Visual system

Question 1

What are the main differences between cones and rods?

Answer 1

Rods are sensitive to low-intensity light, important for night vision
Cones are sensitive to bright light, important for day vision, color vision, and high visual acuity

Question 2

Describe the cells of the retina and the order that they are stimulated in response to light

Answer 2

A chain of three neurons project visual impulses through the optic nerve: Rods and cones receive light in the posterior layer of the retina. They stimulate interneurons, which stimulate ganglion cells at the anterior layer of the retina. Ganglion cells converge to form the optic nerve.

Question 3

What is the fovea?

Answer 3

An area of the retina where light directly stimulates photoreceptors. Only cones are present in this zone.

Question 4

Describe the visual pathway from the retina

Answer 4

Retina –> Optic nerve –> Optic chiasm* –> optic tract —> lateral geniculate nucleus of thalamus –> optic radiations –> visual cortex

*Nasal retina fibers (lateral visual fields) cross at optic chiasm

Question 5

Lesion location causing blind right eye

Answer 5

Right optic nerve

Question 6

Lesion location causing bitemporal hemianopia

Answer 6

Optic chiasm

Question 7

Lesion location causing left homonymous hemianopia

Answer 7

Right optic tract or optic radiation

Question 8

Lesion location causing left homonymous hemianopia with macular sparing

Answer 8

Visual cortex

Question 9

What is the physical exam sign of optic nerve damage?

Answer 9

Equal pupils, neither responding to light shone in the eye ipsilateral to the lesion, but both normal to light in contralateral eye

Question 10

What is the physical exam sign of oculomotor nerve damage?

Answer 10

Dilated ipsilateral pupil that does not respond to light shone into either eye

Question 11

What does a visual acuity of 20/100 mean?

Answer 11

Patient reads at 20 feet what a normal person sees at 100 feet

Question 12

What is the use of a pinhole test?

Answer 12

If vision improves looking through a pinhole, the visual problem is ocular. Neurologic deficits are not corrected with the pinhole test.

Question 13

What is a scotoma?

Answer 13

A pathological blind spot caused by ocular, retinal or optic nerve disorder

Question 14

Why is there a normal blind spot?

Answer 14

There are no rods/cones at the optic disc. This maps to a blind spot in the temporal to center of vision

Question 15

What is optic neuritis?

Answer 15

Sudden visual loss in one eye associated with painful eye movements. The optic disc appears swollen and inflamed.
Often seen with MS patients.

Question 16

What is papilledema?

Answer 16

Increased intracranial pressure causes bilaterally swollen optic discs. Risk for blindness if left untreated.

Question 17

How does a pituitary tumor often present?

Answer 17

The optic chiasm is gradually compressed from below. Early presentation is superior temporal quadrantic defect, which progresses to heteronymous bitemporal hemianopsia.

May also see hormonal dysfunction.

Question 18

What visual deficits are seen in a patient with a PCA occlusion/thrombosis?

Answer 18

Cortical blindness from bilateral occipital infarcts with intact pupillary light reflex and no optic atrophy.