Advanced Subconjunctival Anesthesia for Cataract Surgery.

PURPOSE: Ocular regional blocks generally require blind instrumentation to the posterior orbit, leading to rare but serious complications. However, topical anesthesia does not suppress eye or lid movements and may lead to more surgical complications. Advanced subconjunctival anesthesia (ASCAN) is a technique developed to provide reliable akinesia and anesthesia without anterior dissection or blind intrusion into the posterior orbit, while allowing visualization of the needle-tip position. DESIGN: Nonrandomized case series at a rural health service. METHODS: Advanced subconjunctival anaesthesia was performed on 60 elective adult patients undergoing phacoemulsification surgery. The technique involves piercing the conjunctiva and Tenon's capsule in the superior outer quadrant of the globe with a 25-gauge, 16 mm needle, using either lignocaine 2% plain or in equal mix with bupivicaine 0.5%. Up to 10 mL of anesthetic with hyaluronidase 30 IU/mL is injected in a posterior direction into the sub-Tenon's space. Ocular motor functions were assessed 10 minutes after ASCAN using a Brahma scale. Pain was assessed during surgery and at the end of surgery after subconjunctival injection of antibiotic and steroid using a verbal Numeric Rating Scale. RESULTS: All patients completed surgery without needing supplemental anesthesia. Fifty-eight patients (97%) were pain free, whereas 2 patients experienced transient mild pain. Adequate globe akinesia and reliable lid paralysis was achieved comparable to other studies, with no major surgical or anesthetic complications. CONCLUSIONS: Advanced subconjunctival anesthesia is a visually guided, minimally invasive technique, achieving satisfactory analgesia and akinesia for phacoemulsification surgery.

Ascending vestibular drive is asymmetrically distributed to the inferior oblique motoneuron pools in a subset of hemispheric stroke survivors.

OBJECTIVE: Aberrant vestibular nuclear function is proposed to be a principle driver of limb muscle spasticity after stroke. Although spasticity does not manifest in ocular muscles, we sought to determine whether altered cortical modulation of ascending vestibuloocular pathways post-stroke could impact the excitability of ocular motoneurons. METHODS: Nineteen chronic stroke survivors, aged 49-68 yrs. were enrolled. Vestibular evoked myogenic potentials (VEMPs) were recorded from the inferior oblique muscles of the eye using surface EMG electrodes. We assessed the impact of ascending otolith pathways on eye muscle activity and evaluated the relationship between otolith-ocular function and the severity of spasticity. RESULTS: VEMP responses were recorded bilaterally in 14/19 subjects. Response magnitude on the affected side was significantly larger than on the spared side. In a subset of subjects, there was a strong relationship between affected response amplitude and the severity of limb spasticity, as estimated using a standard clinical scale. CONCLUSIONS: This study suggests that alterations in ascending vestibular drive to ocular motoneurons contribute to post-stroke spasticity in a subset of spastic stroke subjects. We speculate this imbalance is a consequence of the unilateral disruption of inhibitory corticobulbar projections to the vestibular nuclei. SIGNIFICANCE: This study potentially sheds light on the underlying mechanisms of post-stroke spasticity.

Electrophysiologic vestibular evaluation in type 2 diabetic and prediabetic patients: Air conduction ocular and cervical vestibular evoked myogenic potentials.

