Publications by authors named "G Mitchell"

2,865 Publications

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Visual Performance with Spherical and Multifocal Contact Lenses in a Pediatric Population.

Optom Vis Sci 2021 May 8. Epub 2021 May 8.

The Ohio State University College of Optometry, Columbus, Ohio

Significance: This pilot study compared subjective and objective vision of children wearing single-vision and +2.00, +3.00, and +4.00 D add power soft multifocal contact lenses to determine whether the higher add power-thought to provide better myopia control-resulted in visual compromise.

Purpose: This study aimed to determine the maximum add power children wearing center-distance soft multifocal contact lenses could accept objectively and subjectively.

Methods: Myopic children were assigned in random order to wear omafilcon A single-vision or multifocal "D" contact lenses with +2.00, +3.00, or +4.00 D add power for 1 week each. High-contrast distance and near visual acuity, low-contrast distance visual acuity, and contrast sensitivity were measured at each visit along with a quality of vision questionnaire to assess their vision. The Friedman test was performed to evaluate the impact of add power on all outcome measures.

Results: Eleven subjects were enrolled, and nine subjects completed the study. The median age of completed subjects was 11 years. Median logMAR low-contrast distance visual acuity was reduced in the +3.00 (+0.20) and +4.00 (+0.28) D add lenses compared with the +2.00 (+0.16) D add and single-vision lenses (+0.10, P < .001). All three multifocal lenses resulted in reduced contrast sensitivity (+1.35 to +1.40) compared with single-vision lenses (+1.60, P < .001). In general, +3.00 and +4.00 D add lenses resulted in worse glare/starbursts, ghost images, computer vision, changing fixation distance, and overall vision, but results varied. There were no differences among the lenses with respect to subjective assessments of distance vision, near vision, strain or tiredness, contact lens comfort, or sporting activities.

Conclusions: The +3.00 D and higher add powers result in more objective and subjective vision-related issues than single-vision lenses, but the +2.00 D add multifocal lenses were well tolerated.
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http://dx.doi.org/10.1097/OPX.0000000000001695DOI Listing
May 2021

Single-session effects of acute intermittent hypoxia on breathing function after human spinal cord injury.

Exp Neurol 2021 May 2:113735. Epub 2021 May 2.

Department of Physical Therapy, University of Florida, Gainesville, FL, United States; Brooks Rehabilitation, Jacksonville, FL, United States; Breathing Research and Therapeutics Center, University of Florida, Gainesville, FL, United States. Electronic address:

After spinal cord injury (SCI) respiratory complications are a leading cause of morbidity and mortality. Acute intermittent hypoxia (AIH) triggers spinal respiratory motor plasticity in rodent models, and repetitive AIH may have the potential to restore breathing capacity in those with SCI. As an initial approach to provide proof of principle for such effects, we tested single-session AIH effects on breathing function in adults with chronic SCI. 17 adults (13 males; 34.1 ± 14.5 years old; 13 motor complete SCI; >6 months post injury) completed two randomly ordered sessions, AIH versus sham. AIH consisted of 15, 1-min episodes (hypoxia: 10.3% O2; sham: 21% O2) interspersed with room air breathing (1.5 min, 21% oxygen); no attempt was made to regulate arterial CO2 levels. Blood oxygen saturation (SpO2), maximal inspiratory and expiratory pressures (MIP; MEP), forced vital capacity (FVC), and mouth occlusion pressure within 0.1 s (P0.1) were assessed. Outcomes were compared using nonparametric Wilcoxon's tests, or a 2 × 2 ANOVA. Baseline SpO2 was 97.2 ± 1.3% and was unchanged during sham experiments. During hypoxic episodes, SpO2 decreased to 84.7 ± 0.9%, and returned to baseline levels during normoxic intervals. Outcomes were unchanged from baseline post-sham. Greater increases in MIP were evident post AIH vs. sham (median values; +10.8 cmHO vs. -2.6 cmHO respectively, 95% confidence interval (-18.7) - (-4.3), p = .006) with a moderate Cohen's effect size (0.68). P, MEP and FVC did not change post-AIH. A single AIH session increased maximal inspiratory pressure generation, but not other breathing functions in adults with SCI. Reasons may include greater spared innervation to inspiratory versus expiratory muscles or differences in the capacity for AIH-induced plasticity in inspiratory motor neuron pools. Based on our findings, the therapeutic potential of AIH on breathing capacity in people with SCI warrants further investigation.
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http://dx.doi.org/10.1016/j.expneurol.2021.113735DOI Listing
May 2021

Exploration of changes in health-related behaviours among Saudi Arabian undergraduates in the UK.

Health Promot Int 2021 May 5. Epub 2021 May 5.

Environmental Protection, Workplace Environment Division, Environmental Health Unit, Al-Midra Tower, Dhahran 31311, Kingdom of Saudi Arabia.

