There were significant, positive correlations between pain and blood flow in orbicularis oculi (Table 4), but no significant correlations were observed between pain and muscle activity (not shown in table).
The subject population was divided into a Pain group and a Minimal pain group based on the subjects' score on the VAS. The Pain group was defined as having an increase in the VAS score of >10 mm from baseline to after 1 h. The Minimal pain group was defined as having an increase in the VAS score ≤10 mm from baseline to after 1 h. In the literature, an increase from baseline in scores of around 10 mm VAS is regarded clinical important.51,52 In both the Minimal pain group and the Pain group, all the subjects had baseline symptoms <10 mm VAS.
Blood flow was overall significantly higher in the Pain group during the first hour of computer work compared with the Minimal pain group. There were no overall significant differences in muscle activity between the Pain and Minimal pain group. Muscle activity and blood flow showed significant overall time effects in both symptoms groups, except for blood flow in the Minimal pain group (Fig. 8).
Major significant findings are as follows: (1) Visually demanding computer work for 2 h induced several eye-related symptoms such as pain, tiredness, and blurred vision. (2) Orbicularis oculi muscle load was increased and stable during the 2 hours of computer work, whereas muscle blood flow was increased during the first 40 min of the first hour of computer work and the first minute of the second hour of work before returning to baseline. (3) There were positive correlations between tiredness symptoms and muscle load and pain symptoms and muscle blood flow in orbicularis oculi. (4) Subjects who developed eye-related pain showed increased orbicularis oculi blood flow during the first hour of computer work compared with subjects with minimal pain symptoms.
The visually demanding computer work lasted for 2 h and induced several symptoms associated with computer vision syndrome.1,2 Tiredness, pain, and blurred vision symptoms may have increased because glare and small font in addition to performing a high-precision task over time is demanding for convergence and accommodation.2,4,6,9,10 During the computer task, attention-reduced blinking led to increased ocular surface exposure which may have disrupted the precorneal tear film and ultimately resulted in dry and sore eyes.53 – 55 The origin of the pain symptoms “in the eye” may therefore be the cornea, the ciliary body, and extraocular muscles.2 Because of increased squinting to avoid the glare during the 2 h of computer work, the orbital part of the orbicularis oculi muscle contracted, and pain and tiredness symptoms “around the eyes” may therefore have its origin in the orbicularis oculi muscle.21 Tiredness and muscle load were positively correlated in the present study, in accordance with earlier studies.20 – 22
Only tiredness and blurred vision symptoms showed a further increase in intensity after 2 h compared with after 1 h of computer work. In line with this, in a recent study using a similar computer-based office work task [90 min continuous work, stationary computer with a slightly bigger screen (2 in), Arial font 11, no glare], eye strain symptoms increased steeply the first 30 min of computer work followed by a slow increase in the last 60 min.30 However, symptoms in the last hour could also be influenced by ingested caffeine and sugar in the break between the 2 h of computer work.56
Muscle activity in orbicularis oculi was low during computer work (mean values <2% MVC). Musculoskeletal disorders have been related to static muscle loads as low as 0.5 to 1% MVC and to sustained muscle load in general. Hence, focus in occupational ergonomics has been changed from maximum acceptable % MVC limits to time limits for prolonged sustained or repeated muscle contractions.57 – 61 The work performed by the orbital part of orbicularis oculi during the visually demanding computer work task may be characterized as sustained/repetitive low-force contractions as the test subjects more or less continuously squint to improve the visual acuity because of small font on the computer screen and to avoid excessive light from the glare source from entering the eye.20 Several studies have shown that repetitive low-force contractions induce metabolic changes in skeletal muscle linked to pain perception.25,62 – 64 In the trapezius (pars descendens), increased levels of interstitial muscle lactate, potassium, and glutamate were detected during a 20 min repetitive low-force arm exercise.25,62,63 Glutamate stimulates nociceptors by the NMDA receptor, and lactate lowers pH and increased H+ may stimulate several proton-sensitive receptors on the nociceptive nerve ending.23,65,66 Increased interstitial potassium may increase muscle pain sensation by depolarizing nerve endings and afferent fibers.23 It is possible that similar interstitial changes occurred in the orbicularis oculi muscle during the visually demanding work task. This could contribute to the increase in pain and tiredness symptoms during the 2 h of computer work.
