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.

Ghrelin as a novel locally produced relaxing peptide of the iris sphincter and dilator muscles

Ghrelin is a recently described acylated peptide, which works as a somatosecretagogue and has described effects on the smooth, skeletal and cardiac muscle. We examined the production and effects of ghrelin on relaxation of the iris muscles. Contractile effects of 1-5 human ghrelin (frGhr, 10(-9)-6 x 10(-5)M) and 1-5 human des-octanoyl-ghrelin (d-frGhr; 10(-9)-6 x 10(-5)M) were tested on iris rabbit sphincter (n=11 frGhr; n=7 d-frGhr), dilator (n=6 frGhr; n=6 d-frGhr) and rat sphincter (n=6 frGhr; n=8 d-frGhr) precontracted muscles. On rabbit sphincter the effect of frGhr was also tested in presence of: i) L-NA (10(-5)M; n=7); ii) indomethacin (10(-5)M; n=7); iii) DLys(3)GHRP6 (10(-4)M; n=6); and iv) apamin+carybdotoxin (10(-6)M; n=6). Furthermore, on rabbit dilator the effect of frGhr was tested in presence of DLys(3)GHRP6 (10(-4)M; n=7). Finally, ghrelin mRNA production was assessed by "in situ" hybridization in Wistar rat eyes (n=8). In all muscles, frGhr promoted a concentration-dependent relaxation, maximal at 6 x 10(-5)M, 1.5-3 min after its addition, decreasing tension by 34.1+/-12.1%, 25.8+/-4.8% and 52.1+/-10.3% in the rabbit sphincter, dilator and rat sphincter, respectively. In the rabbit sphincter the relaxing effects of frGhr were: (i) enhanced in presence of DLys(3)GHRP6 (118.1+/-21.1%); (ii) blunted by indomethacin; and (iii) not altered by apamin+carybdotoxin (36.4+/-14.4%) or L-NA (52.4+/-11.4%). Relaxing effects of d-frGhr in rabbit (43.3+/-5.2%) and rat (77.1+/-15.3%) sphincter muscles were similar to those of frGhr. In rabbit dilator muscle, d-frGhr did not significantly alter active tension and the relaxing effect of frGhr was blunted by GHSR-1a blockage. Ghrelin mRNA was identified in iris posterior epithelium. In conclusion, ghrelin is a novel, locally produced, relaxing agent of iris dilator and sphincter muscles, an effect that is mediated by GHSR-1a in the former, but not in the latter. Furthermore, in the sphincter it seems to be mediated by prostaglandins, but not by NO or K(Ca) channels.

Botulinum toxin injection of the inferior oblique muscle for the treatment of superior oblique muscle palsy

PURPOSE

To evaluate botulinum toxin injection of the inferior oblique muscle for management of superior oblique (SO) paresis.

METHODS

We undertook a prospective case series of injections of the ipsilateral inferior oblique (IO) muscle with 10-20 units of botulinum toxin type A in patients with a SO muscle palsy/paresis of less than 2 years' duration.

RESULTS

We enrolled 16 patients (18 eyes) with a mean age of 33.7 years. The median time from onset of symptoms until injection was 6 months. The cause of paresis was trauma for 81% of the patients. The mean hypertropia decreased from 6.4(Delta) to 1.9(Delta) at 6 months after treatment. Other clinical measures improved: mean IO overaction from + 1.7 to + 0.6, mean SO underaction from -1.5 to -0.4, mean subjective torsion from 9.3 degrees to 0.4 degrees , and mean head tilt from 8.4 degrees to 1.1 degrees.

CONCLUSIONS

Botulinum toxin injection to IO muscle reduces the symptoms of SO paresis while patients are waiting for recovery.

Use of botulinum toxin in strabismus

PURPOSE OF REVIEW

The history, pharmacology, indications, complications, and success rates of botulinum administration are discussed in this review as they relate to strabismus and associated conditions.

RECENT FINDINGS

Botulinum has been used to treat strabismus and a variety of other ocular conditions for over three decades. Alan Scott initially investigated extraocular muscle paralysis by botulinum injection in 1973, and in the ensuing years botulinum has been evaluated as a treatment for horizontal and vertical strabismus, nystagmus, dissociated vertical deviation, sensory strabismus, ophthalmoplegia, and paradoxical diplopia.

