Children's Accommodation to a Variety of Targets - A Pilot Study.

BACKGROUND: Previous research indicates that a significant proportion of children underaccommodate at 1/3 m. Accommodation may vary with task demand, so children may accommodate appropriately if required, for example, when reading small print. This study explores the range of accommodative responses elicited in typical children, under naturalistic conditions, to a range of targets. METHOD: We identified 24 typically developing children from the University of Reading Child Database. Primary-school children attending UK Year 2 (age 6-7 years) or Year 6 (age 10-11 years) with minimum distance visual acuity of 0.200 logMAR and near visual acuity of 0.100 logMAR were recruited for participation. A remote haploscopic photorefractor was used to assess naturalistic, sustained, binocular accommodative responses to a variety of targets. At 33 cm, accommodative targets included individual letters, age-appropriate text in large print equivalent to early primary-school books, small N5 equivalent print, a visual search task ("Where's Wally?"), a clown picture containing a range of spatial frequencies, and a children's cartoon. Participants were given minimal instructions for task completion. The target presentation order was counterbalanced. The results reported in this study were obtained during a longer testing session involving different target types and fixation distances. RESULTS: The accommodative response observed with each target varied across participants to both the clown target and single letters of a size used in school reading books the accommodative responses were 2.4±0.48 D (range 0.85-2.97 D) and 2.47±0.37 D (range 1.48-3.09 D), respectively. The accommodative response to N5 print (3.06±0.52 D) was statistically better than all other targets other than the visual search and larger print tasks (P<0.05). CONCLUSIONS: Even to demanding N5 text, accommodation is variable between participants, but is better than that to less demanding targets. Tasks experienced by children in everyday or clinical situations will stimulate an unknown amount of accommodation for near fixation.

A rare cause of cystic lung disease - Birt-Hogg-Dubé syndrome.

Birt-Hogg-Dubé syndrome, initially described in 1977, is an autosomal dominant inherited condition characterised by basal pulmonary cysts often resulting in pneumothorax, renal tumours and cutaneous involvement. Lung cysts have been described in up to 90% of patients with a corresponding risk of pneumothorax of 50 times greater than the normal population. We describe here a case of Birt-Hogg-Dubé diagnosed in the 9th decade of life and discuss the radiological findings and clinical implications.

Attention processes in children with movement difficulties, reading difficulties or both.

Reading difficulties (RD) and movement difficulties (MD) co-occur more often in clinical populations than expected for independent disorders. In this study, we investigated the pattern of association between attentional processes, RD and MD in a population of 9 year old school children. Children were screened to identify index groups with RD, MD or both, plus a control group. These groups were then tested on a battery of cognitive attention assessments (TEA-Ch). Results confirmed that the occurrence of RD and MD was greater than would be predicted for independent disorders. Additionally, children with MD, whether or not combined with RD, had poor performance on all attention measures when compared with typically developing children. Children with RD only, were no poorer on measures of attention than typical children. The results are discussed with respect to approaches proposed to account for the co-occurrence of disorders.

Development of the response AC/A ratio over the first year of life.

This study investigated the development of the link from accommodation to vergence in infants by occluding one eye thus removing binocular cues. Occluded adults continue to converge partially demonstrating that the accommodative drive to vergence (the AC/A link) and proximal cues are sufficient to drive vergence. For infants of all ages, AC/A ratios were found to be in the normal adult range. We conclude that infants can use monocular cues to drive vergence and that this occurs before the age when there is a substantial increase in the accuracy of oculomotor processes. There is flexibility in the developing visual system which is able to produce early vergence responses by relying upon alternative cues.

Neural network control systems that learn to perform appropriately.

Setting up a neural network with a learning algorithm that determines how it can best operate is an efficient way to formulate control systems for many engineering applications, and is often much more feasible than direct programming. This paper examines three important aspects of this approach: the details of the cost function that is used with the gradient descent learning algorithm, how the resulting system depends on the initial pre-learning connection weights, and how the resulting system depends on the pattern of learning rates chosen for the different components of the system. We explore these issues by explicit simulations of a toy model that is a simplified abstraction of part of the human oculomotor control system. This allows us to compare our system with that produced by human evolution and development. We can then go on to consider how we might improve on the human system and apply what we have learnt to control systems that have no human analogue.

Variations in accommodation and convergence responses in a minimally controlled photorefractive setting.

