Jeffrey T. Somers

A Form of Inherited Cerebellar Ataxia with Saccadic Intrusions, Increased Saccadic Speed, Sensory Neuropathy, and Myoclonus

Over the past five years, rapid progress has been made in genetically identifying different forms of spinocerebellar atrophy (SCA), for which several characteristic disorders of eye movements have been reported. 1,2 Nonetheless, the genetic disorder in some families has not yet been discovered, and this report concerns one such kinship. We studied a family of Slovenian descent in which 5 of 14 siblings presented with progressive ataxia. Two other siblings, who we did not study, may also show involvement, but neither parent and none of 27 children of the 14 siblings has been affected. We examined the five definitely affected patients several times over a 5year period; electroencephalography, electromyography, and MRI brain imaging were performed. In Patient 5 (the proband), we carried out genetic screening (Athena Laboratories) for Friedreich’s ataxia, spinocerebellar ataxia (SCA) 1-3 and 6-8, and Unverricht-Lundborg progressive myoclonic epilepsy (EMP1). We compared results of eye movement studies with a group of 10 normal subjects (2 female; median age 40 years, range 23–60). We measured horizontal and vertical eye movements using the magnetic search coil technique. We tested fixation, horizontal and vertical saccades, smooth pursuit, and vestibulo-ocular reflex (VOR), and vergence, as previously described. 3,4 All patients and subjects gave informed, written consent, as approved by our Institutional Review Board. Clinical findings of the five affected sibs that we studied are summarized in TABLE 1. Disease onset was insidious; early symptoms could be remembered by all sibs during their early twenties, consisting of gait unsteadiness and difficulty read

Tests of two hypotheses to account for different-sized saccades during disjunctive gaze shifts

Abstractu2002Rapid shifts of the point of visual fixation between objects that lie in different directions and at different depths require disjunctive eye movements. We tested whether the saccadic component of such movements is equal for both eyes (Hering’s law) or is unequal. We compared the saccadic pulses of abducting and adducting movements when horizontal gaze was shifted from a distant to a near target aligned on the visual axis of one eye (Müller paradigm) in ten normal subjects. We similarly compared horizontal saccades made between two distant targets lying in the same field of movement as during the Müller paradigm tests, and between targets lying symmetrically on either side of the midline, at near side of the midline, at near or far. We measured the ratio of the amplitude of the movements of each eye in corresponding directions due to the saccadic component, as well as corresponding ratios of peak velocity and peak acceleration. In response to a Müller test paradigm requiring about 17° of vergence, the change in position of the unaligned eye was typically twice the size of the corresponding movement of the aligned eye. The ratio of peak velocities for the unaligned/aligned eyes was about 1.5, which was greater than for saccades made between distant targets. The ratio of peak acceleration for unaligned/aligned eyes was about 1.0 during shifts from near to far and about 1.3 for shifts from far to near, these values being similar to corresponding ratios for saccades between distant targets. These measurements of peak acceleration indicate that the saccadic pulses sent to each eye during the Müller paradigm are more equal than would be deduced by comparing the changes in eye position. We retested five subjects to compare directly the peak acceleration of saccades made during the Müller paradigm with similar-sized ”conjugate” saccades made between targets at optical infinity. Saccades made during the Müller paradigm were significant slower (P<0.005) than similar-sized conjugate saccades; this indicated that the different-sized movements during Müller paradigm are not simply due differences in saccadic pulse size but are also influenced by the concurrent vergence movement. A model for saccade-vergence interactions, which incorporates equal saccadic pulses for each eye, and differing contributions from convergence and divergence, accounts for many of these findings.

Ocular oscillations induced by shifts of the direction and depth of visual fixation

Shifts of the point of fixation between two targets aligned on one eye that are located near and far (Müller paradigm) stimulates a combined saccadic‐vergence movement. In normal subjects, this test paradigm often induces saccadic oscillations of about 0.3 degrees at 20 to 30 Hz. We measured eye movements using the magnetic search coil technique in 2 patients recovering from viral opsoclonus‐myoclonus syndrome, comparing saccadic‐vergence responses to the Müller paradigm with conjugate saccades between distant targets. Both patients exhibited intermittent conjugate ocular oscillations of about 4 to 5 degrees amplitude at about 10 Hz. Combined saccadic‐vergence movements induced these oscillations twice as often as did conjugate saccades. One patient also exhibited disjunctive ocular oscillations at 10 Hz while sustaining fixation on the near target. The Müller paradigm provides a useful clinical and experimental technique for inducing saccadic oscillations. The probable mechanism is that pontine omnipause neurons, which normally gate saccades, are inhibited during the sustained vergence movement that follows the saccadic component of the response to the Müller paradigm. Ann Neurol 2001;49:24–28

Vestibular and Non-vestibular Contributions to Eye Movements that Compensate for Head Rotations during Viewing of Near Targets

Geometry dictates that when subjects view a near target during head rotation the eyes must rotate more than the head. The relative contribution to this compensatory response by adjustment of the vestibulo-ocular reflex gain (Gvor), visual tracking mechanisms including prediction, and convergence is debated. We studied horizontal eye movements induced by sinusoidal 0.2–2.8xa0Hz, en-bloc yaw rotation as ten normal humans viewed a near target that was either earth-fixed (EFT) or head-fixed (HFT). For EFT, group median gain was 1.49xa0at 0.2xa0Hz declining to 1.08xa0at 2.8xa0Hz. For HFT, group median gain was 0.03xa0at 0.2xa0Hz increasing to 0.71xa0at 2.8xa0Hz. By applying transient head perturbations (peak acceleration >1,000°xa0s−2) during sinusoidal rotation, we determined that Gvor was similar during either EFT or HFT conditions, and contributed only ~75% to the compensatory response. We confirmed that retinal image slip contributed to the compensatory response by demonstrating reduced gain during EFT viewing under strobe illumination. Gain also declined during sum-of-sines head rotations, confirming the contribution of predictive mechanisms. The gain of compensatory eye movements was similar during monocular or binocular viewing, although vergence angle was greater during binocular viewing. Comparison with previous studies indicates that mechanisms for generation of eye rotations during near viewing depend on head stimulus type (rotation or translation), waveform (transient or sinusoidal), and the species being tested.

Spacecraft occupant protection requirements: a review of the recent changes

NASA has recently updated spacecraft design requirements for protecting crewmembers during dynamic spaceflight phases. The details of the update are available in a NASA publication (NASA TM-2013-217380) and are summarized here. Previously, NASAs occupant protection requirements relied primarily on the multiaxial dynamic response criterion, which NASA refers to as the Brinkley Dynamic Response Criteria (BDRC). Although simple to implement, there are several important ground rules that must be met for the injury predictions to be applicable. These include proper restraint, flail controls, proper seating support, pressure suit considerations, head protection including consideration of helmet mass, and spaceflight deconditioning. Even if these ground rules are met, there are limitations to the model that must be addressed, including: model validation, sex differences, age effects, anthropometry effects, and differences between the physical fitness of military test subjects and future crewmembers. To address these limitations, new injury assessment reference values (IARV) have been prescribed for the 5(th) percentile female and 95(th) percentile male Hybrid III anthropomorphic test devices (ATD). These metrics are head-injury criterion, head-rotational acceleration, neck injury criterion, neck-axial-force limits, flail prevention, and lumbar-axial compression force. Using these new ATD IARVs, NASA can have increased confidence that vehicle designs mitigate the risk of injury during dynamic phases of flight.