Is the 15∆ Base in Prism Test Reliable for Detection of Amblyopia in Anisometropic Patients?

PURPOSE: The 15∆ base in prism test (15∆BIPT) introduced by Gobin is often used in The Netherlands to detect fixation preference, especially in young and preverbal children in whom a reliable measurement of the visual acuity (VA) is difficult. It is assumed that the fixation preference detected by the 15∆BIPT can be used to predict the presence of amblyopia. The aim of this retrospective case note review was to investigate the accuracy of the 15∆BIPT in detection of amblyopia in anisometropic patients. METHODS: Four hundred and twelve files of anisometropic patients visiting the orthoptic department of The Rotterdam Eye Hospital were analyzed. Amblyopia was defined as an intraocular difference in VA of 2 or more Snellen lines. The sensitivity, specificity, and positive and negative predictive values of the 15∆BIPT were calculated and the receiver operating characteristic (ROC) curve was plotted. RESULTS: One hundred and fifty-two patients ranging from 3.3-13.1 years of age (median 5.4 years) met the inclusion criteria. One hundred and two patients were diagnosed with amblyopia. Best-corrected median VA of the best eye was 1.0 (range 0.5-1.2) and the worst eye 0.70 (range 0.05-1.2). Sensitivity of the 15∆BIPT (based on detecting amblyopia) was 34.3%. Specificity was 88.0%. The positive predictive value was 85.4% versus a negative predictive value of 39.6%. The area under the ROC curve (AUC) was 0.65 (95% CI 0.56-0.74). CONCLUSION: The low sensitivity, large number of false negatives and the AUC show that the 15∆BIPT can be considered a poor test for detecting amblyopia in anisometropic patients.

An atomic force microscope operating at hypergravity for in situ measurement of cellular mechano-response.

We present a novel atomic force microscope (AFM) system, operational in liquid at variable gravity, dedicated to image cell shape changes of cells in vitro under hypergravity conditions. The hypergravity AFM is realized by mounting a stand-alone AFM into a large-diameter centrifuge. The balance between mechanical forces, both intra- and extracellular, determines both cell shape and integrity. Gravity seems to be an insignificant force at the level of a single cell, in contrast to the effect of gravity on a complete (multicellular) organism, where for instance bones and muscles are highly unloaded under near weightless (microgravity) conditions. However, past space flights and ground based cell biological studies, under both hypogravity and hypergravity conditions have shown changes in cell behaviour (signal transduction), cell architecture (cytoskeleton) and proliferation. Thus the role of direct or indirect gravity effects at the level of cells has remained unclear. Here we aim to address the role of gravity on cell shape. We concentrate on the validation of the novel AFM for use under hypergravity conditions. We find indications that a single cell exposed to 2 to 3 x g reduces some 30-50% in average height, as monitored with AFM. Indeed, in situ measurements of the effects of changing gravitational load on cell shape are well feasible by means of AFM in liquid. The combination provides a promising technique to measure, online, the temporal characteristics of the cellular mechano-response during exposure to inertial forces.

Behavioural consequences of hypergravity in developing rats.

Gravity represents a stable reference for the nervous system. When the individual is increasing in size and weight, gravity may influence several aspects of the sensory and motor developments. To clarify this role, we studied age-dependent modifications of several exteroceptive and proprioceptive reflexes in five groups of rats conceived, born and reared in hypergravity (2 g). Rats were transferred to normal gravity (1 g) at P5 (post-natal day 5), P10, P15, P21, and P27. Aspects of neural development and adaptation to 1 g were assessed until P40. Hypergravity induced a delay in growth and a retardation in the development of contact-righting, air-righting, and negative geotaxis. However, we found an advance in eye opening by about 2-3 days in HG-P5 and HG-P10 rats and an increase in grip-time. No differences were found in tail and grasp reflexes. Our results show that hypergravity leads to a retarded development of motor aspects which are mainly dependent upon the vestibular system.

Effects of hypergravity on the morphological properties of the vestibular sensory epithelium. II. Life-long exposure of rats including embryogenesis.

