Drusen in the Verification Lars

Optic Nerve

Head

by Computerized Tomography

Frisen, MD; Gunilla Schöldström, MD; Pal Svendsen, MD

• Five patients with a clinical diagnosis of buried or exposed drusen of the optic nerve head were examined by computer¬ ized tomography (CT). The relative x-ray absorption within their nerve heads was significantly larger than in controls, reflecting the high calcium content of drusen. Computerized tomography, and particularly high-definition CT, therefore merits consideration in the investigation of disc swelling.

(Arch Ophthalmol 96:1611-1614, 1978)

Thecontaining

elevated

head drusen re¬ mains the leading cause of mistaken diagnoses of papilledema from ele¬ vated intracranial pressure. Both con¬ ditions may be associated with volu¬ minous and poorly demarcated optic nerve heads, vascular abnormalities, and signs of circulatory disturbances.'·' The distinction is usually easier to make in adults, where drusen often are exposed on the surface of the optic disc, and can be recognized by their brilliant reflection of light. In Accepted

optic

nerve

buried

publication Dec 15, 1977. Department of Ophthalmology (Drs Schòldstròm) and the Section of Neuroradiology (Dr Svendsen), University of Goteborg, Sweden. Reprint requests to Ögonkliniken, Sahlgrenska sjukhuset, S-413 45 Goteborg, Sweden (Dr Fris¬ en). From the Frisen and

for

patients, drusen frequently lie deep beneath the surface of the elevated nerve head, where they cannot be directly identified by oph¬ thalmoscopy. The presence of buried drusen is suggested by a scalloped margin of the elevated optic nerve head, absence of an optic cup, early branching of major retinal vessels, circumpapillary ring reflexes, and an orderly radial arrangement of peripa¬ pillary retinal nerve fiber bundles.-4S Fluorescein fundus angiography and ultrasonography often offer addition¬ al diagnostic clues,' but cases remain in which the presence of buried drusen cannot be proved until the drusen increase in size, and insidious atrophy of overlying axons exposes them to view. In this report we show that drusen, because of their calcium content, can be positively identified by computer¬ ized tomography (CT). younger

METHODS Two patients with exposed drusen, three patients with buried drusen, and seven patients without clinical evidence of drus¬ en, were examined neurologically and neuro-ophthalmologically. The appearance of their optic nerve heads was documented photographically at a nominal magnifica¬ tion of x 15. All patients were studied by CT, in the

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the high-definition modes mode uses matrix element cross sections of 1.5 x 1.5 mm; the highdefinition mode has a matrix element size of 0.75 0.75 mm). Special care was taken to center the optic nerve heads within the tomographic slice (nominal height, 10 mm). Maximum attenuation values were deter¬ mined by manipulation of display controls, from full printouts, or both.

ordinary and (the ordinary

FINDINGS

The appearance of the optic nerve head, and the maximum nerve head CT attenuation values for patients with and without drusen, are listed in the Table. Two illustrative cases were selected for fuller review.

Exposed Drusen of Optic Nerve Heads A man (case 1) came to the Depart¬ the

ment of

Ophthalmology, University of Goteborg, Sweden, on many occasions for removal of corneal foreign bodies. During his first visit, at the age of 37 years, superficial papillary drusen were recorded. Visual acuity was 0.4 in the right eye, and 1.0 in the left eye at that time. During the following years, vision in the right eye failed progressively. At the age of 61, the patient consulted a neurologist because of

headaches. The headaches

were

attri-

buted to tension, but the patient was referred to us for evaluation of handmotion vision in the right eye. Acuity in the left eye was still 1.0. He had a prominent afferent pupillary defect in the right eye, and pronounced visual field defects bilaterally (Fig

1).

Prominent grape-like clusters of drusen were seen on both optic nerve heads, and there was pronounced axonal wasting of the peripapillary retinal nerve fiber layer, particularly in zones corresponding with the patient's visual field defects (Fig 2). The results of the remainder of his neuro-ophthalmological examination were normal. The referring neurologist arranged for a CT examination to rule out the possibility of an intracranial tumor. The study showed no intracranial abnormalities, but midorbital sections showed larger than normal attenua¬ tion values in both optic nerve heads, already visible in the standard display mode (Fig 3, top). The measuring display mode revealed a maximum attenuation value of 85 units on the right, and 79 units on the left (Fig 3,

bottom).

Elevated Optic Nerve Heads With Buried Drusen

A an

17-year-old girl (case 3) consulted

ophthalmologist

for frontal head-

Fig 1.—Case

aches and vague blurring of vision in her right eye of three months' dura¬ tion. Bilateral "choked discs" were diagnosed, and she was referred to us.