OBJECTIVE: Chronically increased blood glucose levels may affect the vestibular system by damaging cells and neural structures in diabetes mellitus (DM). We aimed to search the effects of neurovascular degeneration on the vestibular system in type 2 DM and prediabetic patients by using air-conducted ocular (oVEMP) and cervical (cVEMP) vestibular evoked myogenic potentials. DESIGN: Prospective study. STUDY SAMPLE: Thirty diabetic, 30 prediabetic patients, and 31 age- and sex-matched controls having no peripheral or central vestibular disease, were enrolled. All participants were evaluated by audiovestibular tests, oVEMP, and cVEMP. RESULTS: In the diabetic group, mean values of both oVEMP and cVEMP p1, n1 latencies were significantly longer compared to the prediabetic group and the control group, whereas latencies were similar in prediabetic and the control groups. Bilateral neural dysfunction was recognized in both tests and lateralization was not seen in VEMP asymmetric ratios. In the diabetic group, prevalence of pathological p1 and n1 latencies in oVEMP were 30.4% and 37.5%, whereas they were 53.7%, 59.3% in cVEMP, respectively. p1 latencies of cVEMP and oVEMP were positively correlated with HbA1c and fasting plasma glucose level in the diabetic group. CONCLUSION: Subclinical vestibular neuropathy can be a newly defined diabetes-related complication.

Jendrassik maneuver facilitates cVEMP amplitude: some preliminary observations.

BACKGROUND: The cervical vestibular evoked myogenic potential (cVEMP) is an acoustically driven electrophysiological measure of saccular and inferior nerve function that requires tonic sternocleidomastoid muscle (SCM) activity in order to be elicited. The cVEMP is gaining increased interest in the clinical and research communities based on the anatomical specificity it adds to vestibular test batteries, because it is noninvasive, and since it can be performed with instrumentation commonly found in audiology clinics worldwide. PURPOSE: Because maintaining a constant level of tonic background electromyography (EMG) over the entire course of the recording epoch is a requirement for response elicitation, active participation for some individuals including the elderly and those with cervical problems can be difficult. As a way to facilitate the response for some clinical populations, this study addressed whether cVEMPs could be modulated by remote or local changes in EMG related neural activity by applying various maneuvers during the course of the recording epoch. RESEARCH DESIGN: Keeping acoustic stimulation and recording parameters constant, three separate experimental conditions, Jendrassik maneuver, jaw (teeth) clenching, and forced-eye closure, were used to determine whether cVEMP amplitudes could be enhanced from the control condition. STUDY SAMPLE: Nine adults (2 males; 7 females) ranging in age from 24 to 42 yr with normal pure-tone hearing sensitivity and a negative history of otological disease, neurological disease, and head trauma. DATA COLLECTION AND ANALYSIS: Cervical vestibular evoked myogenic potentials were recorded from the SCM using surface electrodes in response to suprathreshold 500 Hz Blackman windowed tone bursts under a control and three experimental conditions. Three separate one-way repeated measures analyses of variance (ANOVAs) were used to evaluate the effects of these maneuvers on P1/N1 peak-to-peak amplitudes and P1 and N1 peak latencies. RESULTS: A significant main effect of experimental condition was shown to increase P1/N1 peak-to-peak cVEMP amplitude. Post hoc analysis found that Jendrassik maneuver versus control was the only the condition that produced significantly increased response amplitudes in comparison to all other post hoc contrasts. P1 and N1 peak latencies were unchanged across the various experimental conditions. CONCLUSIONS: In adults with normal hearing sensitivity and a negative history of otological disease, neurological disease, and head trauma, Jendrassik maneuver increased cVEMP amplitude by over 39% in comparison to the control condition. Such a simple modulation effect warrants further investigation for application in clinical studies.

Characteristics and clinical applications of ocular vestibular evoked myogenic potentials.

Recently, ocular vestibular evoked myogenic potentials (oVEMPs) have been described and added to the neuro-otologic test battery as a new measure for the vestibulo-ocular reflex. oVEMPs represent extraocular muscle activity in response to otolith stimulation e.g. by air-conducted sound or bone-conducted vibration. In response to vestibular stimulation, electromyographic activity of the extraocular muscles can be recorded by means of surface electrodes placed beneath the contralateral eye. oVEMPs are likely to reflect predominantly utricular function, while the widely established cervical vestibular evoked myogenic potentials (cVEMPs) assess saccular function. Thus, measuring oVEMPs and cVEMPs in addition to caloric and head impulse testing provides further evaluation of the vestibular system and enables quick and cost-effective assessment of otolith function. This review summarizes the neurophysiological properties of oVEMPs, gives recommendations for recording conditions and discusses oVEMP alterations in various disorders of the vestibular system. With increasing insight into oVEMP characteristics in vestibular disorders, e.g. Menière's disease and superior semicircular canal dehiscence syndrome, oVEMPs are becoming a promising new diagnostic tool for evaluating utricular function.