This project is an exploratory survey of the ways in which living in the UK has affected the health-related behaviours of Saudi Arabian undergraduate students. The study identifies changes in exercise behaviours, dietary and smoking habits and experiences of stress after their move to the UK. In addition, it identifies what the students perceive to be the drivers of these changes. To achieve this, an online questionnaire was developed collecting quantitative data. This was distributed via Facebook groups specifically for Saudi students in the UK. The results demonstrate that a majority of Saudi Arabian undergraduates felt that their behaviours had become healthier since their move to the UK, mainly as a result of a more active lifestyle, derived from reduced reliance on cars, increased daily walking, and a heightened focus on health promoted by their independence, the availability of healthy foods and exposure to a more health-promoting culture. Although these results are based on a relatively small sample, they are surprising since they contradict previous studies on international students, which have generally found negative lifestyle changes during their studies, including increased stress, poorer dietary habits and a decline in physical activity. Although the exploratory nature of the study means that concrete recommendations cannot be made, it nonetheless offers an impetus for further research into how Saudi Arabian students may be encouraged to adopt healthier lifestyles while studying in the UK.
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http://dx.doi.org/10.1093/heapro/daab055DOI Listing
May 2021

Efficacy and time course of acute intermittent hypoxia effects in the upper extremities of people with cervical spinal cord injury.

Exp Neurol 2021 Apr 28;342:113722. Epub 2021 Apr 28.

Shirley Ryan Ability Lab, Northwestern University, Chicago, IL 60611, United States of America; Department of Physical Medicine and Rehabilitation, Northwestern University, Chicago, IL 60611, United States of America.

Spinal cord injuries (SCI) disrupt neural pathways between the brain and spinal cord, causing impairment of motor function and loss of independent mobility. Spontaneous plasticity in spared neural pathways improves function but is often insufficient to restore normal function. One unique approach to augment plasticity in spinal synaptic pathways is acute intermittent hypoxia (AIH), meaning brief exposure to mild bouts of low oxygen, interspersed with normoxia. While the administration of AIH elicits rapid plasticity and enhances volitional somatic motor output in the lower-limbs of people with incomplete SCI, it is not known if AIH-induced neuroplasticity is equally prevalent in spinal motor pathways regulating upper-extremity motor-function. In addition, how long the motor effects are retained following AIH has not yet been established. The goal of this research was to investigate changes in hand strength and upper-limb function elicited by episodic hypoxia, and to establish how long these effects were sustained in persons with incomplete cervical SCI. We conducted a randomized, blinded, placebo-controlled and cross-over design study consisting of a single AIH or sham AIH session in 14 individuals with chronic, incomplete cervical SCI. In a subset of six participants, we also performed a second protocol to determine the cumulative effects of repetitive AIH (i.e., two consecutive days). In both protocols, hand dynamometry and clinical performance tests were performed pre- and post-exposure. We found that a single AIH session enhanced bilateral grip and pinch strength, and that this effect peaked ~3 h post-intervention. The strength change was substantially higher after AIH versus sham AIH. These findings demonstrate the potential of AIH to improve upper-extremity function in persons with chronic SCI, although follow-up studies are needed to investigate optimal dosage and duration of effect.
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http://dx.doi.org/10.1016/j.expneurol.2021.113722DOI Listing
April 2021

Cervical spinal injury compromises caudal spinal tissue oxygenation and undermines acute intermittent hypoxia-induced phrenic long-term facilitation.

Exp Neurol 2021 Apr 26;342:113726. Epub 2021 Apr 26.

Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida, Gainesville, FL 32610, USA. Electronic address:

An important model of respiratory motor plasticity is phrenic long-term facilitation (pLTF), a persistent increase in phrenic burst amplitude following acute intermittent hypoxia (AIH). Moderate AIH elicits pLTF by a serotonin-dependent mechanism known as the Q pathway to phrenic motor facilitation. In contrast, severe AIH (greater hypoxemia) increases spinal adenosine accumulation and activates phrenic motor neuron adenosine 2A receptors, thereby initiating a distinct mechanism of plasticity known as the S pathway. Since the Q and S pathways interact via mutual cross-talk inhibition, the balance between spinal serotonin release and adenosine accumulation is an important pLTF regulator. Spinal injury decreases spinal tissue oxygen pressure (PtO) caudal to injury. Since AIH is being explored as a neurotherapeutic to restore breathing ability after cervical spinal injury, we tested the hypothesis that decreased PtO in the phrenic motor nucleus after C2 spinal hemisection (C2Hx) undermines moderate AIH-induced pLTF, likely due to shifts in the adenosine/serotonin balance. We recorded C3/4 ventral cervical PtO with an optode, and bilateral phrenic nerve activity in anesthetized, paralyzed and ventilated rats, with and without C2Hx. In intact rats, PtO was lower during severe versus moderate AIH as expected. In chronic C2Hx rats (> 8 weeks post-injury), PtO was lower during baseline and moderate hypoxic episodes, approaching severe AIH levels in intact rats. After C2Hx, pLTF was blunted ipsilateral, but observed contralateral to injury. We conclude that C2Hx compromises PtO near the phrenic motor nucleus and undermines pLTF, presumably due to a shift in the serotonin versus adenosine balance during hypoxic episodes. These findings have important implications for optimizing AIH protocols in our efforts to restore breathing ability with therapeutic AIH in people with chronic cervical spinal injury.
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http://dx.doi.org/10.1016/j.expneurol.2021.113726DOI Listing
April 2021