Friday, 8 October 2010

MRCP revision battle 27.5: Visual field defects

Visual fields pop up frequently in MRCP questions so it pays to learn this well!

The basic anatomy is illustrated below:

Armed this this anatomy lets look at the various patterns of field defect...

1: Bitemporal hemianopia
  • caused by compression of the optic chiasm
  • upper affected more than lower = due to pituitary tumour
  • lower affected more than upper = due to craniopharyngioma
  • I personally remember this as UP  London City

2: Homonymous quadrantanopia

  • superior homonymous quadrantanopia is a lesion in the temporal lobe
  • inferior homonymous quadrantanopia is a lesion in the parietal lobe
  • this can be remembered by thinking PITS - parietal inferior temporal superior
  • if the defect is incongruous it is in the optic tract
  • if the defect is congruous it is in the optic radiation/cortex

3: Homonymous hemianopia
  • injury to the brain (eg bleed, tumour) on the opposite side to the field defect

4: Central scotoma

  • usually caused by optic neuritis

5: Binasal hemianopia
  • rare
  • ?calcification of carotids

Note that the fovea is supplied by both the PCA and MCA so may be spared in a PCA CVA.

In the calcarine sulcus peripheral regions are processed anteriorly while central regions are processed posteriorly.

Finally remember cortical blindness:
  • caused by bilateral occipital infarcts
  • pupillary responses are preserved
  • possibly small macular sparing
  • patient may deny they have vision loss (=Anton's syndrome)

Wow, that was lots for one battle... lets move on to some urine for light relief...