Rats were exposed to a hypergravity (HG) level of 2.5 x g from conception until the age of 14 weeks. The vestibular epithelia of four of these animals and four control animals were immunohistochemically labeled for actin and tubulin. The apical cross-sectional area of epithelial cells of HG exposed rats appeared to be larger in all end organs. Area increase was 7.0% in the utricle (p<0.005) and 8.2% in the crista (p<<0.001). Hair cells and supporting cells appeared to be intact. The cellular arrangement and the proportion of different cell types within the epithelia was normal.

Effects of hypergravity on the morphological properties of the vestibular sensory epithelium. I. Long-term exposure of rats after full maturation of the labyrinths.

The effect of prolonged exposure to hypergravity on the morphology of vestibular epithelia of rats was investigated. At the age of 1 month, i.e., when vestibular end organs are fully maturated, three rats were transferred to a hypergravity environment of 2.5 g inside a large radius centrifuge. After 9 months, vestibular epithelia of these animals and of three control animals were immunohistochemically labeled for actin and tubulin. The apical cross-sectional area of epithelial cells of hypergravity exposed rats appeared to be smaller in all end organs. Area reduction was 1.9% in the saccule (not significant), 5.0% in the utricle (p < 0.005), and 11.6% in the crista (p<<0.001). No indications for a deterioration of vestibular functioning were observed.

Morphometric analysis of the vestibular sensory epithelia of young adult rat.

The appearance of vestibular sensory cells and their progressive development has been the subject of many ontogenetic studies. Because deteriorating hair cells are supposed to play a role in balance disorders of the elderly, the final stage of development (i.e. senescence) has been investigated as well. It is generally assumed that the number of hair cells in crista ampullaris, saccule and utricle slowly but steadily decreases with age. However, actual data covering the period between maturation and senescence are scarce. In the present study, rat vestibular epithelia were labeled for actin and tubulin. Morphology was inspected from immediately after weaning until the age of 12 months. Although, postnatal development was no part of this study some data on one day old epithelia are presented for comparison. At postnatal day 1, hair bundles are still shorter than in mature sensory organs, the width of the zonula adherens is less, and the apical cross-sectional area of the epithelial cells is smaller. After one month, maturation is complete. Total cell density is 400-500 per 0.01 mm2, both in the otolith maculae and in the cristae ampullares. During the first year after maturation, no changes in epithelial morphology were observed and cell density remains constant.

The horizontal vestibulo-ocular reflex of hypergravity rat at different gravity levels.

The horizontal vestibulo-ocular reflex (VOR) of two groups of rats was measured. One group was bred and kept under hypergravity (HG; 2.5 g) conditions, the other group lived under normal gravity (NG; 1 g). Eye position was recorded in response to horizontal rotatory stimuli. Measurements were made under NG (1 g), and during parabolic flight (0.0 and 1.8 g). For both groups, the response to a rotatory stimulus during parabolic flight is similar to the response that was observed under 1 g conditions. In general, however, the VOR of HG rats is reduced by 20-50% relative to the response of NG rats and the phase is shifted by -40 degrees. We conjecture that this amplitude reduction and phase shift are the consequence of living in a rotating system.

The vestibulo-ocular reflex of hypergravity rats.

The vertebrate vestibular system detects linear (otolith organs) and angular (semicircular canals) acceleration. The function of the otolith system is twofold, 1: perception of linear acceleration of the head, and 2: assessment of the spatial orientation of the head relative to the vector of gravity. Because of the latter function, a change of gravity will affect the vestibular input which, in turn, may have a wide range of serious physiological effects, for instance on ocular reflexes. The function of the vestibulo-ocular reflex (VOR) is to stabilize the visual image on the retina. Measurement of this VOR provides a method to investigate the (processing within the) vestibular system. Discrimination between gravity and linear acceleration, caused by movement of the head, is not possible. Therefore, information from the otolith system must be constantly compared with additional information from other sensory systems in order to solve the inherent ambiguity between tilt and translation. In this processing, cues from the semicircular canals also play a role. During parabolic flight, experiments can be performed at altered gravity levels for brief periods of time. On earth, the only effective possibility to manipulate gravity for longer periods of time is a centrifuge. Together with experiments in weightlessness during orbital flight, these methods form useful tools to investigate the influence of gravity on physiology. In our laboratory, rats have been kept inside a centrifuge at 2.5 g during their entire life-span (i.e. including gestation).

