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Editors Selection IGR 13-1
RNFL Imaging: Effect of head tilt on RNFL and macular thickness
Comment by Mitra Sehi on:
48297 The effect of head tilt on the measurements of retinal nerve fibre layer and macular thickness by spectral-domain optical coherence tomography, Hwang YH; Lee JY; Kim YY, British Journal of Ophthalmology, 2011; 95: 1547-1551
In a prospective study, Hwang et al. (1870) examined the impact of right and left head tilt on the accuracy of retinal nerve fiber layer (RNFL) and macular thickness values compared with the primary head position in 30 right eyes of 30 healthy young volunteers using Spectral- domain optical coherence tomography (SDOCT; Cirrus; Carl Zeiss Meditec, Dublin, CA). They found that the right head tilt induced counter-clockwise (mean 8.27°) rotation of the optic disc and temporal RNFL thickening, whereas, the left head tilt induced clockwise (mean 8.47°) rotation of the optic disc and temporal RNFL thinning. A significant change in the position of superior and inferior RNFL peak was observed in both head tilt positions. The right head tilt induced superior outer macular thickening and inferior outer macular thinning. The left head tilt induced superior outer macular thinning, nasal outer macular thickening and inferior outer macular thickening. The authors mention that the outer macular thickness changes associated with head tilt were significant, but not greater than the changes caused by OCT measurement variability and therefore, may not be as great as it is on RNFL thickness. The inner macular thickness did not change with either head tilt position.
Although the authors measured the angle of optic disc rotation with each head tilt position accurately, they did not quantify the actual angle of head tilt in each position. The authors fail to mention whether both eyes were open during SDOCT imaging. They argued that the visual stimulus used for the Cirrus SDOCT measurements is a monocular non-spatial fixation target and therefore, they did not account for the compensatory mechanisms. However, It has been demonstrated that the head tilt induces compensatory eye movements including excyclovergence mediated by vestibulo-ocular response (VOR), and the ocular counterrolling (OCR) reflex. The OCR reflex is responsible for the ocular torsion around the anterior-posterior y-axis to balance the direction of the head tilt and is mediated by the response of the semicircular canals, and the change of direction of the gravitational force when tilting the head.1,2 Therefore, with the head tilt, the peripheral spatial visual cues to a lesser degree and the VOR and OCR reflex to a greater degree have compensatory impacts. This study is clinically relevant in cross-sectional evaluation of glaucomatous damage, and also in the longitudinal evaluation of structural progression, and it raises awareness regarding this source of noise in imaging. However, larger studies that accurately measure the angle of head tilt, the compensatory response and the residual impact on the RNFL and macular thickness are still required.
References
- Leigh RJ, Zee DS. The Neurology of Eye Movements. 3rd ed. Oxford, UK: Contemporary Neurology Series; 1999:21-43.
- Pansell T, Ygge J, Schworm HD. Conjugacy of torsional eye movements in response to a head tilt paradigm. Invest Ophthalmol Vis Sci. 2003;44(6):2557-64.
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