Comparison of metal and flexible sub-Tenon cannulas.

PURPOSE: To compare metal posterior, flexible posterior, and flexible medium sub-Tenon cannulas for akinesia and anesthesia. SETTING: King Khaled Specialist Eye Hospital, Riyadh, Saudi Arabia. DESIGN: Prospective blind randomized comparison. METHOD: Patients having cataract surgery under sub-Tenon block received a mixture of bupivacaine and lidocaine with hyaluronidase. The patients were divided into 3 groups based on whether a metal posterior sub-Tenon cannula, flexible posterior sub-Tenon cannula, or flexible medium sub-Tenon cannula was used. Pain during injection, akinesia, anesthesia, and complications were recorded. RESULTS: The study enrolled 159 patients. The akinesia score, superior oblique muscle activity, lid movements, and pain during injection were significantly lower in the metal posterior cannula group 2 minutes after injection (P=.002, P=.008, P=.097, and P=.001, respectively); there was no significant difference at 4 minutes, 6 minutes, 8 minutes, and 10 minutes. The incidence of intraoperative pain was low in all groups (P=.607). Chemosis was significantly less in the flexible posterior cannula group (P=.003); however, there was no significant difference in the site of chemosis (P=.460). There was no significant difference in the incidence, site, or severity of subconjunctival hemorrhage (P=.087, P=.072, and P=.331, respectively). No serious complications occurred. CONCLUSIONS: Rigid posterior, flexible posterior, and flexible medium cannulas produced effective and equivalent anesthesia. The flexible cannulas may be safer than the rigid cannulas. FINANCIAL DISCLOSURE: No author has a financial or proprietary interest in any material or method mentioned.

Maturation of prepulse inhibition (PPI) in childhood.

It is known that sensorimotor gating measured by the prepulse inhibition of the startle reflex (PPI) matures during childhood. Since certain disorders in children, for example, enuresis, show a significant loss in PPI, the PPI as a tool for investigating brainstem reflex control mechanism gains in importance. Therefore, it is crucial to know the natural course of PPI maturation in childhood. A total of 122 healthy children aged from 3-10 years and 10 healthy adults were examined. PPI was initiated by a 120 ms and a 60 ms prepulse and was measured by the EMG of M. orbicularis oculi. For the respective prepulse intervals, the PPI level in each age group increased from 3 to 9 or 10 years and showed a similar course. The findings confirm and extend knowledge about the maturation of PPI during childhood and emphasize the importance of age-dependent standard values when investigating PPI in children.

Pro- and antisaccades in children elicited by visual and acoustic targets - does modality matter?

BACKGROUND: Children are able to inhibit a prepotent reaction to suddenly arising visual stimuli, although this skill is not yet as pronounced as it is in adulthood. However, up to now the inhibition mechanism to acoustic stimuli has been scarcely investigated METHODS: Reflexive (prosaccade) and inhibitory (antisaccade) responses to visual and acoustic targets were examined with an eye tracker system in 31 children between seven and twelve years of age using a gap-overlap task and two target eccentricities. RESULTS: Acoustically cued saccades had longer reaction times than visually cued saccades. A gap effect (i.e., shorter reaction time in the gap than the overlap condition) was only found for visually elicited saccades, whereas an eccentricity effect (i.e., faster saccades to more laterally presented targets - 12° vs. 6° or rather 90° vs. 45°) was only present in the acoustic condition. Longer reaction times of antisaccades compared to prosaccades were found only in the visual task. Across both tasks the typical pattern of elevated error rates in the antisaccade condition was found. Antisaccade errors declined with age, indicating an ongoing development of inhibitory functions. CONCLUSIONS: The present results lay the ground for further studies of acoustically triggered saccades in typically as well as atypically developing children and it might thus be possible to upgrade physiological diagnostic tools.

