Describe cortical visual loss and the higher visual disorders.

Cortical Visual Loss and Higher Visual Disorders

• Cortical blindness results from bilateral occipital lesions — most often bilateral posterior cerebral artery occlusion from basilar disease, and also from cardiac arrest, eclampsia, posterior reversible encephalopathy syndrome, trauma and hypoglycaemia
• The diagnostic combination IS blindness with normal pupils and A normal fundus. The pupillary light reflex leaves the pathway at the pretectum, well before the cortex, so it is entirely preserved — and a patient who cannot see with normally reacting pupils and healthy discs has a cortical problem, or none at all
• ANTON syndrome (visual anosognosia) — the cortically blind patient denies being blind and confabulates visual descriptions, bumping into furniture while insisting the room is dim or their glasses are wrong. It is one of the most striking phenomena in neurology, and explains why such patients are sometimes referred as having a psychiatric illness
• The riddoch phenomenon — preserved perception of movement within a blind field, with no perception of static form or colour. Patients describe sensing that something moved without seeing what it was
• BALINT syndrome — from bilateral parieto-occipital lesions, comprising simultanagnosia (inability to perceive more than one object at a time, so a scene cannot be taken in as a whole), optic ataxia (inability to reach accurately under visual guidance) and oculomotor apraxia (inability to direct gaze voluntarily to a target)
• Visual agnosia — failure to recognise objects despite adequate vision; and prosopagnosia, the specific inability to recognise faces, from bilateral or right fusiform lesions — patients identify people by voice, gait or clothing
• ALEXIA without agraphia — from a lesion of the left occipital lobe and the splenium of the corpus callosum: the patient cannot read but can write, and is unable to read back what they have just written. accompanied by a right homonymous hemianopia
• Charles bonnet syndrome — formed visual hallucinations in a patient with poor vision from any cause, with intact insight and NO psychiatric disease. Patients see people, animals, patterns or lilliputian figures, and conceal it for fear of being thought mad. Asking about it and explaining it is the treatment

Migraine, Functional Visual Loss and Related Phenomena

• Migraine with aura — the commonest cause of transient visual disturbance in a young patient. The classical aura is a scintillating scotoma beginning near fixation and marching outward over 20 to 30 minutes, with fortification spectra — shimmering zigzag lines resembling the plan of a fortified town — and leaving a scotoma behind it, followed by headache
• Three features distinguish it from a transient ischaemic attack: it is binocular and homonymous (though patients invariably describe it as one eye); it builds and marches over minutes rather than appearing complete instantly; and it lasts 20 to 30 minutes rather than seconds to a few minutes. Amaurosis fugax is monocular, abrupt and brief
• Acephalgic migraine — the aura without headache, common in later life and a frequent cause of alarm; and retinal migraine, which is genuinely monocular and rare, and should not be diagnosed until embolic causes are excluded
• Functional (non-organic) visual loss — commoner than is recognised, particularly in adolescents and after trauma or litigation. The diagnosis is made positively, by demonstrating vision the patient denies, not by exclusion alone
• Useful tests — the absence of a relative afferent pupillary defect in claimed severe monocular loss; optokinetic nystagmus, which cannot be suppressed and requires vision of at least 6/60; the mirror test, in which a moving mirror induces following movements; fogging and prism dissociation techniques, which trick the patient into reading with the "blind" eye; a tubular (non-expanding) field on perimetry at different distances, which is physiologically impossible; and a normal visually evoked potential
• Management — avoid confrontation; reassure that no structural disease has been found and that recovery is expected; suggest a face-saving explanation such as eye strain; and involve psychological services where there is distress or a clear precipitant. Organic disease must still be excluded, since functional and organic disease may coexist
• Visual snow — continuous tiny flickering dots across the whole visual field, like television static, with palinopsia, photophobia and floaters, in young patients often with migraine. It is a recognised entity, not a psychiatric symptom, and reassurance with explanation is the mainstay
• Normal pupils with blindness means cortical or functional, since the light reflex leaves the pathway before the cortex
• Anton syndrome makes the patient deny blindness, which is why such patients are sometimes referred as psychiatric
• Migraine aura is binocular and homonymous, though patients invariably describe it as affecting one eye
• Aura marches over twenty minutes, whereas amaurosis fugax is abrupt, monocular and lasts only minutes
• Diagnose functional loss positively, by demonstrating vision the patient denies, rather than by exclusion alone
• A tubular field is physiologically impossible, since a normal field expands with testing distance
• Optokinetic nystagmus cannot be suppressed, and its presence establishes vision of at least 6/60
• Avoid confrontation in functional visual loss, and offer a face-saving explanation with expectation of recovery
• Organic and functional disease may coexist, so the examination must still be complete
• Explain Charles Bonnet syndrome without being asked; patients conceal the hallucinations and the explanation itself is therapeutic