The association between multifocal electroretinograms and OCT retinal thickness in retinitis pigmentosa patients with good visual acuity.

AIMS: To investigate relationships between retinal morphology and retinal function in patients with retinitis pigmentosa (RP) using optical coherence tomography (OCT) and multifocal electroretinography (mfERG). METHODS: In all, 14 patients with RP who had visual acuities of 0.2 logMAR or better and Humphrey central fields of 10 degrees or larger participated in the study along with 16 normal control subjects. The amplitudes and timings of the mfERG responses were compared with spatially corresponding measures of retinal layer thickness from OCT within the macula region (central 12 degrees ). RESULTS: Eyes with RP showed thinning of the photoreceptor retinal (PR) layer and thickening of mid-inner retinal (MIR) layers beyond the fovea. mfERG amplitude was reduced in all regions, whereas mfERG timing was only significantly delayed at a retinal eccentricity of 6-12 degrees and was otherwise preserved within the foveal and parafoveal retina (0-6 degrees). PR layer thickness was correlated with mfERG amplitude across the macula region. mfERG timing was correlated with the total change in retinal thickness (combined PR thinning and MIR thickening) at an eccentricity of 6-12 degrees. CONCLUSIONS: The relationship between mfERG timing and retinal thickness in RP is dependent on the retinal eccentricity. Preserved timing in the central retina (0-6 degrees ), despite significant disruption to retinal laminar structure, could be suggestive of inner retinal remodelling or functional redundancy. Cone system activity derived from mfERG amplitude appears to be related to the thickness of the photoreceptor layer in the macula region.

Review of interface pressure measurement to establish a protocol for their use in the assessment of patient support surfaces.

The interface pressure (IP) between a body and support surface is one objective measure used in the evaluation of the wide variety of pressure relieving equipment that exists for the prevention and treatment of pressure sores. We present a review of the literature associated with making these measurements and discuss the acute need to be able to use them to choose the most effective 'anti-pressure sore devices' for patient care, as well as discussing the difficulties associated with this decision making. We focus on interface pressure monitors describing their development and discussing the questions that remain about the accuracy and relevance of their measurements. An outline of the strengths and limitations of the most commonly used pressure sensors is given. There remains a gap in research in this area that is clinically specific and the need to be able to assess these anti-pressure sore devices with patients in a hospital environment is highlighted. The variants involved in assessing patients on various mattresses and the difficulties associated with this are described. A protocol is suggested in an attempt to improve the use of such devices and establish a better method for quantifying the clinical effectiveness of different support surfaces.

Reduced duration of a visual motion aftereffect in congenital nystagmus.

Congenital nystagmus (CN) is a primarily horizontal, involuntary, conjugate eye movement which can be observed soon after birth or during the first half-year of life. Individuals with CN rarely complain of oscillopsia. Using a motion aftereffect (MAE), we investigated if individuals with CN have abnormalities in motion perception and if any such abnormality could be due to nystagmus or to compensatory mechanisms to avoid oscillopsia. In task A, patients (n=10) and control subjects (n=10) indicated the direction, duration and relative velocity of MAEs. The subjects binocularly viewed a high contrast, grey scale grating (0.23 cyc/deg; visual angle: 18.3 deg) moving upward or downward at 1, 3, and 6 deg/sec for 60 sec. Vertical optokinetic nystagmus (OKN) was monitored. In task B, patients (n=8) and control subjects (n=8) viewed similar spatial frequency gratings (visual angle: 40.7 degs; 0.5, 0.2, 0.08 cyc/deg) which moved at 4, 10, and 16 deg/sec for 60 sec. In task C, five control subjects, with induced vestibular nystagmus, viewed a grating (0.2 cyc/deg; visual acuity: 28.5 deg), moving upward for 40 sec. In all three tasks, after adaptation with the moving grating, subjects viewed the then static grating and reported the duration and direction of the MAE. One CN patient and eight control subjects reported MABs at all three test velocities in task A. When patients exhibited OKN, the gain was close to one, as in the control group. In task B, seven of the eight patients and all of the control subjects had MABs at the faster adaptation velocities. CN patients had less MAEs at an adaptation velocity of 4 deg/sec and when MAEs were observed, the duration of the illusory motion was reduced by approximately 48%. Control subjects, with induced vestibular nystagmus, reported MAEs at 4 deg/sec (task C). These findings indicate that nystagmus cannot be the only factor accounting for the suppression of motion perception and suggest that compensatory mechanisms used to avoid oscillopsia contribute to the differences found between the groups.