Neuro-ophthalmologic

abnormali¬

ties were limited to her ocular fundi. Both optic nerve heads were broad and elevated, but there was no hyper¬ emia, venous congestion, or hemor¬ rhage. Circumpapillary ring reflexes were prominent bilaterally. The peri¬ papillary retinal nerve fiber layer had normal details without a hint of edema. Skull films were normal. The Maximum Attenuation Values in

working diagnosis

was

buried drus¬

en.

Follow-up examinations on several occasions during the next two months showed no sign of change. The patient then began to note photopsias pro¬ jected to the blindspot area in the right visual field. Renewed examina¬ tion disclosed increased prominence of the right optic nerve head that was associated with a deep peripapillary hemorrhage, and the formation of a row of small waxy exudates at the lower temporal disc margin (Fig 4). Except for a slight increase in the size

Computerized Tomography

of

Optic Nerve

Heads Attenuation Values'"

Case No./ Age, yr 1/61 2/20 3/17 4/55 5/58 6/23 7/27 8/65 9/34 10/76

Standard Mode

Condition of Optic Nerve-head

85

Exposed drusen, atrophy Exposed drusen, atrophy

High-definition Mode 85 98

Buried drusen

62 68

Buried drusen Buried drusen

61

81 101

Papilledema Severe atrophy

46 50

56 54

70

Normal Normal

11/47

Normal Normal

12/36

Normal

28 51 15

67 61 65 53 21

"Attenuation values are given in Hounsfield units. The difference between the drusen group and < 0.004 in high-definition the normal group is statistically significant (P < 0.04 in ordinary mode, mode, using the Wilcoxon-Mann-Whitney test).

1. Visual field defects due to drusen of

optic

nerve

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head.

Fig 2.—Case 1, Right optic disc. Note clusters of drusen. Lack of peripapillary radial striations and abnormally distinct definition of retinal vessels suggest severe wasting of axons.

Fig 3.—Case 1. High-definition computer¬ ized tomogram of orbits. Top, Ordinary display. Both optic nerve heads are lighter than normal (light picture indicates high attenuation). Bottom, Measurement dis¬ play mode shows that attenuation within both optic nerve heads exceeds 79 Hounsfield units.

Fig 5.—Case

Fig 4.—Case 3, Right optic disc. Disc is abnormally prominent and surrounded by deep peripapillary hemorrhage. There are small waxy deposits in lower temporal border area. Normal appearance of retinal nerve fiber layer suggests anomaly rather than papillede¬ ma

from raised intracranial pressure.

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3. High-definition computer¬ ized tomogram of orbits. Top, Ordinary display. Both optic nerve heads are lighter than normal. Bottom, Measurement dis¬ play mode shows that attenuation within both optic nerve-heads exceeds 100 Hounsfield units.

of the right blindspot,

no further abnormalities were found. There were no chorioretinal folds. The orderly arrangement of retinal nerve fiber bundles was retained. A drusen-related deep nerve head hemorrhage" was thought to be the most likely explanation, but a neuro¬ logical consultation was obtained. No further abnormality was elicited, but a CT scan was suggested. This scan revealed normal intracranial condi¬ tions, but tomographic slices that included the optic nerve heads showed greater than normal amounts of at¬ tenuation (Fig 5, top). The maximum reading in the nerve-head area was 118 units on the right, and 114 on the left (Fig 5, bottom). There was no dilation of the optic nerve sheaths. In the following months, the right optic nerve head regressed to its orig¬ inal prominence. The photopsias and the waxy deposits disappeared. No drusen became exposed during the six-month observation period.

COMMENT

The CT

picture reflects the relative absorption of tissues within a matrix of box-shaped volume ele¬ x-ray

ments, so-called voxels.7 The level of

absorption, or attenuation, within sin¬ gle voxels is defined in arbitrary Hounsfield units

pure air attenuation of —1,000 HU; a pure water voxel has an attenuation of 0 HU. Bone ranges up to +1,000 HU. Various tissues range between these reference values, most¬ ly in the —50 to + 50 range. Maximum attenuation in normal optic nerve-heads ranges between 15 and 68 HU. Papilledema from ele¬ vated intracranial pressure, and optic atrophy, fall within the same range (Table). The large range is presum¬ ably due to a combination of quantum statistics and eye movements. The attenuation within a given voxel of immobile tissues has a standard devia¬ tion of 3 to 4 HU in the ordinary mode, and 10 HU in the high-definition mode, due to variations in photon inci¬ dence on the detectors. The standard deviation will be larger in mobile

