Anatomical differences in optic nerve, chiasma and tractus opticus in human albinism as demonstrated by standardised clinical and MRI evaluation

Background: There has been long-standing clinical and electrophysiological evidence that in patients with albinism the visual pathways cross atypically: most fibres from one eye cross to the contralateral visual cortex. Purpose of the study: to determine whether the size and configuration of the optic chiasm in human albinos is different from normally pigmented controls. Patients and methods: 17 patients (11 female, mean age 35.8 years) with oculocutaneous albinism underwent a standardised graded morphological and functional evaluation. Magnetic resonance images were reformatted to the region of the optic chiasm and analysed using observer-independent morphometry. In addition, fMRI of the visual cortex was performed during VEP analysis (1.5 Tesla Siemens Vision). Morphological and fMRT results were compared to an age-correlated group of n = 16 normally pigmented healthy volunteers with normal visual acuity and stereopsis. Results: 65% of the patients (n = 10) showed signs of dysplasia of the optic nerve head. Statistical morphometry showed distinct differences in chiasmal morphology between albinos and normally pigmented probands (smaller optic nerves, different angles of optical entry into the chiasm and of the beginning of the Tractus optici leaving the chiasm, overall chiasmal width and height). Conclusions: Optic nerve head anomalies are frequent in albinism and influence visual outcome. Size and configuration of the optic chiasm in human albinos is distinctly different from normally pigmented control persons and reflects the atypical crossing of optic fibres.