Head tilt can affect RNFL, macular thickness measurements

This study from the November issue of the British Journal of Ophthalmology evaluated the effects of head tilt on retinal nerve fiber layer (RNFL) and macular thickness measurements obtained with spectral-domain optical coherence tomography (OCT). The results demonstrate that even a moderate degree of head tilt (about a quarter of a clock hour) can cause significant shifts in sectoral thicknesses.

The study's findings are important to consider when performing both initial OCT analysis and follow-up scans. Head tilt can cause a normal optic nerve to show abnormal sector thinning when compared with the normative database of the instrument, while head tilt on subsequent scans can suggest apparent thinning of some sectors.

In this study a single, well-trained examiner took RNFL and macular thickness measurements in 30 right eyes from 30 healthy young subjects with the Cirrus HD OCT. The measurements were made at a baseline head position and at right and left head tilt positions.

Right and left head tilt induced counterclockwise (mean 8.27 degrees) and clockwise (mean 8.47 degrees) rotation of the optic disc. Right head tilt caused superior-temporal RNFL thickening, inferior-temporal RNFL thinning, superior outer macular thickening and inferior outer macular thinning (all p values < 0.05). Left head tilt induced superior-temporal RNFL thinning, inferior-temporal RNFL thickening, superior outer macular thinning, nasal outer macular thickening and inferior outer macular thickening (all p values < 0.05).

The results suggest that it is critical for clinicians to review the temporal-superior-nasal-inferior-temporal (TSNIT) RNFL curves on follow-up scans to determine if a head tilt/shift has occurred. A left or right shift in the TSNIT curve may account for some apparent thinning of individual sectors.

Although this study used the Cirrus OCT, its findings should be applicable to other OCT instruments that yield sectoral thickness measurements.