voxel is

given

(HU). A

an

tissues of inhomogeneous attenuation, because the computed attenuation within a given voxel is averaged over both time and space. Eye movements associated with blinking, for instance, will cause the optic nerve-head to move in and out of the tomographic plane during the period of measure¬ ment (60 seconds in our unit). The maximum attenuation values within the optic nerve-heads of our drusen cases are clearly outside the normal range in the high definition mode (Table). This must have been the case also in the patient with giant drusen (astrocytic hamartoma) re¬ ported by Daily et al," the only previous report of CT-imaging of drusen that we have been able to find in the literature. The high attenuation can be attributed to the high calcium content of drusen.'" The values are still too low to reflect the true atten¬ uation of drusen, since none of our patients had drusen aggregates large enough to fill voxels completely. The role of the space-averaging effect mentioned is clearly shown by the larger attenuation obtained in the high definition CT mode (Table). It is likely that CT units with smaller voxel sizes would have produced still higher readings. Conversely, patients with smaller drusen aggregates should pro¬ duce lower readings. Diagnostic sensi¬ tivity is inversely related to voxel size.

Although

an

abnormally large

tenuation within

at¬

swollen nerve-head strongly suggests the presence of buried drusen, it is worthy of note that drusen do not confer immunity to papilledema from raised intracranial pressure (except in cases with a very severe loss of axons). The possibility of a combination of drusen and papil¬ ledema should always be considered. Important information in this regard can be obtained by examining the CT image of the retrobulbar optic nerve: distension of the optic nerve sheath occurs when the intracranial pressure is raised."1 A diagnosis of innocuous drusen by CT therefore demands an abnormally large attenuation within the optic nerve head in combination a

with a normal appearance of the retrobulbar optic nerve. Extensive documentation has al¬ ready been made of the ways in which CT may assist in the diagnosis of many conditions that are associated with swollen optic discs, eg, optic nerve and optic nerve sheath tu¬ mors.'"14 The potential of this novel technique to reveal other intraorbital and intracranial lesions needs no comment here: that CT has revolution¬ ized many aspects of neuro-ophthalmic diagnosis is already well known. It is of little surprise that CT, by virtue of its capacity to reveal buried drusen in the optic nerve head, extends also into ophthalmoscopic diagnostic domains. References FB, Hoyt WF: Clinical ophthalmology. Baltimore, Williams & 1. Walsh

Co, 1969, vol 1, pp 567-607, 673-681.

NeuroWilkins

2. Erkkilä H: Optic disc drusen in children. Acta Ophthalmol, suppl 129, 1977, pp 1-301. 3. Friedman AH, Beckerman B, Gold DH, et al: Drusen of the optic disc. Surr Ophthalmol 21:375390, 1977. 4. Hoyt WF, Pont ME: Pseudopapilledema: Anomalous elevation of optic disk. Pitfalls in diagnosis and management. JAMA 181:191-196, 1962. 5. Hoyt WF, Knight CL: Comparison of congenital disc blurring and incipient papillede¬ ma in red-free light: A photographic study. Invest Ophthalmol 12:241-247, 1973. 6. Sanders TE, Gay AJ, Newman M: Hemorrhagic complications of drusen of the optic disk. Am J Ophthalmol 71:204-217, 1971. 7. Hounsfield GN: Computerized transverse axial scanning (tomography): I. Description of system. Br J Radiol 46:1016-1022, 1973. 8. Daily MJ, Smith LJ, Dickens W: Giant drus¬ en (astrocytic hamartoma) of the optic nerve seen with computerized axial tomography. Am J Ophthalmol 81:100-101, 1976. 9. Kurus E, Kurus C: Das histologische Prob¬ lem der Drusenpapille. Klin Monatsbl Augenheilkd 163:683-699, 1973. 10. Sanders MD: Computer-assisted tomogra¬ phy (EMI scan) in orbital and neuro-ophthamologic diagnosis. Trans Pac Coast Otoophthalmol Soc 56:17-42, 1975. 11. Ambrose JA, Lloyd GA, Wright JE: A preliminary evaluation of fine matrix computer¬ ized axial tomography (EMI scan) in the diagno¬ sis of orbital space-occupying lesions. Br J Radiol 47:747-751, 1974. 12. Gawler J, Sanders MD, Bull ,IWD, et al: Computer assisted tomography in orbital disease. Br J Ophthalmol 58:571-587, 1974. 13. Hilal SK, Trokel SL, Coleman DJ: High resolution computerized tomography and B-scan ultrasonography of the orbits. Trans Am Acad Ophthalmol 81:607-617, 1976. 14. Wende S, Aulich A, Nover A, et al: Computed tomography of orbital lesions. Neuroradiology 13:123-134, 1977.

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Otolaryngol

Drusen in the optic nerve head. Verification by computerized tomography.

Drusen in the Verification Lars Optic Nerve Head by Computerized Tomography Frisen, MD; Gunilla Schöldström, MD; Pal Svendsen, MD • Five patients...
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