Visual fields and lesions of the visual pathways (CN II)

This appears in Question 7.2 from the second paper of 2008. The discussion of visual pathway lesions lends itself especially well to explanation by means of a massive insane-looking eyeball diagram, which I have put together many years ago in med school. This summary page combines the insanity of colourful eyeball diagrams with the sober calm of tables. For a thorough exploration of bedside visual field testing technique, one can review Chapter 116 by R.H Spector from Clinical Methods (1990). And for a banquet of juicy detail, one should spend some quality time with "Topical diagnosis of chiasmal and retrochiasmal disorders" by Levin, from Walsh and Hoyt clinical neuro-ophthalmology, 6th ed. Lastly, if one has all the time in the world, one could use it to become familiar with Kidd Newman and Biousse's Neuro-ophthalmology.

Lesions of the visual pathways, with anatomical relationships

In brief description, the anatomy of the visual pathways is as follows:

• Retina
• Optic nerve
• Optic chiasm (and there, the nasal visual field fibers cross over to the opposite side)
• Optic tract
• Lateral geniculate nucleus
• Optic radiation
• Primary and secondary visual cortices

Like most anatomical topics in neurology, this is one of those things which is better explained with a huge confusing diagram. Here is one I have had since medical school. Surprisingly, at one point this thing had actually improved my understanding of the visual pathways.

Better images of the optic nerve and visual pathways are available from Cranial Nerves Illustrated.

Blood supply to the visual pathway

The retina and extracranial optic nerve receive their blood supply from the ophthalmic artery, which is a branch of the internal carotid artery. The intracranial optic nerve and optic chiasm are all supplied by branches from the anterior cerebral, anterior communicating, and the superior hypophyseal artery - all of which also arise from the internal carotid. The optic tract is predominantly perfused by branches of the posterior communicating and anterior choroidal arteries.

The lateral geniculate nucleus is supplied by the anterior and posterior choroidal arteries. The anterior choroidal is a branch of the MCA, and the posterior choroidal is a branchof the PCA - thus, the LGN receives blood from two major arterial territories, and is unlikely to be wiped out by a single large infarct. The optic radiation is also supplied by both the middle and the posterior cerebral arteries.

The primary visual cortex is supplied mainly by the posterior cerebral artery, with watershed areas which encompass areas processing peripheral visual information (with the result that PCA infarcts can occasionally spare the macula).

Visual field testing

This requires a level of cooperation and concentration which is scarce among the ICU population. Each eye must be tested independently.

Instruct the patient to look you in the eye. Instruct them to keep staring at your eye, and to report as soon as they see your wiggling finger. The finger should be equidistant from both patient and examiner. When the examiner notices the finger has entered their peripheral vision, the patient theoretically should also be able to see it. This way, the four quadrants of the visual fields are tested.

Ideally, one should have a large red-topped hat pin for this, but this instrument is a part of the physician trainee paraphernalia, and would be scoffed upon by the ICU crowd. In any case, the hat pin would also be useful in assessing for a scotoma. The wiggling finger is perhaps too crude for this.