Chiasma Lower nasal fibres cross low and anteriorly Upper nasal fibres cross high and posteriorly Macular fibres also cross in the posterior part of the chiasm.
Location of chiasma Central fixation -80%- above the sella Pre fixed chiasm-10%-located anteriorly- so pitutary tumour involves the optic tract first [lower temporal fields first] Post fixed chiasm-10%-located posteriorly- so optic nerve gets involved first [upper temporal fields first]
Pitutary adenoma Visual fields ; bitemporal hemianopia,junctional scotoma, bitemporal hemianopic scotoma Colour vision; early red deficit Visual acuity tends to reduce Optic disc- bow tie atrophy rarely papilloedema Extraocular movements: cranial nerve palsies,see saw nystagmus,spasm nutans.
hemifield slip- due to the failure ofcontrolling phoria by fusion.Post fixation blindness.
OPTIC RADIATIONS The corresponding retinal elements lie progressively closer, so congruous hemianopia. Passes through the temporal lobe and pareital lobe and ends in the visual cortex.
TEMPORAL LOBE Controlateral congruous homonymous superior quadrantanopia[pie in the sky] Controlateral hemisensory disturbance Mild hemiparesis Paraxysomal olfactory and uncinate fits. Formed visual hallucinations
PAREITAL LOBE Controlateral congruous homonymous inferior quadrantanopia[pie on the floor] Visual perception difficulties Right-left confusion Acalculia Assymmetric OKN.[OKN response diminished towards the side of the lesion.]