SUMMARY

Botulinum administration is a strabismus treatment option that may be considered for some strabismus patients; in select cases botulinum is a reasonable and preferable alternative to surgery. Current clinical recommendations regarding botulinum should be considered in the context of the existing comparative studies that are summarized in this review. Since there are very limited data directly comparing the success rates of botulinum with standard strabismus surgery, additional, well designed prospective studies are needed. Data from such studies would enable recommendations for strabismus management to be formulated using an evidence-based approach.

Increasing muscle strength as a treatment for strabismus: sustained release of insulin-like growth factor-1 in rabbit extraocular muscle

PURPOSE

Currently, no drug treatment is available for strengthening underacting extraocular muscles (EOM) in strabismus. We showed previously that single injections of insulin-like growth factor (IGF-1) result in significant but short-term increases in muscle force generation. This study examined the effects of sustained release of IGF-1 on force generation in rabbit superior rectus muscles.

METHODS

In adult rabbits, slow-release pellets containing IGF-1 were implanted on the global side of one superior rectus muscle. After 1 week, or 1, 2, 3, or 6 months, treated and control muscles were examined for force generation using an in vitro physiology apparatus. All muscles were prepared for histology and mean myofiber cross-sectional areas were determined.

RESULTS

One and 3 months after pellet implantation, treated muscles generated significantly greater force than contralateral control muscles, whereas at 2 months, no significant difference was found. Force per cross-sectional area (mN/cm(2)) at 3 months also increased significantly in the treated muscles. Mean muscle cross-sectional area increased significantly after 1, 2, and 3 months of sustained exposure to IGF-1 compared with controls. After an additional 3 months without IGF-1 exposure, mean cross-sectional areas were significantly greater than controls but significantly reduced compared with areas at 1, 2, and 3 months.

CONCLUSIONS

IGF-1 appears to be highly effective in increasing muscle force generation. Because slow release of IGF-1 results in sustained increases in EOM force generation, it may be a potentially useful alternative to surgical resection procedures because it avoids many of the potential long-term biomechanical hazards of resection surgery.

Imaging of the superior rectus in superior rectus overaction after retrobulbar anesthesia

OBJECTIVE

Vertical diplopia after cataract surgery caused by an overaction of an extraocular muscle is more common when the superior rectus muscle is involved, whereas contracture is more common when the inferior rectus muscle is involved. However, no documented imaging has been presented. The aim of this report was to study the superior rectus in such a patient with magnetic resonance imaging (MRI).

DESIGN

Observational case report.

METHODS

Ophthalmologic examination and thin-sectioned MRI across the superior rectus muscle were performed in a patient with superior rectus overaction after cataract surgery.

MAIN OUTCOME MEASURES

Ocular alignment, ocular movement, and the superior rectus muscle on MRI.

RESULTS

Magnetic resonance imaging disclosed focal thickening of the superior rectus muscle near the orbital apex in a patient who showed superior rectus overaction after retrobulbar anesthesia for cataract extraction.

CONCLUSIONS

The focal thickening of the superior rectus muscle in this patient is consistent with the theory that segmental contracture leads to overactive muscles after retrobulbar anesthesia.

Increased extraocular muscle strength with direct injection of insulin-like growth factor-I

PURPOSE

Previous work has demonstrated the effectiveness of insulin-like growth factor (IGF)-II in increasing force generation in extraocular muscle (EOM). Studies in the literature have suggested that IGF-I would be even more effective than IGF-II. This study was performed to assess the effects on muscle mass and force generation of IGF-I injection in adult rabbit superior rectus muscle.

METHODS

Adult rabbits received a single injection of IGF-I at one of several doses into one superior rectus muscle. One week after treatment, the rabbits were euthanatized, and the superior rectus muscle from each orbit was removed. Force generation was measured using an in vitro apparatus, and injected muscles were compared with the contralateral control. A second group of animals were injected similarly, and the muscles were examined at 1 week for changes in cross-sectional area of individual myofibers.

