Retinotopic Organization in the Visual Pathway (continued)
Clinical Examples
| Example 1 |
Symptoms: The patient is having his semiannual physical examination. As he is diabetic, the physician examines his retinas and performs a confrontation test of his visual fields. An abnormality is detected in his left fundus (Figure 15.12) but the confrontational field test detects nothing.
Perimetry testing is requested.
Perimetry Test Results: The results indicate the right eye's visual field is normal and that there is peripheral a scotoma (i.e., loss of vision that does not follow the boundaries of the visual field quadrants) in the left eye's temporal hemifield (Figure 15.13).
Side & Retinotopicity of damage: The visual loss
- is limited to the left eye
- is in the temporal (left) hemifield
- is associated with retinal abnormalities in the nasal hemiretina of the left eye
So you conclude that the visual defect involves
- retinal damage in the left eye
- damage located in the nasal half of the left retina (Figure 15.14, Lesion 1)
- damage related to the patient's diabetes - diabetic retinopathy
Retinal Damage: A defect involving only the visual field of one eye indicates possible damage in the retina or optic nerve. If the visual loss is confined to one eye, it is called a monocular visual field defect. Often retinal lesions are small and do not follow the boundaries of the visual field quadrants. Such a visual field disorder is called a scotoma. A retinal visual field defect is most severe when vision in the central field is affected, as in the case of macular degeneration. In macular degeneration, the patient will report difficulty reading and seeing clearly and visual field testing will demonstrate that the patient has a central scotoma (i.e., is blind in the visual field center).
| Example 2 |
Symptoms: The patient complains of a sudden headache and loss of vision in his left eye. Ophthalmoscope examination does not reveal abnormalities in the left eye1. However, confrontation testing indicates a severe loss of vision in the left eye.
The patient is referred for immediate neuroradiographic tests and perimetry testing.
Perimetry Test Results: The results indicate the right eye's visual field is normal and that there is a large visual loss encompassing nearly all of the left eye's visual field (Figure 15.15).
Side & Retinotopicity of damage: The visual loss
- does not appear to relate to changes in the retina of the left eye
- is limited to the left eye
- encompasses nearly the entire the visual field of the left eye
So, you conclude that the visual defect is
- retrobulbar (beyond the retina or eye) (Figure 15.14, Lesion 2)
- probably limited to optic nerve damage (only one eye affected)
Neural imaging results indicate an aneurysm on the left ophthalmic artery, which is compressing the left optic nerve (Figure 15.16). Compression of the nerve prevents action potentials from the retina to travel to the lateral geniculate nucleus of the thalamus. Long-term compression may damage the nerve, however, of greater concern is the potential rupture of the aneurysm, which could cause extensive brain damage.
Figure 15.16 |
Optic Nerve Damage: Each optic nerve contains the axons of retinal ganglion cells from one eye, e.g., the right nerve from the right eye. Damage to one optic nerve will produce a monocular visual field defect. Destruction of one optic nerve (e.g., crushed by a tumor on the orbital surface of the frontal cortex) will result in the total loss of vision in the ipsilesional eye.
| Example 1 | Example 2 | Example 3 | Example 4 | Example 5 |
