Horizontal vestibuloocular reflex evoked by high-acceleration rotations in the squirrel monkey. I. Normal responses.

The horizontal angular vestibuloocular reflex (VOR) evoked by high-frequency, high-acceleration rotations was studied in five squirrel monkeys with intact vestibular function. The VOR evoked by steps of acceleration in darkness (3,000 degrees /s(2) reaching a velocity of 150 degrees /s) began after a latency of 7.3 +/- 1.5 ms (mean +/- SD). Gain of the reflex during the acceleration was 14.2 +/- 5.2% greater than that measured once the plateau head velocity had been reached. A polynomial regression was used to analyze the trajectory of the responses to steps of acceleration. A better representation of the data was obtained from a polynomial that included a cubic term in contrast to an exclusively linear fit. For sinusoidal rotations of 0.5-15 Hz with a peak velocity of 20 degrees /s, the VOR gain measured 0.83 +/- 0.06 and did not vary across frequencies or animals. The phase of these responses was close to compensatory except at 15 Hz where a lag of 5.0 +/- 0.9 degrees was noted. The VOR gain did not vary with head velocity at 0.5 Hz but increased with velocity for rotations at frequencies of >/=4 Hz (0. 85 +/- 0.04 at 4 Hz, 20 degrees /s; 1.01 +/- 0.05 at 100 degrees /s, P < 0.0001). No responses to these rotations were noted in two animals that had undergone bilateral labyrinthectomy indicating that inertia of the eye had a negligible effect for these stimuli. We developed a mathematical model of VOR dynamics to account for these findings. The inputs to the reflex come from linear and nonlinear pathways. The linear pathway is responsible for the constant gain across frequencies at peak head velocity of 20 degrees /s and also for the phase lag at higher frequencies being less than that expected based on the reflex delay. The frequency- and velocity-dependent nonlinearity in VOR gain is accounted for by the dynamics of the nonlinear pathway. A transfer function that increases the gain of this pathway with frequency and a term related to the third power of head velocity are used to represent the dynamics of this pathway. This model accounts for the experimental findings and provides a method for interpreting responses to these stimuli after vestibular lesions.

Horizontal vestibuloocular reflex evoked by high-acceleration rotations in the squirrel monkey. II. Responses after canal plugging.

The horizontal angular vestibuloocular reflex (VOR) evoked by high-frequency, high-acceleration rotations was studied in four squirrel monkeys after unilateral plugging of the three semicircular canals. During the period (1-4 days) that animals were kept in darkness after plugging, the gain during steps of acceleration (3, 000 degrees /s(2), peak velocity = 150 degrees /s) was 0.61 +/- 0.14 (mean +/- SD) for contralesional rotations and 0.33 +/- 0.03 for ipsilesional rotations. Within 18-24 h after animals were returned to light, the VOR gain for contralesional rotations increased to 0. 88 +/- 0.05, whereas there was only a slight increase in the gain for ipsilesional rotations to 0.37 +/- 0.07. A symmetrical increase in the gain measured at the plateau of head velocity was noted after animals were returned to light. The latency of the VOR was 8.2 +/- 0. 4 ms for ipsilesional and 7.1 +/- 0.3 ms for contralesional rotations. The VOR evoked by sinusoidal rotations of 0.5-15 Hz, +/-20 degrees /s had no significant half-cycle asymmetries. The recovery of gain for these responses after plugging was greater at lower than at higher frequencies. Responses to rotations at higher velocities for frequencies >/=4 Hz showed an increase in contralesional half-cycle gain, whereas ipsilesional half-cycle gain was unchanged. A residual response that appeared to be canal and not otolith mediated was noted after plugging of all six semicircular canals. This response increased with frequency to reach a gain of 0.23 +/- 0.03 at 15 Hz, resembling that predicted based on a reduction of the dominant time constant of the canal to 32 ms after plugging. A model incorporating linear and nonlinear pathways was used to simulate the data. The coefficients of this model were determined from data in animals with intact vestibular function. Selective increases in the gain for the linear and nonlinear pathways predicted the changes in recovery observed after canal plugging. An increase in gain of the linear pathway accounted for the recovery in VOR gain for both responses at the velocity plateau of the steps of acceleration and for the sinusoidal rotations at lower peak velocities. The increase in gain for contralesional responses to steps of acceleration and sinusoidal rotations at higher frequencies and velocities was due to an increase in the gain of the nonlinear pathway. This pathway was driven into inhibitory cutoff at low velocities and therefore made no contribution for rotations toward the ipsilesional side.

Relationship of the utriculus and sacculus to the stapes footplate: anatomic implications for sound-and/or pressure-induced otolith activation.

One hundred thirty human temporal bones that were sectioned in the vertical plane were examined to evaluate the relationship between the stapes footplate and the otolith organs. The shortest distance between the footplate and the utriculus was 0.58+/-0.10 mm in the posterior third of the oval window, 1.04+/-0.20 mm in the middle third, and 1.51+/-0.20 mm in the anterior third. The distance from the sacculus to the footplate was 1.33+/-0.20 mm in the middle third of the oval window and 1.31+/-0.18 mm in the anterior third. Membranous connections extending between the utriculus and the footplate were found in 26% of temporal bones. These membranous connections in coexistence with additional anatomic factors such as stapes hypermobility and/or dehiscence of bone within labyrinthine structures may predispose patients to sound- and/or pressure-induced otolith activation. The findings may have implications for different causes of the Tullio phenomenon.