PURPOSE: A remote haploscopic photorefractor, designed for assessment of accommodation and convergence in infants and clinical groups, was used to determine heterophoria accommodative convergence/accommodation (AC/A) ratios in normal naïve adults. These were compared with conventional clinical measures. METHODS: Twenty-one naïve subjects were used to compare occluded and unoccluded prism cover test responses with the remote haploscopic photorefractor using a text and picture target. RESULTS: Although luminance was generally low for both targets, binocular vergences were appropriate for target demand in both studies. Binocular accommodation showed greater lag for the highest target accommodative demand and the less demanding target. Occlusion not only reduced vergence response, but also frequently caused a marked reduction in accommodation, especially to the picture target. Normal mean AC/A values were found, but with wide variations between individual subjects. CONCLUSIONS: Although mean accommodation, vergence, and AC/A values were comparable with published data, we suggest that in these conditions using naïve subjects, accommodation is frequently inaccurate, especially on occlusion, without concomitant loss of vergence, at least at low light levels. Accommodative convergence may play a less important part in, and other cues contribute more to, the near reflex than has been previously suggested.

The response to prism deviations in human infants.

Previous research has suggested that infants are unable to make a corrective eye movement in response to a small base-out prism placed in front of one eye before 14-16 weeks [1]. Three hypotheses have been proposed to explain this early inability, and each of these makes different predictions for the time of onset of a response to a larger prism. The first proposes that infants have a 'degraded sensory capacity' and so require a larger retinal disparity (difference in the position of the image on the retina of each eye) to stimulate disparity detectors [2]. This predicts that infants might respond at an earlier age than previously reported [1] when tested using a larger prism. The second hypothesis proposes that infants learn to respond to larger retinal disparities through practice with small disparities [3]. According to this theory, using a larger prism will not result in developmentally earlier responses, and may even delay the response. The third hypothesis proposes that the ability to respond to prismatic deviation depends on maturational factors indicated by the onset of stereopsis (the ability to detect depth in an image on the basis of retinal disparity cues only) [4] [5], predicting that the size of the prism is irrelevant. To differentiate between these hypotheses, we tested 192 infants ranging from 2 to 52 weeks of age using a larger prism. Results showed that 63% of infants of 5-8 weeks of age produced a corrective eye movement in response to placement of a prism in front of the eye when in the dark. Both the percentage of infants who produced a response, and the speed of the response, increased with age. These results suggest that infants can make corrective eye movements in response to large prismatic deviations before 14-16 weeks of age. This, in combination with other recent results [6], discounts previous hypotheses.

Developing visual systems and exposure to virtual reality and stereo displays: some concerns and speculations about the demands on accommodation and vergence.

Little is known about the developmental plasticity of the vergence and accommodative systems, an important issue since abnormalities can lead to visual problems, e.g. strabismus. One way of artificially altering the links between accommodation and vergence is to vary the stimulus to vergence while fixing the accommodative stimulus, as is found in virtual reality displays. While it would be of interest to study developmental plasticity in this situation, since many children are exposed to games machines which use this arrangement, no studies to date have tackled this issue. There is, however, some indication that long-term VR viewing in adults can lead to visual problems. It seems important to determine the safety of these systems for the developing human visual system before they come into common use. In this paper, adaptation of the accommodation and vergence systems and the effect of VR viewing in adults is discussed. The sparse literature on adaptation in children is then reviewed, and suggestions made for approaches that would enhance our knowledge of plasticity of accommodation and vergence in children.

Comparison of measures of visual acuity in infants: Teller acuity cards and sweep visual evoked potentials.

PURPOSE: This study compares the development of acuity in the same infants during one testing session using Teller acuity cards (TAC) and sweep visual evoked potentials (sVEP). We asked whether different testing methods in two centers would produce different developmental time courses. METHODS: Forty-eight infants were tested in two centers. The standard procedure for TAC was used. For sVEP acuity, the amplitude response curve derived from time-locked cortical activity was used to extrapolate to zero response, giving an acuity estimate for each infant. RESULTS: sVEP acuity was generally higher than TAC acuity. The rate of development was steeper for TAC than sVEP acuity with TAC starting at a much lower level. The ratio of sVEP to TAC acuity decreased exponentially with age reaching an asymptote of about 1.44 at 6 months. CONCLUSIONS: Results were indistinguishable between centers suggesting that comparison of acuity measures obtained using variations of these methods across centers is possible.