The basis for using bone-conducted vibration or air-conducted sound to test otolithic function.

Extracellular single neuron recordings of primary vestibular neurons in Scarpa's ganglion in guinea pigs show that low-intensity 500 Hz bone-conducted vibration (BCV) or 500 Hz air-conducted sound (ACS) activate a high proportion of otolith irregular neurons from the utricular and saccular maculae but few semicircular canal neurons. In alert guinea pigs, and humans, 500 Hz BCV elicits otolith-evoked eye movements. In humans, it also elicits a myogenic potential on tensed sternocleidomastoid muscles. Although BCV and ACS activate both utricular and saccular maculae, it is possible to probe the functional status of these two sense organs separately because of their differential neural projections. Saccular neurons have a strong projection to neck muscles and a weak projection to the oculomotor system. Utricular afferents have a strong projection to eye muscles. So measuring oculomotor responses to ACS and BCV predominantly probes utricular function, while measuring neck muscle responses to these stimuli predominantly probes saccular function.

Coupling between spontaneous (resting state) fMRI fluctuations and human oculo-motor activity.

The recent discovery of incessant spontaneous fluctuations in human brain activity (also termed resting state fMRI) has been a focus of intense research in brain imaging. The spontaneous BOLD activity shows organized anatomical specialization as well as disruption in a number of brain pathologies. The link between the spontaneous fMRI fluctuations and human behavior is therefore of acute interest and importance. Here we report that a highly significant correlation exists between spontaneous BOLD fluctuations and eye movements which occur subliminally and spontaneously in the absence of any visual stimulation. Of the various eye movement parameters tested, we found robust and anatomically consistent correlations with both the amplitude and velocity of spontaneous eye movements. Control experiments ruled out a contribution of spatial and visual attention as well as smooth pursuit eye movements to the effect. The consistent anatomical specificity of the correlation patterns and their tight temporal link at the proper hemodynamic delay argues against a non-neuronal explanation of the effect, such as cardiac or respiratory cycles. Our results thus demonstrate a link between resting state and spontaneously emerging subconscious oculo-motor behavior.

The ocular vestibular-evoked myogenic potential to air-conducted sound; probable superior vestibular nerve origin.

OBJECTIVE: Intense air-conducted sound (ACS) elicits an ocular vestibular-evoked myogenic potential (oVEMP), and it has been suggested that it does so by stimulating saccular receptors and afferents in the inferior vestibular nerve and so activating a crossed sacculo-ocular pathway. Bone conducted vibration (BCV) also elicits an oVEMP probably by activating utricular receptors and a crossed utriculo-ocular pathway. Are there two separate pathways mediating oVEMPs for ACS and BCV? If saccular receptors and afferents are primarily responsible for the oVEMP to ACS, then the oVEMP to ACS should be normal in patients with reduced or absent utricular function--unilateral superior vestibular neuritis (SVN). If utricular receptors and afferents are primarily responsible for oVEMP n10, then oVEMP to ACS should be reduced or absent in SVN patients, and in these patients there should be a close relationship between the size of the oVEMP n10 to BCV and to ACS. METHODS: The n10 component of the oVEMP to 500 Hz BCV and to 500 Hz ACS was recorded in 10 patients with unilateral SVN but who had saccular and inferior vestibular nerve function preserved, as shown by their normal cVEMP responses to ACS. RESULTS: In SVN patients with normal saccular and inferior vestibular nerve function, the oVEMP n10 in response to ACS was reduced or absent. Across SVN patients there was a very close correspondence between the size of oVEMP n10 for ACS and for BCV. CONCLUSIONS: The n10 component of the oVEMP to ACS is probably mediated predominantly by the superior vestibular nerve and so most likely by utricular receptors and afferents. SIGNIFICANCE: The n10 component of the oVEMP to either ACS or BCV probably indicates mainly superior vestibular nerve function.