Trauma to the external auditory canal and temporal bone.

Sources of trauma to the external auditory canal range from impaction of small foreign bodies to severely comminuted temporal bone fractures sustained after blunt or penetrating forces. This article provides an overview of commonly encountered injuries. The proposed systematic approach to evaluation and management is meant to serve as a guide to individualizing patient care.

alpha-Difluoromethylornithine delays behavioral recovery and induces decompensation after unilateral labyrinthectomy.

Biochemical and pharmacologic studies suggest a role for the ornithine decarboxylase-polyamine system as a modulator of behavioral changes during vestibular compensation. alpha-Difluoromethylornithine specifically blocks the rate-limiting step of polyamine biosynthesis. To assess the effects of alpha-difluoromethylornithine on the acute phase of postural compensation, guinea pigs were divided into groups subjected to either unilateral labyrinthectomy only (n = 7), alpha-difluoromethylornithine (500 mg/kg/day) for 4 days before labyrinthectomy (n = 10), equivalent volumes of saline for 4 days before labyrinthectomy (n = 8), and sham operations (n = 5). Yaw head tilt and roll head tilt, trunk curvature, and air-righting reflex were measured at baseline and at regular intervals up to 4 weeks. alpha-Difluoromethylornithine significantly delayed recovery of normal air-righting but had no effect on yaw head tilt, roll head tilt, and trunk curvature. We also evaluated effects of alpha-difluoromethylornithine in compensated guinea pigs. Fully compensated animals from phase 1 were randomly assigned to receive alpha-difluoromethylornithine (500 mg/kg/day) or saline once daily for 4 days. Only 33% of alpha-difluoromethylornithine animals maintained air-righting, compared with 100% of saline-treated animals (p = 0.003). Maximum trunk curvature was greater in the alpha-difluoromethylornithine group (p = 0.02). Thus alpha-difluoromethylornithine not only delayed the time course for postural recovery after unilateral labyrinthectomy, it also transiently disrupted the maintenance of the compensated state.

Prospective study of resident-performed stapedectomy.

Because of the diminishing number of patients diagnosed with otosclerosis, the adequacy of residency training in stapedectomy techniques remains controversial. A prospective study, conducted from 1986 to 1991, assessed whether or not surgical outcome obtained by residents, under close supervision by otologic faculty, could be improved using a single stapedectomy technique. Comparison of pre- and postoperative hearing results (pure-tone averages of 500, 1000, and 2000 Hz) from 49 cases revealed closure of the air-bone gap to within 10 dB in 68 percent of procedures. Complications included failure to improve the conductive hearing loss, tympanic membrane perforations, transient facial nerve weakness, subluxation of the incus, and adhesions. The failure to improve the success rate in resident-performed procedures relates to the individual learning curve and the limited number of training cases.

Assessment of pubertal maturity in boys, using height and grip strength.

A method is described for the convenient, noninvasive assessment of pubertal maturity in boys, which is intended for use in studies of developmentally related sports injury. Ninety-eight boys were evaluated for Tanner stage, age, height, weight, body mass index, and maximum (right or left hand) and average grip strength. Height and average grip strength were selected by discriminate analysis as independent predictors of maturity. Immature boys (Tanner stages 1-3) fell below average grip strength of 25 kg and were less than 65 inches (165 cm) in height, whereas boys who exceeded both of these measurements were mature (Tanner stage 4 or 5). This method was validated in a second sample of 99 boys with 100% specificity and sensitivities of 93.5% (immature boys) and 81% (mature boys). A subset of mature boys met the height criterion but did not achieve the average grip strength criterion for maturity. We propose the use of height and grip strength as a practical, noninvasive method to classify physical maturity in boys. In addition, we speculate that the subset of boys who are tall, but weak, may be at increased risk for certain types of sports injuries.

Soccer injuries and their relation to physical maturity.

A prospective study of injuries encountered during participation in a summer soccer camp for youths aged 6 through 17 years revealed an injury incidence of 10.6 per 1000 hours for girls (107 injuries in 458 girls) and 7.3 per 1000 hours for boys (109 injuries in 681 boys). For both sexes, the incidence of injury increased at age 14 years. One certified athletic trainer (W.D.C.) assessed and documented all injuries during the course of the study. Seventy percent of injuries involved the lower extremities. Contusions represented 35.2%, strains 27.8%, sprains 19.4%, and fractures and dislocations 2% of all reported injuries. The ankle was the most frequent site of injury in both sexes. Quadriceps strain was a common injury in boys. The boys with the highest incidence of injury were tall (greater than 165 cm) and had a weak grip (less than 25 kg), suggesting that skeletally mature but muscularly weak boys may be more susceptible to injury while playing soccer with peers of the same chronological age.