A randomized controlled trial of laparoscopic extraperitoneal hernia repair as a day surgical procedure.

BACKGROUND: A randomized controlled trial was conducted in a day surgery setting comparing a standardized variant of the Shouldice hernioplasty with extraperitoneal laparoscopic herniorrhaphy. METHODS: The laparoscopic repair was technically challenging, evidenced by conversion from extraperitoneal to transabdominal repairs in 6.25% of patients. It was free from the inherent dangers of intraperitoneal laparoscopy. Surgical morbidity was low and comparable to that for patients randomized to the open repair. RESULTS: Outcome following laparoscopic extraperitoneal herniorrhaphy varied depending on the parameter measured. It was comparable to the open repair with respect to postoperative activity levels and the number of days required for return to work but inferior to the open repair in terms of operation time and time to hospital discharge. The extraperitoneal approach was superior to the open repair with respect to postoperative pain levels and analgesic requirements. No attempt was made to compare recurrence rates due to the short follow-up period. CONCLUSIONS: Laparoscopic extraperitoneal herniorrhaphy should not supercede conventional hernia repair until subjected to further trials with the aid of larger study populations and greater technical expertise; the results of long-term recurrence rates are awaited.

Binocular alignment and vergence in early infancy.

Vergence to static targets presented at five distances between 25 and 200 cm from the subject was measured in 631 infants aged between 17 and 120 days. Photographic images of the eyes were magnified and measured to yield information on the monocular and binocular eye positions for each target. Vergence data were fit by a linear function and compared to the vergence calculated from target distance and each infant's measured interpupillary distance. Differences in vergence across targets were also evaluated for each subject by calculating the change in angle of rotation for each eye. Many of even the youngest infants showed good ocular alignment both monocularly and binocularly, although the youngest infants showed the greatest variability in vergence. However, the median difference in vergence angle between the eyes for even the youngest group was < 4 deg (6.8 prism D), and some of this difference is attributed to versional eye movements and to slightly off-axis head position across trials. The average infant of 1-2 months showed substantially better vergence than has been reported in some recent studies. Apparently, oculomotor constraints are not a significant barrier to the development of the higher forms of binocularity that begin to emerge in the months immediately following the interval studied here, and may form the substrate for later developments in binocular vision.

Calibration of the Hirschberg test in human infants.

PURPOSE: The Hirschberg ratio has been defined as the ratio of the change in angular position of the line of sight to the change in location of the first Purkinje image relative to the pupil center. This study was designed to determine empirically whether the adult value of the Hirschberg ratio (approximately 22 prism diopters/mm) is a suitable value to use for all ages. Because many structures in the eye are growing during the 1st year, it is possible that there could be some change in the Hirschberg ratio during this period. METHODS: A photographic technique was used to calculate the Hirschberg ratio for 323 infants between the ages of 27 and 365 days and for ten adult subjects. The study also measured angle lambda, the angle between the line of sight and the pupillary axis, in these populations. RESULTS: The Hirschberg ratio did not change with age during infancy and was similar for adults and infants both in value and in variance. Angle lambda, however, declined rapidly, from an extrapolated value of about 8.4 degrees at birth to near 5 degrees at 5 months, a change that is assumed to reflect axial growth of the eye. CONCLUSIONS: The results suggest that the same average Hirschberg ratio can be used to estimate angle of strabismus across age. The change in angle lambda is important to consider when evaluating angle of deviation from measures using the Hirschberg test.

Development of accommodation and convergence in infancy.

Paraxial photorefraction was used to assess the development of accommodation and convergence in a large sample of infants under 1 year of age. The infants viewed small dolls placed at various distances (200-25 cm). The majority of infants at all ages demonstrated appropriate convergence for target distance, regardless of manifest refractive error. However, accommodation lagged behind convergence in development. Infants under 2 months tended to demonstrate either flat accommodation responses with a fixed plane of focus at around 30 cm, or accommodated appropriately for near targets, but failed to relax their accommodation sufficiently for the more distant targets. Thus, the focussing error increased with increasing target distance. Since the manifest refractive error was estimated by extrapolating the accommodation function to 0 diopters demand, these infants demonstrated spuriously myopic behavior. After 2 months, the majority of infants showing emmetropic behavior had accommodation responses that changed appropriately with target distance. However, infants with myopic or hyperopic manifest refractive errors displayed a variety of accommodative styles.