Scalp-recorded induced γ-band responses to auditory stimulation and its correlations with saccadic muscle-activity.

We previously showed that the transient broadband induced gamma-band response in EEG (iGBRtb) appearing around 200-300 ms following a visual stimulus reflects the contraction of extra-ocular muscles involved in the execution of saccades, rather than neural oscillations. Several previous studies reported induced gamma-band responses also following auditory stimulation. It is still an open question whether, similarly to visual paradigms, such auditory paradigms are also sensitive to the saccadic confound. In the current study we address this question using simultaneous eye-tracking and EEG recordings during an auditory oddball paradigm. Subjects were instructed to respond to a rare target defined by sound source location, while fixating on a central screen. Results show that, similar to what was found in visual paradigms, saccadic rate displayed typical temporal dynamics including a post-stimulus decrease followed by an increase. This increase was more moderate, had a longer latency, and was less consistent across subjects than was found in the visual case. Crucially, the temporal dynamics of the induced gamma response were similar to those of saccadic-rate modulation. This suggests that the auditory induced gamma-band responses recorded on the scalp may also be affected by saccadic muscle activity.

[Recording cervical and ocular vestibular evoked myogenic potentials. Part 2: influencing factors, evaluation of findings and clinical significance]. / Die Ableitung zervikaler und okulärer vestibulär evozierter myogener Potenziale. Teil 2: Einflussfaktoren, Bewertung der Befunde und klinische Bedeutung.

VEMP measurements are subject to various influencing factors: patient age, threshold, sound intensity and frequency. Using air (AC) and bone conduction (BC) the vestibular receptors and afferents of the otolith organs can be activated to varying degrees. Recordings of cervical (cVEMP) and ocular VEMP (oVEMP) are clinically possible. AC-cVEMP are primarily an indicator of the sacculocollic reflex pathway. Together with findings on the vestibuloocular reflex (VOR) and complimentary otolith tests, VEMP enable otolith function analysis of each side separately. In addition, the distinction between combined or isolated canal and otolith dysfunction in terms of subtyping and patterns of damage in mono- and bilateral disorders, such as vestibular neuritis or bilateral vestibulopathy, is possible. Moreover, VEMP is relevant in terms of prognostic and therapeutic considerations as well as expert assessments.

[Recording cervical and ocular vestibular evoked myogenic potentials: part 1: anatomy, physiology, methods and normal findings]. / Die Ableitung zervikaler und okulärer vestibulär evozierter myogener Potenziale: Teil 1: Anatomie, Physiologie, Methodik und Normalbefunde.

Vestibular evoked myogenic potentials (VEMP) have gained in clinical significance in recent years, now forming an integral part of neurootological examinations to establish the functional status of the otolith organs. They are sensitive to low-frequency acoustic stimuli. When stimulated, receptors in the sacculus and utriculous are activated. By means of reflexive connections, myogenic potentials can be recorded when the relevant muscles are tonically activated. The vestibulocolic (sacculocollic) reflex travels from the otolith organs over the central circuitry to the ipsilateral sternocleidomastoid muscle. Myogenic potentials can be recorded by means of cervical VEMP (cVEMP). The vestibuloocular reflex crosses contralaterally to the extraocular eye muscle. Ocular VEMP (oVEMP) are recorded periocularly, preferably from the inferior oblique muscle. Various stimulation methods are used including air conduction and bone conduction.

Frequency tuning in the tone burst-evoked myogenic potentials in extraocular muscles in normal human subjects.

