Exanopic Central Scotoma, a Pitfall in Diagnosis

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Ordinarily, amblyopia exanopsia is associated with strabismus. However, it may occur in patients who are not obviously cross-eyed. In either event, the loss of vision in one eye is associated with a central scotoma. In neurologic diagnosis a central scotoma in the field of vision of one eye usually indicates involvement of the optic nerve itself. Hence, a central scotoma that is really due to amblyopia exanopsia may prove to be misleading. Since this situation is not generally recognized, and especially since it is not mentioned in the authoritative work on perimetry by Traquair,2 it is

Ordinarily, amblyopia exanopsia is associated with strabismus. However, it may occur in patients who are not obviously cross-eyed. In either event, the loss of vision in one eye is associated with a central scotoma. In neurologic diagnosis a central scotoma in the field of vision of one eye usually indicates involvement of the optic nerve itself. Hence, a central scotoma that is really due to amblyopia exanopsia may prove to be misleading. Since this situation is not generally recognized, and especially since it is not mentioned in the authoritative work on perimetry by Traquair,2 it is thought worth while to present the following case in illustration.

CASE REPORT

A twenty-four year old man was referred with the diagnosis of brain tumor. His complaint was severe headache for one and one-half months. Five years previously he had been in an automobile accident and had been unconscious for one minute but had no persistent symptoms. Two and one-half years before admission he suddenly noticed blurring of vision in the right eye whenever he looked at a target, and this condition persisted unchanged, so that he began sighting with his left eye. Seven months previously, while reading, he had severe headache, which was promptly relieved by glasses. Since then he had had occasional mild headaches. For six weeks prior to admission he suffered severe, persistent, right-sided headache. This headache was maximal in the right frontal region and seemed to extend backward. It was exacerbated by straining or jarring. Recently he had had episodes of feeling faint, especially on standing upright or on bending his head forward, and once he had lost consciousness momentarily.

Clinical examination revealed bilateral acutely choked discs, tenderness over the anterior part of the right temporal fossa, a central scotoma in the right eye on the confrontation test, and a pulse rate of 52. Roentgenograms of the skull revealed erosion of the floor of the sella, and displacement of the pineal body backward and to the left.

The working diagnosis at this time was probable meningioma of the right sphenoidal ridge. However, ophthalmic examination revealed that the visual acuity of the right eye without glasses was 5/200 and with a plus 6.00 sphere was 20/100. The left eye had no error of refraction and had a visual acuity of 20/15. There was bilateral papilledema of 4 diopters with one small recent hemorrhage at the edge of each disc. The blind spots were enlarged. Otherwise the visual fields were normal except for a central scotoma, 3 or 4 degrees in diameter, in the right field, which was revealed only by the 1 mm. and 3 mm. test objects at a distance of two meters with the plus 6.00 correction in place. In view of the marked degree of hyperopia in the right eye as well as emmetropia in the left, the central scotoma was considered to be due to amblyopia exanopsia rather than to involvement of the optic nerve. What was apparently a large central scotoma on the confrontation test was actually a small central scotoma associated with marked blurring of near vision due to the marked degree of farsightedness.

Therefore, the diagnosis was revised to: increased intracranial pressure due to right supratentorial lesion, probably subdural hematoma. An extracerebral process, subdural hematoma, was considered likely because of the relatively great lateral displacement of the pineal body in the absence of hemiplegia or hemianopsia. Accordingly, burr holes were placed over the parietal bosses to explore for subdural hematomata, and, in the event of negative exploration, to allow for ventriculography. A very large liquid subdural hematoma was found on the right side and was drained. Subsequent roentgenograms revealed air at the site of the evacuated hematoma which extended widely over the frontal, parietal, and temporal lobes. The postoperative course was uneventful. The patient's headaches were relieved and the pineal body returned to the mid-line in six days. At this time ophthalmic re-examination revealed no change from the pre-operative status, except slight diminution in the degree of choking of the optic discs. Subsequently the patient became free of symptoms, and the papilledema disappeared. He was sent back to full military duty.

Re-examination was carried out six months later because of discomfort in the right frontal region. The examination of the eyes and nervous system revealed no change. The scalp was depressed over the burr holes; spinal fluid was normal in every respect; encephalogram revealed minimal enlargement of the lateral ventricles without deformity or displacement. The fourth ventricle was normal in size, shape and position. Cortical markings were normal on both sides. The patient resumed military duty free from symptoms.

DISCUSSION

As is generally known, disparity of the retinal images produced by the two eyes results in the suppression of one image. When such suppression occurs continually in one eye in early life, there results a diminution of visual acuity in that eye, so-called amblyopia exanopsia. What is not generally known is that such loss of vision is due to a small central scotoma, usually with a radius of about three degrees.1 As in other defects of vision in one eye, the patient may become aware of his defect suddenly in adult life. In cases of obvious strabismus there is usually no difficulty in diagnosis. In other cases, careful study of the extra-ocular muscles and refraction will be required. The recognition of the nature of the central scotoma in this case was essential for its proper neurosurgical management.

SUMMARY

The central scotoma associated with amblyopia due to an error of refraction should be recognized as such, and not confused with that due to involvement of the optic nerve. An illustrative case is presented.

REFERENCES

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References

1.

PeterL. C. The extra-ocular muscles. A clinical study of normal and abnormal ocular motility. Philadelphia: Lea & Febiger19413rd ed.368 pp.PeterPhiladelphia: Lea & Febiger

2.

TraquairH. M. An introduction to clinical perimetry. London: Henry Kimpton19424th ed.TraquairLondon: Henry Kimpton

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