The effects of hyperventilation on postural control mechanisms.

The effect of hyperventilation on postural balance was investigated. Voluntary hyperventilation increased body sway in normal subjects, particularly in the sagittal plane. The possibility that this hyperventilation-induced unsteadiness is due to interference with lower limb somatosensory input, vestibular reflexes or cerebellar function was assessed. (i) The effect of hyperventilation on peripheral compound sensory action potentials (SAPs) and somatosensory evoked potentials (SEPs) (recorded centrally, from the scalp) elicited by electrical stimulation of the sural nerve was measured in six normal adults. A reduction in the scalp SEP amplitude and an increase in the peripheral SAP amplitude were observed during hyperventilation, which reversed during the recovery period. These changes indicate increased peripheral neural excitability which could lead to a higher level of ectopic activity; the latter would interfere with central reception of peripheral input. (ii) The click-evoked vestibulo-collic reflex was recorded to study the effect of hyperventilation on vestibulo-spinal activity. EMG recordings from both sternocleidomastoid muscles of six healthy subjects were made in response to loud clicks presented to either ear. Neither the amplitude nor the latency of the response were altered significantly by hyperventilation. (iii) Eye-movement recordings were obtained in the six normal subjects to assess the effect of hyperventilation on the vestibulo-ocular reflex and its visual suppression, the latter being a function largely mediated by the cerebellum; no changes were detected. (iv) Three-dimensional eye-movement recordings and body-sway measurements were obtained in six patients with longstanding unilateral vestibular loss in order to evaluate if hyperventilation disrupts vestibular compensation. In all patients, a horizontal nystagmus either appeared or was significantly enhanced for > or = 60 s after voluntary hyperventilation. Sway was also enhanced by hyperventilation in these patients, particularly in the frontal plane. This study suggests that hyperventilation disrupts mechanisms mediating vestibular compensation. The increase in sway may be, at least partly, mediated by deranged peripheral and central somatosensory signals from the lower limbs. Hyperventilation seems to spare vestibular reflex activity and cerebellar-mediated eye movements.

The perception of body verticality (subjective postural vertical) in peripheral and central vestibular disorders.

The perception of body verticality (subjective postural vertical, SPV) was assessed in normal subjects and in patients with peripheral and central vestibular lesions and the data were compared with conventional neuro-otological assessments. Subjects were seated with eyes closed in a motorized gimbal which executed cycles of tilt at low constant speed (1.5 degrees s-1), both in the frontal (roll) and sagittal (pitch) planes. Subjects indicated with a joystick when they entered and left verticality, thus defining a sector of subjective uprightness in each plane. The mean angle of tilt (identifying a bias of the SPV) and the width of the sector (defining sensitivity of the SPV) were then determined. In normal subjects, the angle of the "verticality' sector was 5.9 degrees for pitch and roll. Patients with bilateral absence of vestibular function, patients with vertigo, i.e. acute unilateral lesions, benign paroxysmal positional vertigo (BPPV) and Ménière's disease, and patients with positionally modulated up-/downbeat nystagmus all had enlarged sectors (i.e. loss in sensitivity). Mean sector angle in these groups ranged from 7.8 to 11 degrees and the abnormality was present both in pitch and roll, regardless of the direction of nystagmus or body sway. Patients with chronic unilateral peripheral vestibular lesions and those with position-independent vertical nystagmus had normal sensitivities. No significant bias of the SPV was found in any patient group, not even those with acute unilateral vestibular lesions who had marked tilts of the subjective visual vertical (SVV). Complementary experiments in normal subjects tested under galvanic vestibular or roll-plane optokinetic stimulation also failed to show biases of the SPV. In contrast, a significant bias in the SPV could be induced in normal subjects by asymmetric cycles of gimbals tilt, presumably by proprioceptive adaptation. The following conclusions can be drawn. (i) The perception of body verticality whilst seated is mainly dependent on proprioceptive/contact cues but these are susceptible to tilt-mediated adaptation. (ii) Vestibular input improves the sensitivity of the SPV, even in vestibular disorders, as long as the abnormality is stable. (iii) There can be marked dissociation between vestibulo-motor (ocular and postural) phenomena and the perception of body verticality, and between the SPV and SVV. (iv) The postural sway asymmetries in patients with peripheral and central vestibular lesions, like those induced by galvanic or optokinetic stimulation in normal subjects, are not consequences of changes of the SPV.