RESULTS

EOMs demonstrate significant numbers of cells expressing the IGF receptor. After the EOMs were injected with IGF-I, there were significant increases both in muscle force generation and cross-sectional area at all doses tested in this study. Doses of 10 and 25 microg IGF-I were most effective.

CONCLUSIONS

Direct muscular injection of IGF-I effectively increases EOM force generation without the potential biomechanical hazards of surgery such as permanently altered muscle length or insertional position on the globe.

Use of high-dose botulinum A toxin in benign essential blepharospasm: is too high too much?

BACKGROUND

Botulinum toxin (Botox) is the mainstay treatment for benign essential blepharospasm. Current treatment practice appears restricted by several reports demonstrating adverse effects and resistance to high-frequency, higher-dose therapy. This study aimed to explore whether high-dose, high-frequency treatments could be used without developing secondary resistance and without significant side-effects in patients refractory to conventional Botox doses.

METHODS

From a cohort of 120 patients being treated with Botox therapy for benign essential blepharospasm and idiopathic hemifacial spasm, case notes from six patients were retrospectively examined. In these patients, therapy had exceeded the recommended 50 units per side for a duration greater than 12 months and at less than 3 monthly intervals. Patterns in subjective severity grading and percentage of improvement as well as reported side-effects were analysed.

RESULTS

All patients described greater than 60% improvement and 0-2 severity grading over a 3- to 15-year period with no evidence of secondary resistance. Side-effects were minor, transient and less frequently reported at higher doses.

CONCLUSION

In a select group of patients, Botox therapy can be used effectively at doses higher than recommended over long periods with minimal side-effects and little evidence of secondary resistance.

Unrecognized side effect of statin treatment: unilateral blepharoptosis

A 43-year-old man receiving statin monotherapy (10 mg atorvastatin) for hypercholesterolemia had unilateral blepharoptosis as the result of isolated myositis of the levator muscle. Statin-induced myositis in the levator muscle should be considered in the differential diagnosis of acquired unilateral blepharoptosis of unknown cause.

Accommodative spasm after laser-assisted in situ keratomileusis (LASIK)

PURPOSE

To report a case of accommodative spasm following laser-assisted in situ keratomileusis (LASIK).

DESIGN

Observational case report.

METHODS

Ophthalmic examination including manifest and cycloplegic refraction to assist in diagnosis.

RESULTS

Two months following LASIK surgery, a 41-year-old healthy woman complained of intolerable headaches that had interfered with her daily activities. The patient sought medical consultation, and an MRI of the brain was normal. Best-corrected visual acuity (BCVA) was 20/20 and 20/25 with manifest refraction of -2.25 -0.50 x 170 and plano in the right and the left eye, respectively. Cycloplegic refraction was plano in the right and +0.75 sphere in the left eye, and she was corrected to 20/20 bilaterally. After being treated with cyclopentolate 1% one drop at bedtime to each eye for approximately six weeks, the headache completely resolved.

CONCLUSIONS

Accommodative spasm should be considered in patients undergoing LASIK surgery complicated with fluctuating refraction of uncertain causes.

Transient diplopia following maxillary local anesthetic injection

A 36-year-old female patient developed diplopia and an ipsilateral lateral rectus paresis following local anesthetic administration to remove a left maxillary second molar. Complete resolution occurred within 3 hours. The clinical examination and management plan are reviewed for this uncommon occurrence. The relevant anatomical pathways are discussed and illustrated with photographs.

The role of botulinum toxin A in augmentation of the effect of recession and/or resection surgery

PURPOSE

The aim of this study is to assess the effectiveness of intraoperative botulinum toxin A (BTA) injection as an adjunct to the surgical treatment of large-angle esotropia or exotropia.

METHODS

Ten patients were included in the study. Mean age of the patients was 27+/-20 years. Of these 10 patients, 7 were esotropic and 3 exotropic. The average preoperative esodeviation was 73.6+/-16.5 prism diopters and exodeviation was 76.7+/-5.8 PD. Five units of BTA were injected intraoperatively into one of the recessed horizontal rectus muscles in all of the patients.

