of the internal capsule. From this point forward the anesthesia diminishes, and hemiplegia with little or no diminution of the sensitiveness of the skin suggests the region of the middle and anterior lobes of the brain. An absolute hemianopsia aids little of itself in the localization of the lesion, as it may be produced by one located in the cortex or elsewhere in the brain, but a relative hemianopsia we expect to find to be produced by a lesion in the cortex. Sometimes we can determine whether the trouble is in front of or behind the geniculate bodies by means of Wernicke’s test of the pupillary reaction. This test must be made in a dark room with only just enough illumination to enable us to perceive whether any movement takes place in the pupil or not. A small beam of light is cast into the eye in such a way that it will fall upon the hemiopic part of the retina, and then turned so as to fall on the functionating part as nearly as possible at the same distance from | the fovea. If the pupil does not contract when the light falls on the hemiopic portion, but does when it falls elsewhere on the retina, we know that the lesion is situated in front of the geniculate body; while if the reaction of the pupil is the same whenever the beam of light falls upon any part of the retina that is at a certain distance from the fovea, the lesion is behind the geniculate body. This test is one which is difficult to make, and does not give us as much certainty as could be desired, because light has a strong tendency to become diffused after it has entered the eye, and so to affect all parts of the retina equally. We have to combat this tendency by using as small a beam of light as will excite a reaction when cast upon the healthy 436 DIAGNOSIS FROM OCULAR SYMPTOMS retina, and then be careful to throw it in exactly the right direction. Some cases have been met with in which the light when cast on the hemiopic side induced a slighter reaction of the pupil than when it fell on the other side, but the effect of diffusion would seem to be hard to exclude in such a case. The test can be used only when the hemianopsia is absolute. A concentric contraction of the preserved portion of the field does not necessarily indicate that the lesion is situated in the cortex, but it does so rather commonly.<Callout type="warning" title="Difficult Test">This test requires precise control over light and pupil reactions, making it challenging to perform accurately.</Callout><Callout type="important" title="Absolute Hemianopsia Required">The test can only be used when the hemianopsia is absolute, not relative.</Callout>
Key Takeaways
- The pupillary reaction test helps in localizing brain lesions causing hemianopsia.
- Absolute hemianopsia is necessary for the test to be effective.
- Light diffusion can affect the accuracy of the test.
Practical Tips
- Practice using a dimly lit environment and precise light control when performing this test.
- Ensure that the patient's pupils are examined in both eyes, as asymmetry might indicate a lesion.
- Use a small beam of light to minimize diffusion effects on the retina.
Warnings & Risks
Difficult Test
This test requires precise control over light and pupil reactions, making it challenging to perform accurately.
Light Diffusion Risk
The strong tendency of light to become diffused can affect the accuracy of the test results.
Modern Application
While the techniques described in this chapter are historical and may require adaptation for modern use, the principles of using ocular symptoms to diagnose neurological conditions remain relevant. Modern medical imaging has improved diagnostic precision but understanding these foundational tests can still be valuable in emergency situations where immediate diagnosis is critical.
Frequently Asked Questions
Q: What does a relative hemianopsia indicate about the location of the lesion?
A relative hemianopsia suggests that the lesion is located in the cortex, as it indicates that the pupillary reaction differs when light falls on the functioning part of the retina compared to the hemiopic part.
Q: Why can't this test be used for relative hemianopsia?
This test can only be used for absolute hemianopsia because it relies on comparing reactions between the healthy and affected parts of the retina, which is not possible with a relative hemianopsia.
Q: What precautions should be taken when performing this test?
Precautions include ensuring that the light beam is as small as possible to minimize diffusion effects on the retina, using only enough illumination to perceive pupil reactions, and carefully controlling the direction of the light.