Abstract

Axial and coronal CT sections of the orbital optic nerve appeared normal in four out of five male patients, aged between 39 and 49 years, who had bilateral ophthalmoscopicalty visible optic nerve drusen. One patient had no clinical signs of increased intracranial pressure, but CT showed bilateral, symmetrical enlargement of the orbital opticnerves with no change after contrast enhancement. X-rays revealed a large sella and normal optic foramina. Orbital optic nerve thickness was also measured with standardized echography and was found to be greater than in the other four patients whose optic nerves measured within normal values. The usefulness of A- and B-scan echography for the evaluation of the optic nerve and the controversial pathogenesis of optic disc drusen are discussed.

A 39-year-old man who had never undergone ophthalmological examination was admitted to our Clinic with cranial trauma and a large lid tear.

Ophthalmoscopical and retinal fluorescein angiographic findings and preinjection autofluorescence evidenced bilateral expsed drusen of the optic nerve head. Both eyes bad full vision and inferonasally restricted visual fields.

Skull x-rays revealed a large sella with clear-cut margins and normal optic foramina. Computed axial tomography unexpectedly revealed a bilateral narrow-diffuse symmetrical enlargement of the orbital optic nerve; no change was observed after contrast enhancement (Fig. 1). ln the coronal projections, the optic nerve outlines appeared well circumscribed.

No cafe au lait spots were observed at the neurological examination. which was negative. The patient. who complained of moderate transient headache, refused both lumbar puncture and carotid arteriography. During the four-yeaI follow-up, no changes were observed in the optic disc, visual field. visual acuity and CT picture and no proptosis was recorded.

The optic nerves, measured by means of standardized 7200 MA A-scan, were found to he thicker than normal· 6.5 microseconds in the right eye and 6 microseconds in the left eye (Fig. 2). No distension of nerve sheaths by increased subarachnoidal fluid was observed and no differences were ob­ served between the optic nerve obtained in primary-gaze and abduction positions.

Pseudotumor cerebri was excluded on the basis of fluorescein angiographic and A-scan investigations. CT-scans were inconsistent with the diagnosis of optic nerve glioma or meningioma (Jacobiec et al. 1984). A diagnosis of bilateral astroglial proliferation of the optic nerves associated with optic nerve head drusen was suggested (Doro et al. 1982).

This case prompted us to submit four 39 to 49-year-old male patients with bilateral optic nerve drusen to standardized A-scan echography and CT to ascertain whether they had optic nerve enlargement.

The visual acuity and field defects of all five patients are summarized in Table1. The pre-injection autofluorescence and the retinal fluorescein angio­grams of the four patients evidenced optic nerve head drusen. Axial and coronal sections of the orbital optic nerves of the four patients appeared normal; in three patients, there were obvious drusen of the optic nerve bead. The optic nerve widths measured with standardized A-scan echography, ranged from 4 to 5.5 microseconds (Table 2). In all patients, the optic nerves were higher reflective than in five controls.

Discussion

Retinal fluorescein angiography is necessary for, and usually provides enough evidence to make a diagnosis of drusen of the optic nerve head (Moro et al. 1979). 8-scan and CT are useful in the detection of exposed and mostly buried optic nerve head drusen. Standardized A-scan is the most accurate means for measuring the optic nerve width (Ossoinig et al. 1981); a width of 4.7 ± 0.9 microseconds is considered normal (Guthoff et al. 1982} and the optic nerve widths observed in four of our cases may, therefore, be considered normal. In four of our patients, CT images of the retrobulbar optic nerve appeared normal, as did those of the five patients with optic nerve head drusen reported by Frisen et al. (1978). The diameter of the optic nerve was found to be greater than normal in both eyes of our first patient with drusen of the optic nerve head associated with CT evidence of bilateral optic nerve enlargement.

The pathogenesis of drusen of the optic nerve has long been a controversial subject. Recently Spencer (1978) suggested that an axoplasmic transport alteration may be the basic pathogenetic mechanism for formation of drusen in the optic nerve; the mild axonal swelling and the inconstant moderate gliosis observed histologically by Tso (1981} in patients with optic nerve head drusen may account for the increased optic nerve diameter found in our first patient. Further investigations using standardized A-scan are needed to establish how frequently optic disc drusen are associated with increased optic nerve thick-ness.

References

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