RESULTS

The average follow-up was 14+/-10 months (range, 8 to 40 months). The average final deviation in the esotropia group was 13+/-9.6 PD of esotropia. The average final deviation in the exotropia group was 4.7+/-5 PD of exotropia. The final deviation of the 70% patients was within 10 PD of esotropia or exotropia.

CONCLUSION

The results of this study suggest that the combination of BTA injection with recession may increase the expected correction of a conventional horizontal rectus muscle surgery.

[Contralateral amaurosis and extraocular muscle palsies after retrobulbar injection]

CASE REPORT

We report a rare complication of retrobulbar anesthesia in ophthalmic surgery-amarurosis and extraocular muscle palsies in the contralateral eye. Our patient did not suffer permanent sequelae from the injection.

DISCUSSION

Numerous complications resulting from retrobulbar injections in the eye and orbit have been reported. One possible explanation of this case is the inadvertent penetration of the subdural or subarachnoid space surrounding the optic nerve and the injection of anesthetic into that space. The drug then tracks along the ipsilateral optic nerve to the chiasm and then to the contralateral optic nerve. Several methods of decreasing the probability of such a complication are discussed.

Long Time-Constant Behavior of the Oculomotor Plant in Barbiturate-Anesthetized Primate

The mechanics of the extraocular muscles and orbital tissue (“oculomotor plant”) can be approximated by a small number of viscoelastic (Voigt) elements in series. Recent analysis of the eye's return from displacement in lightly anesthetized rhesus monkeys has suggested a four-element plant model with time constants (TCs) of ∼0.01, 0.1, 1, and 10 s. To demonstrate directly the presence of long (1,10 s) TC elements and to assess their contribution quantitatively, horizontal eye displacement was induced in Cynomolgus monkeys under deep barbiturate anesthesia that prevented interference from spontaneous eye movements. The displacement was maintained for either a prolonged (30 s) or brief (0.2 s) period before release. Return to resting position took 20–30 s after prolonged displacement but only 1–2 s after brief displacement, consistent with the presence of long TC elements that would only be substantially stretched in the former condition. Quantitative fitting of the release curves after prolonged displacement indicated that the two long TC elements contribute a substantial proportion (∼30%) of the total plant compliance. A model based on the estimated compliance values is shown to account quantitatively both for our release data and for Goldstein and Robinson's data on hysteresis of ocular motoneuron firing rates measured after centripetal saccades following prolonged eccentric fixation. Long time-constant elements in the plant thus make a substantial contribution to some types of eye movement, and their inclusion in plant models can help interpret the firing patterns of single units in the oculomotor system.

Botulinum toxin treatment of extraocular muscles in rabbits results in increased myofiber remodeling

PURPOSE

Botulinum toxin A (Botox) is commonly used for strabismus treatment. Although other muscles atrophy after intramuscular injection with Botox, extraocular muscles (EOMs) do not. A continuous process of myonuclear addition in normal uninjured adult myofibers in rabbit EOMs was studied. In this study, the effect of Botox-induced muscle paralysis on myofiber remodeling in adult EOMs was examined.

METHODS

The superior rectus muscles of adult rabbits were each injected with 5 units of Botox. The contralateral muscle received injections of saline only. Bromodeoxyuridine (BrdU) was administered for various periods after Botox treatment, followed by various BrdU-free periods. Myonuclear addition, the number of BrdU-positive satellite cells, and the number of MyoD-positive satellite cells were quantified, as were alterations in expression of immature myosins.

RESULTS

Intramuscular injection of Botox resulted in a significant increase in both the number of BrdU-positive myonuclei and satellite cells. MyoD expression in both satellite cells and myonuclei was significantly increased after Botox injection in EOMs. In Botox-treated EOMs, an increased number of myofibers positive for the neonatal myosin heavy chain (MyHC) isoform was detected in the orbital layer.