Blood flow in orbicularis oculi was significantly increased during the first 40 min of the first hour of computer work and the first minute of the second hour of work before returning to baseline (Fig. 7). The return of blood flow to baseline before the end of computer work could not be explained by corresponding changes in muscle activity, which was significantly increased and stable during both hours of work (Fig. 6). In accordance, a study using a similar computer-based office work task also showed a significant fall in trapezius blood flux after 30 min of computer work without similar changes in muscle load.30 Increased microcirculation in relation to increase in muscle activity is well documented in the literature.25,64 The following reduced blood flow response in this study could be due to time-dependent changes in interstitial vasoactive metabolites in the muscle, such as potassium. Potassium induces vasodilation by hyperpolarization of smooth muscle cells involving activation of inward rectifier potassium channels and the Na+/K+ ATPase.67 – 69 Depolarization with KCl and blockade of Ca2+-activated K+ channels inhibit initial vasodilation in soleus muscle and gastrocnemius muscle in rats.70 This indicates that potassium is involved in the early phase of reactive hyperaemia in skeletal muscle.71,72 During a 60 min period of exercise (4 twitches/sec) in dog anterior calf muscles with blood flow held constant, venous potassium first increased before a decreasing trend after about 20 to 30 min of exercise.71 The same decreasing trend in venous potassium level after 30 min of exercise has been shown during low-intensity isometric knee extension (5% MVC) lasting for 60 min.73 Blood flow in the second hour of work in this study was lower compared with the first hour, despite sustained muscle activity. In the 5% MVC isometric knee extension study, subjects rested for 10 min after the 60 min exercise before another 10 min exercise at same intensity level. During the 10 min break, venous potassium levels fell below rest values before start of the first 60 min exercise session. In the following 10 min exercise, potassium concentration increased but did not reach the levels in the first exercise session.73 Consequently, changes in skeletal muscle blood flow during prolonged low-force exercise could be influenced by changes in interstitial metabolites, such as potassium. In future experiments, muscle oxygenation should be measured along with blood flow to further investigate how the changes in blood flow affects muscle metabolism.74
Heart rate and mean arterial pressure were not recorded in this study. Earlier studies using mental stress tasks, including demanding computer work, have reported significant increases in heart rate, mean arterial pressure, and muscle blood flow, probably due to an active coping response/central command.30,75 – 77 The changes in blood flow seen in this study may therefore be partly caused by a central neural response and not solely linked to changes in muscle activity.77 In addition, the observed changes in blood flow could be influenced by local heating.78 Increased heart rate due to mental stress may shunt blood to the skin to dissipate heat79 and consequently muscle blood flow could have been affected because of inhibited convection and increased temperature under the PPG probe.38 Temperature in the test room in this study only increased by 1°C during testing, and this would probably not influence muscle blood flow significantly.80
Orbicularis oculi muscle blood flow in the second hour of computer work could also be affected by drinks/snacks ingested during the break between the 2 h. Some of the subjects drank one cup of coffee/tea in the break. Caffeine influences the vasodilator effect of adenosine.72,81 – 83 However, previous human studies showing vasoconstrictor effects of caffeine on blood flow during exercise have used doses ≥6 mg/kg.82,84 A single cup of coffee provides a caffeine dose of <3 mg/kg.85
In this study, we showed a positive correlation between eye-related pain and blood flow in orbicularis oculi, together with no significant association between pain and muscle activity. In the first hour of computer work, subjects who developed pain symptoms had significantly higher muscle blood flow compared with subjects with minimal pain symptoms. A recent study on trapezius showed a significant association between development of pain over time and blood flow and no significant interactions between pain ratings and muscle load during a 90 min computer task.30 In a follow-up study, this reference group was compared to subjects with chronic shoulder and neck pain doing the same 90 min computer exercise, and during the task, trapezius blood flow increased to the same extent in both pain-afflicted subjects and controls despite higher pain sensitivity during exercise in the subjects with chronic shoulder and neck pain. The study showed significant positive correlations between pain ratings and trapezius blood flux and no significant associations between pain and EMG recordings in the active trapezius of subjects with chronic shoulder and neck pain.26 In patients with myalgia, there have been done a number of studies on muscle activity and metabolism related to development of pain. Recordings in trapezius during 8 h of repetitive manual work in women with chronic shoulder and neck pain showed higher pain intensity and consistently elevated blood flow compared with pain-free female colleagues but no differences in muscle load between the groups.24 In accordance, women with chronic work-related myalgia (MYA) doing 20 min of repetitive low-force exercise (moving short wooden sticks back and forth on a pegboard) exhibited significantly increased pain intensity compared with female controls (CON), but trapezius muscle blood flow during exercise increased to the same extent in both groups. However, the MYA group showed higher levels of interstitial potassium during exercise.25,63 In a similar study using the same protocol but with a heavier load (100 g sticks compared with 23 g sticks), pain ratings and relative muscle load was higher in the MYA group together with reduced blood flow during exercise compared with CON. Potassium levels did not differ between the groups.74 When comparing the “light” (23 g sticks) and “heavy” version (100 g sticks) of the repetitive exercise, the relative increase during exercise in potassium and blood flow was higher in the “light” compared with the “heavy” version in the MYA group.25,63,74 This may indicate differences in potassium levels depending on muscle activity pattern which could affect blood flow.25,63,74 Interestingly, in the present study at the time points during computer work were the association between pain and blood flow were strongest (Table 4), potassium levels have also been shown to be at the highest during low-force exercise.73
An additional or alternative explanation for the positive correlation between pain and blood flow may be that subjects experiencing pain have an increased mental stress level compared with subjects with minimal pain, inducing a greater sympathoinhibitory effect in the muscle and muscle vasodilation.77 Vasodilation has previously been directly linked to pain sensation in the blood vessel-nociceptor interaction hypothesis.27
In conclusion, eyestrain during visually demanding computer work is related to the orbicularis oculi muscle. Muscle pain development during demanding, low-force exercise may be related to increased muscle blood flow. Subjects who develop pain may have a different muscle activity pattern, producing metabolites linked to both increased pain sensation and vasodilation, and/or increased mental stress level producing vasodilation and pain compared with subjects with minimal pain development.23,72,73,77 The mechanisms behind this pain development may be different compared to during contractions at higher muscle loads where microcirculation is attenuated.86,87
We thank the students at Department of Optometry and Visual Science, Buskerud University College, for participating as test subjects in the study. We also thank the students Sina Delavari, Steffen Amlie Hole, Nina Haarslev Johannessen, Anders Olsrud, Anne Therese Putkowski, and Joakim Sand for contributing with Figure 5. A special thanks to Professor Eric Rinvik at Institute of Basic Medical Sciences, Department of Anatomy, University of Oslo, for organizing anatomical studies of the orbicularis oculi muscle.
This work was supported by grant number 176541/V10 from The Norwegian Research Council.
This paper was presented as a poster at the 40th European Muscle Conference in Berlin, Germany, in September 2011.
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