Vestibular-induced behaviour of rats born and raised in hypergravity.

One group of rats were bred and kept under hypergravity (HG) conditions (2.5 g) in a centrifuge. Another group were bred and kept under normal gravity conditions (1 g). Rats from both groups were dropped from a supine position into a water basin under infrared illumination leaving only gravity (1 g for both groups) for orientation. The airrighting reflex and reappearance at the water surface were examined. The success rate for airrighting of HG rats is 47% versus 45% for controls, and is performed about equally fast by both groups. The success rate of HG and control hamsters is /=80%, respectively [22]. This interspecific difference does not appear to support the conjecture that altered behaviour is caused by a structural change of vestibular end organs during ontogenetic development under HG. The success rate for surfacing of control rats is 100%. Surfacing of young HG rats is less successful (36% at age 6 weeks) and requires more time. On average, surfacing of adult rats of both groups is about the same. Apparently, the repeated stay of centrifuge-bred rats at 1 g for experiments and daily care suffices to recalibrate and improve their orientation, which is essential for surfacing.

Neuronal encoding of sound direction in the auditory midbrain of the rainbow trout.

Acoustical stimulation causes displacement of the sensory hair cells relative to the otoliths of the fish inner ear. The swimbladder, transforming the acoustical pressure component into displacement, also contributes to the displacement of the hair cells. Together, this (generally) yields elliptical displacement orbits. Alternative mechanisms of fish directional hearing are proposed by the phase model, which requires a temporal neuronal code, and by the orbit model, which requires a spike density code. We investigated whether the directional selective response of auditory neurons in the midbrain torus semicircularis (TS; homologous to the inferior colliculus) is based on spike density and/or temporal encoding. Rainbow trout were mounted on top of a vibrating table that was driven in the horizontal plane to simulate sound source direction. Rectilinear and elliptical (or circular) motion was applied at 172 Hz. Generally, responses to rectilinear and elliptical/circular stimuli (irrespective of direction of revolution) were the same. The response of auditory neurons was either directionally selective (DS units, n = 85) or not (non-DS units, n = 106). The average spontaneous discharge rate of DS units was less than that of non-DS units. Most DS units (70%) had spontaneous activities < 1 spike per second. Response latencies (mode at 18 ms) were similar for both types of units. The response of DS units is transient (19%), sustained (34%), or mixed (47%). The response of 75% of the DS units synchronized to stimulus frequency, whereas just 23% of the non-DS responses did. Synchronized responses were measured at stimulus amplitudes as low as 0.5 nm (at 172 Hz), which is much lower than for auditory neurons in the medulla of the trout, suggesting strong convergence of VIIIth nerve input. The instant of firing of 42% of the units was independent of stimulus direction (shift <15 degrees), but for the other units, a direction dependent phase shift was observed. In the medial TS spatial tuning of DS units is in the rostrocaudal direction, whereas in the lateral TS all preferred directions are present. On average, medial DS units have a broader directional selectivity range, are less often synchronized, and show a smaller shift of the instant of firing as a function of stimulus direction than lateral DS units. DS response characteristics are discussed in relation to different hypotheses. We conclude that the results are more in favor of the phase model.

Mapping of sound direction in the trout lower midbrain.

In the trout lower midbrain 35% of the auditory neurons are directionally selective (DS). Most of these neurons have a higher directional selectivity than the sensory hair cells. DS units and non-DS units occur in vertical clusters, with the former more dorsally. The direction of preference is topographically mapped. Apparently, auditory space mapping is a common feature in the midbrain of vertebrates.

Photopic spectral sensitivities of the red and the yellow field of the pigeon retina.

The spectral sensitivities of the red field and the yellow field in the retina of the homing pigeon (Columba Livia) were determined on the basis of ERG responses. Between 450 and 550 nm the relative spectral sensitivity of the yellow field turned out to be higher than that of the red field. The results are in agreement with spectral sensitivity data, obtained by behavioural threshold procedures.