CONCLUSIONS

Botox-induced EOM paralysis resulted in a significant short-term increase in satellite cell activation and myonuclear addition in single myofibers in adult rabbit EOMs compared with control muscles. The appearance of MyoD-positive myonuclei suggests that protein synthesis becomes upregulated after Botox injection, and this, in turn, may help explain the minimal effects on myofiber size in EOMs after Botox injection. Understanding the effect of Botox on satellite cell activation and myonuclear addition in existing myofibers may suggest new ways to maximize the clinical effectiveness of Botox in patients with strabismus.

Comparison of the effectiveness of mitomycin-C and Viscoat on delayed adjustable strabismus surgery in rabbits

PURPOSE

To determine a way to prevent the formation of adhesions and delay the time of suture adjustment in the course of adjustable strabismus surgery, an animal study was performed to assess and compare the effects of mitomycin C (Mit-C) and viscoelastic material Viscoat (sodium hyaluronate 3% and chondroitin sulphate 4%, Alcon, Fort Worth, TX).

METHODS

Right eyes of 47 rabbits were divided into three groups. After recession of the superior rectus muscle (SRM), Mit-C was administered beneath and over the SRM in Group M (16 eyes), Viscoat was administered beneath and over SRM in Group V (16 eyes), and ringer lactate was administered in Group C (15 eyes). SRM then recessed 5 mm with adjustable strabismus surgery technique. Animals in each group were subdivided into 1 and 2, where 1 = adjustment 1 week postoperatively and 2 = adjustment 3 weeks postoperatively. Delayed adjustment was performed in M1 group (6 eyes), V1 group (6 eyes), and C1 group (7 eyes) after 1 week; in M2 group (6 eyes), V2 group (6 eyes), and C2 group (6 eyes) after 3 weeks. Histopathologic examinations were performed for the remaining 10 eyes without suture adjustment at the end of first and third weeks after adjustable strabismus surgery. The possible length and the necessary force to adjust and the degree of adhesions were recorded.

RESULTS

When length and strength of the adjustment, adhesions between muscle and sclera, and adhesions between muscle and conjunctiva were taken into consideration, there was no statistically significant difference among M, V, and C groups at postoperative weeks 1 and 3. The force needed for adjustment in M1 group was statistically lower than M2 group. CONCLUSIONS. The intraoperative use of Mit-C (0.4 mg/mL) may decrease adhesion formation in the early postoperative period, especially in the first week. The intraoperative use of Viscoat was not effective in reducing postoperative adhesions and delaying adjustment.

Transynaptic effects of tetanus neurotoxin in the oculomotor system

The question whether general tetanus arises from the independent sum of multiple local tetani or results from the actions of the transynaptic tetanus neurotoxin (TeNT) in higher brain centres remains unresolved. Despite the blood-borne dissemination of TeNT from an infected wound, the access to the central nervous system is probably prevented by the blood-brain barrier. However, several long-term sequelae (e.g. autonomic dysfunction, seizures, myoclonus, and sleep disturbances) present after the subsidence of muscle spasms might be indicative of central actions that occur farther away from lower motoneurons. Subsequently, the obvious entry route is the peripheral neurons followed by the transynaptic passage to the brain. We aimed at describing the pathophysiological correlates of TeNT translocation using the oculomotor system as a comprehensive model of cell connectivity and neuronal firing properties. In this study, we report that injection of TeNT into the medial rectus muscle of one eye resulted in bilateral gaze palsy attributed to firing alterations found in the contralaterally projecting abducens internuclear neurons. Functional alterations in the abducens-to-oculomotor internuclear pathway resembled in part the classically described TeNT disinhibition. We confirmed the transynaptic targeted action of TeNT by analysing vesicle-associated membrane protein2 (VAMP2) immunoreactivity (the SNARE protein cleaved by TeNT). VAMP2 immunoreactivity decreased by 94.4% in the oculomotor nucleus (the first synaptic relay) and by 62.1% presynaptic to abducens neurons (the second synaptic relay). These results are the first demonstration of physiological changes in chains of connected neurons that are best explained by the transynaptic action of TeNT on premotor neurons as shown with VAMP2 immunoreactivity which serves as an indicator of TeNT activity.