OBJECTIVE: In this cross-sectional study, we investigated the frequency tuning properties of sound-evoked vestibular myogenic potentials in extraocular muscles (OVEMPs) in normal human subjects. METHODS: Acoustic tone bursts (130 dB pSPL, 10 ms plateau, 1 ms rise/fall) with nine frequencies from 125 to 4000 Hz were presented monoaurally to 12 normal subjects while they sat upright and maintained centre gaze. Using surface electrodes, the OVEMPs were recorded at four locations surrounding the contralateral eye (superior, inferior, nasal, and temporal) and were referenced to an electrode placed at the nape of the neck over C7. To measure the amplitudes of the OVEMP, signals from the electrodes were amplified and sampled at 10 kHz and were averaged over 250 repetitions. RESULTS: We found that the OVEMPs recorded at the four sites exhibited similar well-defined frequency tuning with peak amplitude at ≈ 1000 Hz. CONCLUSION: Although several studies have examined the frequency tuning of the vestibular evoked myogenic potential measured from the sternocleidomastoid muscles (SVEMP), the reported results are quite variable as regards frequency, which produces peak amplitude. The well-defined OVEMP frequency tuning provides an alternative to the SVEMP for assessing vestibular function via acoustic stimulation. Further studies are needed to identify the extent to which each of the five vestibular end-organs is activated by sound and their contribution to the sound-evoked vestibular myogenic potentials.

Frequency tuning properties of ocular vestibular evoked myogenic potentials.

Loud air-conducted sound or bone-conducted vibration produces vestibular-dependent electromyographic responses both in the cervical muscles (cervical vestibular evoked myogenic potentials) and in the extraocular muscles (ocular vestibular evoked myogenic potentials; oVEMPs). Although previous frequency tuning measurements of cervical vestibular evoked myogenic potentials have provided important information for a clinical application, those of oVEMPs have rarely been studied. We explored the frequency tuning properties of oVEMPs in 12 healthy participants. Our results indicate that the best frequencies of the oVEMPs to air-conducted sound and bone-conducted vibration are 500 and 250 Hz, respectively. The difference in the best frequencies between the two stimuli may originate from the difference in the end organs (saccule or utricle), preferentially activated by each stimulus.

The effect of gaze direction on the ocular vestibular evoked myogenic potential produced by air-conducted sound.

OBJECTIVE: To assess the effects of vertical and horizontal gaze, head rotation, body position, and vision on the ocular vestibular evoked myogenic potential (OVEMP) produced by air-conducted (AC) sound. METHODS: Ten normal subjects were stimulated by 500 Hz 2 ms AC tone bursts at 136-142 dB peak SPL. OVEMPs were recorded from electrodes placed beneath the eyes. Angles of vertical gaze ranged from maximal downward to upward gaze in increments of 5-10 degrees . Horizontal gaze was measured during elevation and ranged from 20 degrees adduction to 20 degrees abduction. RESULTS: Increasing vertical gaze increased OVEMP amplitude, especially for the contralateral eye (neutral vs maximal upward gaze; contra: 1.0 vs 2.6 microV; ipsi: 0.8 vs 0.9 microV; P<0.001). OVEMPs from the contralateral eye peaked significantly earlier in the upward gaze positions (contra: 9.2 ms; ipsi: 10.4 ms; P<0.001), but peaked later during downward gaze (contra: 14.2 ms; ipsi: 11.4 ms; P=0.014). There were small effects of horizontal gaze and supine body position, but no effects of head rotation or vision. CONCLUSIONS: OVEMP amplitudes are strongly modulated by gaze position. Truncal position also affects OVEMP amplitude. SIGNIFICANCE: This study quantifies the effect of gaze on the OVEMP and demonstrates the importance of controlling for gaze in clinical and experimental studies.

Ocular vestibular-evoked myogenic potentials in children using air conducted sound stimulation.

