Letter to the Editor: The lens is stable during accommodation

Dear Editor, Using wavefront aberrometry, Radhakrishnan and Charman (2007) reported a very small, progressive increase in with-the-rule astigmatism during accommodation. The authors measured less than 1.0 dioptre of associated astigmatism. As these changes were small, an ANOVA is required to assess whether there is any relationship between the magnitude of the subjects astigmatism and its change during accommodation. The authors and Cheng et al. (2004a) used similar methodology and consequently the small aberrometric measurements of with-the-rule astigmatism observed during accommodation in both studies may reflect the following uncontrolled or confounding variables: 1. Narrowing of the palpebral fissure during aberrometry induces with-the-rule-astigmatism (Han et al., 2007). 2. The cornea is not symmetrical and its apex is not coincident with the optic axis of the eye (Navarro et al., 2006). Consequently, movement of the eye, relative to the aberrometer, may have caused the observed astigmatism. As little as 3.5 of eye movement, relative to the aberrometer, can induce changes in measurements of astigmatism (Cheng et al., 2004b). 3. Non-randommovements associated with the experiment itself can be responsible for the observed increase in with-the-rule astigmatism during accommodation. During monocular accommodation, the viewing eye cyclotorts and undergoes vergence-induced positional change. Buehren et al. (2003), using videokeratoscopy, have demonstrated 4 of excyclotorsion of the viewing eye during monocularly induced accommodation of 7 dioptres. Additionally, Steffen et al. (2000), by tracking both eyes simultaneously with three axes magnetic field search coils, have demonstrated that the horizontal primary position of the viewing eye changed by a mean 4.4 during 5 dioptres of monocularly induced accommodation. These are non-random eye movements not subject to statistical averaging. The authors attribute their observation of a small, progressive increase in with-the-rule astigmatism during accommodation to increasing lens instability with increasing accommodation. Careful experiments, using a calibrated, validated, very high resolution dual Purkinje image eyetracker (DPI), that also tracked the distance between the cornea and the DPI, have demonstrated the lens remains in a stable position at up to 7 dioptres of accommodation with concurrent eye saccades and/or rapid head movements (Schachar et al., 2007). Consistent with these findings, Kirschkamp et al. (2004), using Purkinje imaging, found that the lens was stable during accommodation and Sokolowska and Thorn (2003) found lens stability increased with increasing accommodation. Sokolowska and Thorn (2003) state that during accommodation to 25 cm, horizontal tilt decreased significantly while vertical tilt changed little and concluded there was a significant decrease in lens tilt. They indicate that the lens remains rigidly in position during accommodation (Confirmed by personal communication with Professor Frank Thorn, on 31 May 2007). Furthermore, lens focus has been observed to be stable under differing gravitational conditions, a variable which might be anticipated to affect an unstable lens. Atchison et al. (1994) did not find a statistically significant difference in the mean accommodative amplitude of young subjects when they moved their heads so that their gaze was 20 , 45 or 90 above or below the horizontal plane, see column 4 of Table 3 of their reference. Note that all accommodative amplitude values pertaining to head movements were <0.441 dioptres, the amplitude change required for statistical significance in their study. Sixty-five subjects between 8 and 44 years of age were strapped to a modified neurosurgical bed (Schachar and Cudmore, 1994). Their heads were fixed to the bed with a large tightly fitting forehead clamp. The near point of the subjects was measured while the subjects were in the supine and prone position. Schachar andCudmore (1994) concluded the lens was stable. The 2 mm near point recession that occurred in the prone positionwas constant between subjects, R = 0.007, and as these investigators stated, was probably due to compression of the forehead skin against the clampwhen the subjects were in the prone position and not to movement of the lens. Astronauts report no change in accommodative amplitude at high levels of gravitational force (8gs) Ophthal. Physiol. Opt. 2007 27: 520–521