Effect of hyaluronidase on ocular motility and eyelid function in sub-Tenon's anaesthesia: randomised controlled trial

PURPOSE

To assess the effect of hyaluronidase on eye and eyelid movements when used as an adjunct in sub-Tenon's anaesthesia.

METHODS

A total of 60 patients who had sub-Tenon's anaesthesia prior to phacoemulsification surgery were divided into two equal groups in a double-masked randomised controlled fashion. Of these, Group A had 4 ml lignocaine 2%, while Group B had 4 ml lignocaine 2% with the addition of sodium hyaluronidase 75 IU/ml. Ocular motility, levator, and orbicularis oculi function were measured in all patients at 5 and 8 min. Levator function was scored from 0 (no function) to 3 (complete function) while orbicularis function was scored from 0 to 2. The score for ocular motility was the sum in four positions of gaze, each position scoring from 0 to 2. Results were compared using a nonparametric test.

RESULTS

Group B achieved significantly better ocular and lid akinesia than Group A both at 5 and 8 min with P<0.01. The median scores for levator function at 5 and 8 min were 2 for Group A and 0 for Group B. For orbicularis function, the median scores at both time intervals were 2 for Group A and 1 for Group B. For ocular motility, the median score for Group A at 5 min was 3 and at 8 min was 2.5; for Group B at 5 min was 0.5 and at 8 min was 0.

CONCLUSIONS

The addition of hyaluronidase in sub-Tenon's anaesthesia has a significant effect in improving ocular and lid (levator and orbicularis) akinesia.

[Electromyograph assistance and Mendonça's forceps--a comparison between two methods of botulinum toxin A injection into the extraocular muscle]

PURPOSE

To compare two methods of botulinum toxin A (BTA) injection into the extraocular muscle (EOM): the electromyographically (EMG) guided injection and the injection using Mendonça's forceps.

METHODS

Twenty-nine (29) patients with strabismus and low visual acuity in one eye were examined at the Department of Ophthalmology of UNIFESP. They were divided into 2 groups -- group I with 17 patients receiving the botulinum toxin A injection using Mendonça's forceps, and group II with 12 patients receiving the toxin with electromyographical guidance. The patients of both groups were examined on the 7th and 14th day after intervention and the outcome of both methods of botulinum toxin A injection were compared. Friedman and Mann-Whitney correlation tests were used in the statistical analysis of the data.

RESULTS

Although the follow-up examinations on the 7th and 14th days showed a different behavior between both methods, there was no statistically significant difference between the mean values of the correction attained in both groups at the end of this study.

CONCLUSION

No statistically significant difference was shown between the two groups. Mendonça's forceps can be a safe alternative to electromyography to locate an extraocular muscle for botulinum toxin A injection.

Paradoxical Use of Frontalis Muscle and the Possible Role of Botulinum A Toxin in Permanent Motor Relearning

PURPOSE

To report 7 patients with paradoxical use of the frontalis muscle despite postsurgical correction of ptosis with good postoperative eyelid position. Successful treatment with botulinum A toxin facilitated motor relearning and cessation of muscle contraction.

DESIGN

Interventional case series.

PARTICIPANTS

Seven patients, in 2 eye-plastic clinics, who underwent successful surgical correction of upper eyelid ptosis.

METHODS

Review of clinical history, clinical photographs, treatment, and follow-up.

MAIN OUTCOME MEASURES

Frontalis muscle contraction and upper eyelid position.

RESULTS

Patients underwent successful surgical correction of ptosis but continued using the frontalis muscle despite good eyelid position postoperatively. Frontalis contraction ceased spontaneously in 2 patients, but required botulinum A toxin injection in 5. The effects of a single treatment of botulinum A toxin lasted from 3 months to 2 years, longer than the expected effect of the toxin.

CONCLUSION

Patients with long-standing eyelid ptosis may paradoxically continue utilizing the frontalis after successful surgical correction and despite good postoperative eyelid position. Cessation of frontalis contraction can be achieved with a single injection of botulinum A toxin. We hypothesize that chemodenervation, achieved with the toxin, may influence the central nervous system to relearn the set point for muscle contraction and may be associated with permanent motor relearning. Spontaneous resolution of muscle contraction can occur in the first months after surgery.