was sufficient to elicit both the R1 and the R2 responses, but that activation of A delta afferents could enhance both responses. Different neural circuits, however, produce the R1 and R2 responses. Transganglionic tracing with wheatgerm agglutin or choleragenoid subunit of cholera toxin bound to HRP revealed that primary afferents from the supraorbital branch of the trigeminal nerve terminated densely in the dorsal horn of spinal cord segment C1 and in the caudalis-interpolaris border region of the spinal trigeminal nucleus. Injections of HRP into the orbicularis oculi motoneuron region of the facial nucleus showed that both of these regions projected to the facial nucleus. Hemisections at the level of C1 eliminated the R2 blink response, but not the R1 response, evoked by stimulation of the supraorbital branch of the trigeminal nerve. Subsequent hemisections at the level of the obex eliminated the R1 response. Microinjections of the GABAB agonist baclofen into the spinal trigeminal nucleus at the level of the obex abolished the R1 but not the R2 response. Thus, the spinal trigeminal nucleus produces the R1 component, whereas the R2 component originates in the C1 region of the spinal cord.

Extraocular muscles: basic and clinical aspects of structure and function

Although extraocular muscle is perhaps the least understood component of the oculomotor system, these muscles represent the most common site of surgical intervention in the treatment of strabismus and other ocular motility disorders. This review synthesizes information derived from both basic and clinical studies in order to develop a better understanding of how these muscles may respond to surgical or pharmacological interventions and in disease states. In addition, a detailed knowledge of the structural and functional properties of extraocular muscle, that would allow some degree of prediction of the adaptive responses of these muscles, is vital as a basis to guide the development of new treatments for eye movement disorders.