Relations among autonomic nerve dysfunction, oesophageal motility, and gastric emptying in gastro-oesophageal reflux disease.

Recent studies suggest that vagal nerve dysfunction may be important in the aetiology of gastro-oesophageal reflux disease. Delayed oesophageal transit and slowed gastric emptying occur frequently and may also be of pathogenic importance. In 48 patients with gastro-oesophageal reflux disease we studied the prevalence of and relations between autonomic nerve dysfunction (as assessed by cardiovascular reflex tests) and oesophageal transit, oesophageal motility, gastric emptying, and endoscopic grade of oesophagitis. Of the 48 patients, 21 (44%) had abnormal autonomic nerve function, which was predominantly parasympathetic. Oesophageal transit was delayed in 28% of the patients and gastric emptying of the solid component of the meal was delayed in 46%. Oesophageal transit was significantly (p less than 0.007) slower in patients with abnormal autonomic nerve function. The percentage of synchronous oesophageal contractions was related to the score for autonomic nerve dysfunction (r = 0.40, p less than 0.05). There was no significant relation of autonomic nerve dysfunction to either delayed gastric emptying or endoscopic grade of oesophagitis. We conclude that in gastro-oesophageal reflux disease there is a high prevalence of parasympathetic nerve dysfunction which relates to delayed oesophageal transit and abnormal peristalsis and may therefore be of pathogenic importance.

Spatial discrimination in children with poor vergence control.

In a 1987 study of children with reading difficulties, Stein, Riddell, and Fowler observed a relationship between poor vergence control, as indicated on a synoptophore vergence test, and the children's problems with learning to read. They suggested that poor vergence control led to impaired accuracy of spatial localisation and that this impeded learning to read. Here we have compared the accuracy of spatial localisation on a nonlinguistic computer game by children having good and poor vergence control. The children with poor vergence control made significantly more errors when locating targets than children with good vergence control. These results lend further support to the hypothesis that some children do not learn to read because they are unable to determine accurately the positions of letters in words.

Distal femoropopliteal bypass using a composite graft of PTFE and non-reversed saphenous vein.

The results of femoropopliteal bypass to the infragenicular popliteal artery, in the absence of suitable saphenous vein have, in the main, been disappointing. We present a new type of composite graft, for use when the distal anastomosis is below the knee, which avoids the potential problems of prosthetic graft alone. The graft consists of a proximal segment of 6 mm expanded PTFE (Gore-Tex; or Impra), anastomosed to transposed non-reversed autologous saphenous vein. Forty-two patients were studied following unilateral, below knee composite femoropopliteal graft surgery for severe claudication or critical ischaemia. Pressure indices were calculated along with intraoperative flow rate, and all patients were followed up at regular intervals to assess graft patency. During the study period three patients died and graft occlusion occurred in a further eight. Analysis of the cumulative patency curve revealed that the majority of occlusions occurred in the first 3 months. The patency at 12 and 18 months was encouraging with values of 84% and 79% respectively. Comparison of pressure indices revealed a significant increase following surgery (P less than 0.001). The postoperative pressure index appeared to predict the grafts likely to occlude and the intraoperative flow rates mirrored a similar trend. Grafts which occluded had a significantly lower pressure index and flow rate (P less than 0.01, P less than 0.002, respectively). Our results suggest that for infragenicular femoropopliteal bypass grafting where full length in-situ vein graft is not possible; a composite graft using PTFE with non-reversed vein is a good alternative.

Disordered vergence control in dyslexic children.

By means of a synoptophore vergence eye movements were recorded in dyslexic and normal children while they were attempting to track small targets moving in simulated depth. Of the dyslexic children 64% were unable to make proper vergence movements when macular sized fusion targets (2 1/2 degrees) were employed, but their vergence control was better for larger (7 degrees) targets. The normal readers and the remaining dyslexics showed normal vergence responses for both large and small moving fusion stimuli. The results suggest that many dyslexics suffer a disorder of visuomotor control and perception for stimuli falling on the macula; this may explain their characteristic visual problems when reading. Hence recording vergence eye movement responses to small moving fusion stimuli may be useful in the investigation and treatment of children with reading difficulties.