Previous epidemiological research has shown that sustained oculomotor load during near-work is associated with ocular symptoms and concurrent musculoskeletal symptoms in the neck-scapular area. Such studies however do not allow for a conclusion about causal relationships between these two groups of symptoms. In experimental studies it was shown that prolonged oculomotor load results in fatigue of intra- and extra-ocular muscles and as a consequence, in an impoverished retinal image. This type of oculomotor fatigue may be counteracted by extra efferentation to the eye muscles before the occurrence of any sensory consequences (i.e., the quality of the retinal image may be sustained by reflex increase of the tone of eye muscles).
The end result may be “irrelevant" co-activation of the muscles in the neck, scapular area and upper back. Recent studies demonstrated activation of the trapezius muscles in a time-integrated dose-response manner during different levels of prolonged accommodation-vergence strain (Richter et al. 2010a,b). This co-activation is hypothesized to occur because of the integration at one cortical location of the neural sensorimotor flow from the centers of attention, vision and motion (Corneil et al. 2002; Richter et al. 2005).
The consequences of this reflex muscle activation for postural adjustments, e.g. in terms of eye-head-torso gaze orientation response, have not been yet investigated (Richter 2008). It may also be inferred that sustained oculomotor load during near-work leads to prolonged periods of low-level and low-variation muscle activity that is thought to be a possible cause for musculoskeletal disorders in the neck-shoulder area.
Also, it was previously shown that musculoskeletal disorders of the neck can affect motor control of eye and head movements (Treleaven 2008). It can be thus hypothesized that sustained oculomotor load might affect motor control strategies of gaze orienting, which would manifest in changed patterns of eye and head movements. On the individual level, two strategies for eye-head movement were described, in which healthy people could be classified in predominantly eye-movers and head-movers (Aitsebaomo and Afanador 1982; Bard et al. 1992; Fuller 1992).
Since it is known that in persons with chronic neck pain the mobility in the neck is reduced (Sjolander et al. 2008), such people may show a predominant strategy of gaze orienting via eye movements. At the same time, eye-head movement strategies might be differently affected by sustained oculomotor load in neck pain patients and in healthy persons (including asthenopics). In general, a simultaneous measurement of eye-head movements is found to be a suitable and clinically relevant approach in neck pain patients (Grip et al. 2009).
Dr. Wolfgang Jaschinski, Leibniz-Institut für Arbeitsforschung, Technische Universität Dortmund
Dr. Mikael Forsman, Institutionen för Folkhälsovetenskap, Karolinska Institutet
Svend Erik Mathiassen