Reorientation of visually evoked postural responses by different eye-in-orbit and head-on-trunk angular positions.

We examined the question of whether the position of the eyes in the head and of the head on the trunk influence the direction of visually elicited postural reactions. Normal subjects stood on a force platform viewing a large disc, rotating in the roll plane, always maintained orthogonal to the line of sight. The disc was presented at 0 degree, 30 degrees and 90 degrees to the right or left with respect to the mid-frontal plane of the subject's body and was viewed with various combinations of horizontal eye-in-orbit and head-on-trunk deviations. It was found that the main direction of body sway was always reoriented to be parallel to the disc (e.g. viewing the disc at 30 degrees oriented sway responses at a mean angle of 33 degrees). The largest sway responses were obtained when the disc was parallel to the sagittal plane of the body and was viewed with an ipsilateral eye-neck deviation totalling 90 degrees (head-on-trunk 60 degrees+eye-in-orbit 30 degrees). When eye and head deviations cancelled each other (i.e. eye-in-orbit +30 degrees combined with head-on-trunk -30 degrees), directional effects on sway also cancelled each other out. This result demonstrates that signals of eye-in-orbit and head-on-trunk position have the capability to redirect visuo-motor commands to the appropriate postural muscles. This allows vision to regulate postural balance whatever the position of the eyes in space. We speculate that this function is mediated by eye and neck proprioceptive signals (or alternatively by efference copy) with access to gain control mechanisms in the visuo-postural system.

The effect of eye/head deviation and visual conflict on visually evoked postural responses.

Three interrelated experiments on visually evoked postural responses (VEPR) are presented to investigate the effect of lack of coplanarity between retinal and body coordinates (Experiment I) and the effect of directionally conflicting information in the visual stimulus. Experiment I showed that the direction of VEPR is modified by eye-in-orbit and head-on-trunk position signals, presumably of proprioceptive origin. Experiments II and III showed that VEPR can be critically suppressed by the presence of conflict within the visual stimulus (Experiment II: a linear, tangential component of visual motion acting in the opposite direction to the main angular component of a roll-motion display; Experiment III: a non congruent "improbable" visual motion parallax linear motion stimulus). A conceptual model of the postural system is presented, incorporating a gain control unit for the visuo-postural loop with inputs from the ocular/cervical proprioceptive system and from intra- and inter-sensory conflict detectors (comparators).

EMG-responses to sudden onset free fall.

Recordings of axial and limb muscles in reaction to a free fall induced startle were performed in subjects while they were lying on a tilting couch with their eyes closed. Young normals (n = 24, aged 31.1 +/- 6.6 years) showed an activation sequence consisting of sternomastoid (N.XI: 57 ms), abdominal muscles (T10: 65 ms), quadriceps (L3: 75 ms) and deltoid (C5: 78 ms) and tibialis anterior (L4: 80 ms). The sequence of activation is not compatible with the current hypothesis of the startle being produced by a single volley spreading rostrally and caudally from the lower brainstem. Instead it is suggested that the startle is a patterned response organized by a putative reticular generator capable of spatio-temporal sequencing. Two avestibular patients had responses at mildly delayed latencies, showing that these can be elicited by non-vestibular inputs. Similar testing of 11 subjects aged 70-80 years showed a latency delay of ca. 26% in the EMG response but a similar activation sequence. The amount of delay in the elderly can only partially be attributed to age-dependent motor conduction slowing and suggests a prolongation of central processing time. In patients with advanced stages of akinetic-rigid syndromes abnormalities were seen in cases with an involvement of the brainstem reticular formation.