If a large pituitary or parapituitary tumour compresses the optic chiasm, it may cause visual field defects. The nasal visual fields are transmitted by temporal retinal fibres. The temporal visual fields are transmitted by the nasal retinal fibres and these fibres cross at the optic chiasm. Therefore, if, as frequently is the case, a pituitary tumour grows upwards centrally towards the chiasm, the nasal central fibres become compressed, thus initially causing an upper outer quadrant field defect. If the pressure on the chiasm continues, a complete bitemporal hemianopia may occur. With long-standing compression, this will progress to irreversible optic atrophy and blindness. The growth of the tumour and the resultant visual field defects may, however, be asymmetrical. Further upward extension may cause a hypothalamic disturbance and produce disturbed consciousness, abnormal thirst, hyperphagia, or abnormal temperature regulation.
