Visual System

Question 1

how are lesions of the visual system described?

Answer 1

• in terms of their visual field deficits

- visual field deficits are described from the perspective o the pt’s right or left

Question 2

what are the four quadrants of the retina and what visual quadrants do they receive?

Answer 2

1. upper nasal visual quadrant–projects to lower temporal retinal quadrant
2. lower nasal visual quadrant–projects to upper temporal retinal quadrant
3. upper temporal visual quadrant–projects to lower nasal retinal quadrant
4. lower temporal visual quadrant–projects to upper nasal retinal quadreant

Question 3

where do axons of the ganglion cells terminate?

Answer 3

• majority terminate in the lateral geniculate body

- rest terminate in the superior colliculus as part of the tectal and visual reflex pathways

Question 4

rules of retinotopic projection

Answer 4

• fibers from the temporal hemiretina do not cross in the optic chasm
• fibers from the nasal hemiretina cross in the optic chimes

Rule of L’s

• info from the lower hemiretina projects to:
  
  1. lateral part of the lateral geniculate body
  2. loop of meyer
  3. lingual gyrus

Question 5

homonymous visual fields

Answer 5

• refer to corresponding halves of visual fields such as the temporal field of one eye and the nasal field of other
• Left temporal and R Nasal
• R temporal and L Nasal

Question 6

heteronymous visual fields

Answer 6

• refer to non corresponding visual fields
• temporal field in the L and temporal field in the R
• nasal field in the L and temporal field in the R

Question 7

hemianopsia

Answer 7

• blindness in one half of the visual field

* *nasal hemianopsia of the L eye indicates a lesion of the temporal hemiretina of the L eye

Question 8

quadrantanopia

Answer 8

• blindness of a quadrant of the visual field
• most common involves the upper nasal quadrant of one eye and the upper temporal quadrant of the other eye called superior quadrantanopia–homonymous condition

Question 9

binasal hemianopsia

Answer 9

• cause: bilateral lesion of the lateral aspect of the optic chiasm
• results: heteronymous blindness in the nasal fields of each eye
• may also be unilateral due to atherosclerosis of ICA
• ex: if bilateral lesion of the lateral aspect of optic chasm, blindness in the R and L nasal visual fields

Question 10

unilateral nasal hemianopsia

Answer 10

• cause: unilateral lesion of the lateral aspect of the optic chiasm
• results: nasal hemianopsia (blindness) of the ipsilateral eye
• ex: lesion of the lateral aspect of the L optic chiasm results in (right) nasal hemianopsia of the left eye
  
  • blindness in the nasal field of the left eye

Question 11

bitemporal hemianopsia

Answer 11

• cause: midline lesion of the medial portion of the optic chiasm
  
  • one type is due to a pituitary tumor
• results: bitemporal hemianopsia (blindness)
• ex: so if lesion to the medial aspect of the optic chasm, then blindness occurs in the temporal fields of both eyes

Question 12

contralateral homonymous hemianopsia

Answer 12

• cause: unilateral lesion of the lateral geniculate body, complete optic radiations, or visual cortex
• results: contralateral homonymous hemianopsia
• ex: left homonymous hemianopsia due to a lesion on the R visual pathway (ie. optic tract, lateral geniculate body, optic radiations)

Question 13

contralateral superior quadrantanopia

Answer 13

• cause: unilateral lesions of the loop of Meyer
  
  • possibly due to a tumor of infarction of the posterior temporal lobe
• results: homonymous deficit
• ex: left superior quadrantanopia–due to a lesion in the R loop of Meyer causing blindness in the L temporal visual field and R nasal visual field

Question 14

incongruent contralateral homonymous hemianopsia with macular sparing

Answer 14

• cause: unilateral lesion of the visual cortex
  
  • possibly due to obstruction of the posterior cerebral artery
• results: contralateral hemianopsia that is not symmetrical in both eyes and does not occur in the macula
• lesions of the visual cortex are incongruous while lesions of the LGB or optic radiations are congruous
• ex: left incongruent homonymous hemianopsia with macular sparing–due to a lesion with the R primary visual cortex
  
  • causes blindness in the L half of each eye (L temporal field and R nasal field) leaving the macula spared–not symmetrical on each side

Question 15

visual agnosia

Answer 15

• pts are unable to visually recognize objects or pictures

- may be due to bilateral damage in the visual assoc cortices secondary to anoxia

Question 16

associative visual agnosia

Answer 16

• infarction of the L occipital lobe and posterior corpus callous secondary to occlusion of the PCA
  
  • disconnects the language area from the visual assoc cortex
• pt cannot name or describe an object in the visual field, but they can recognize and demonstrate their use
• visual perception intact
• pts are usually alexic (unable to read) and writing ability may be affected (agraphia)

Question 17

prosopagnosia

Answer 17

• once a visual field is processed in the primary and associative visual cortices, it may be correlated with memory
• bilateral lesions in the occipito-temporal regions may result in bizarre disturbance of fine visual discrimination and ability to recognize extremely familiar faces

Question 18

light reflex

Answer 18

-pupillary compression assoc with a normal response to shining an exam light into the pt’s eye is a PS mediated response that is processed thru the PRETECTUM

-shining a light in an eye results in ipsilateral constriction of the pupil–>direct light reflex
light into R eye–>R retina–>optic N–>optic tract–>brachium of superior colliculus–>superior colliculus–>pretectum–>Edinger Westphal Nucleus–>Oculomotor N–>Ciliary Ganglion–>pupillary constrictor M–>ipsilateral pupillary constriction

-unilateral stimulus will also normally elicit pupillary constriction of the contralateral eye–consensual pupillary reflex
light into R eye->R retina–>optic N–>optic tract–>brachium of superior colliculus–>superior colliculus–>pretectum–>posterior commissure–>Edinger Westphal Nucleus–>Oculomotor N–>Ciliary Ganglion–>pupillary constrictor M–>ipsilateral pupillary constriction

Question 19

pupillary dilation response

Answer 19

• dec in the amount of light reaching retina results in bilateral reflex dilation of the pupils
• sympathetic response mediated by brainstem and upper SC

dec light–>retina–>optic N–>superior colliculus–>pretectum–>reticular formation–>lateral reticulospinal tract–>preganglionic sympathetic neurons–>superior cervical ganglion–>pupillary dilator Ms–>pupillary dilation

Question 20

accomodation

Answer 20

• cortically mediated visual response
• originates in frontal eye field of frontal lobe
• triad of accommodation: convergence of vision, pupillary constriction, thickening of lens

Question 21

Argyll Robertson pupil

Answer 21

• result of syphilis infection
• pupils are unreactive to light but constrict during accommodation
• due to destruction of pretectum–important in light reflex but not accommodation
• accomodating but unreactive–Prostitute’s Pupil

Question 22

Holmes Adie Pupil

Answer 22

• benign condition
• may be due to lesion of ciliary ganglion
• may confused with Argyll Robertson b/c of similar rxn to light, but distinguished by different rxns to accomodation

Question 23

voluntary movements of the eye

Answer 23

• controlled by frontal eye fields in the posterior portion of the middle frontal gyrus
• corticotectal fibers descend to superior colliculus which in turn influences the motor neurons of III, IV, and VI

Question 24

convolutional movements of eye

Answer 24

• controlled by occipital eye fields in the visual assoc cortex
• fibers from the occipital cortex descend as corticotectal fibers to superior colliculus which influence the LMNs of III, IV, VI