OBJECTIVE: This study compared ocular vestibular-evoked myogenic potentials (oVEMPs) in children and adults using air conducted sound stimulation to determine when oVEMP characteristic parameters in children reach adult levels. The relationships between oVEMP characteristics and structural factors were also investigated. METHODS: In total, 15 healthy children (aged 3-13 years) and 15 healthy adults (aged 24-33 years) underwent a combined oVEMP and cVEMP test under 105 dBnHL acoustic stimulation. RESULTS: Mean nI latency, pI latency, nI-pI interval and amplitude of oVEMPs of children did not differ significantly from those of adults. In contrast to oVEMPs, a significant difference in p13 cVEMP latency existed between children and adults. Correlating the structural factors with the characteristic parameters of oVEMPs did not show significant relationship. However, p13 and n23 latencies of cVEMPs in children were significantly related to the age, head girth, body height and body weight. CONCLUSIONS: The non-invasive oVEMP test can quantitatively measure the vestibulo-ocular reflex (VOR) system, which has similar characteristic parameters in children aged >3 years and adults. For children aged <3 years, modified methodologies for testing oVEMPs are warranted. SIGNIFICANCE: The oVEMP test may serve as an effective diagnostic tool for evaluating the integrity of the VOR system for those >3 years old.

Low-vision rehabilitation by means of MP-1 biofeedback examination in patients with different macular diseases: a pilot study.

Macular disease is one of the main causes of visual impairment. We studied the efficacy of low-vision rehabilitation by means of MP-1 biofeedback examination in patients with different macular disease. Five patients were enrolled (3 female and 2 male, mean age 53.8 years) and a total of 9 eyes was examined: 2 eyes with vitelliform dystrophy, 1 with a post-traumatic macular scar, 2 with Stargardt disease, 2 with myopic macular degeneration, 2 with cone dystrophy. All the patients underwent the following tests: visual acuity, reading speed, fixation test, MP-1 microperimetry. Low-vision rehabilitation, which lasted 10 weeks, consisted of 10 training sessions of 10 min for each eye, performed once a week using the MP-1 biofeedback examination. Statistical analysis was performed using Student's t-test. p values less than 0.05 were considered statistically significant. After training all patients displayed an improvement in visual acuity, fixation behaviour, retinal sensitivity and reading speed. Fixation behaviour within the 2 degrees diameter circle improved and was statistically significant for reading speed (p = 0.01). Reading speed improved from a mean value of 64.3 to 92 words/min. Our results show that audio feedback can, by increasing attentional modulation, help the brain to fix the final preferred retinal locus. Audio feedback facilitates stimuli transmission between intraretinal neurons as well as between the retina and brain, which is where the highest level of stimuli processing occurs, thereby probably supporting a "remapping phenomenon".

Toward an implantable functional electrical stimulation device to correct strabismus.

PURPOSE: To investigate the feasibility of electrically stimulating the lateral rectus muscle to recover its physiologic abduction ability in cases of complete sixth cranial (abducens) nerve palsy. METHODS: In the feline lateral rectus muscle model, the effects of a charge-balanced, biphasic, current-controlled stimulus on the movement of the eye were investigated while stimulation frequency, amplitude, and pulse duration was varied. Eye deflection was measured with a force transducer. Denervated conditions were simulated by injection of botulinum toxin A. RESULTS: Three chemically denervated and 4 control lateral rectus muscles were analyzed. In control lateral rectus muscles, the minimum fusion frequency was approximately 170 Hz, and the maximum evoked abduction was 27 degrees. The minimum fusion frequency was unchanged after 4 weeks of chemical denervation. Stimulation of chemically denervated lateral rectus muscle resulted in 17 degrees of abduction. For both innervated and chemically denervated lateral rectus muscle, frequencies greater than 175 Hz yielded very little increase in abduction. Modulating amplitude produced noticeable movement throughout the tested range (0.2 to 9 mA). CONCLUSIONS: Results from the feline lateral rectus muscle showed that electrical stimulation is a feasible approach to evoke a contraction from a denervated lateral rectus muscle. The degree of denervation of the feline lateral rectus muscle was indeterminate. Varying the stimulation amplitude allowed greater eye movement. It is very likely that both frequency and amplitude must be modulated for finer control of static eye position.