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Original Contribution

Progressive Optic Neuropathy in Idiopathic Intracranial Hypertension After Optic Nerve Sheath Fenestration

Wilkes, Byron N MD; Siatkowski, R Michael MD

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Journal of Neuro-Ophthalmology: December 2009 - Volume 29 - Issue 4 - p 281-283
doi: 10.1097/WNO.0b013e3181c2530b
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Abstract

A 16-year-old overweight African American woman presented with headache and decreased vision in both eyes for 2 weeks. Best-corrected visual acuity was 20/25 in the right eye and 20/40 in the left eye. Automated static perimetry showed generalized depression with enlarged blind spots bilaterally (Fig. 1). She identified 13/14 Ishihara plates with the right eye and 12/14 with the left eye. Ophthalmoscopy revealed marked optic disc edema bilaterally with moderate hemorrhage.

FIG. 1
FIG. 1:
At presentation gray scale Humphrey 24-2 shows nerve fiber bundle defects.

Results of brain MRI were normal. Lumbar puncture revealed an opening pressure of 35 cm H2O with normal chemistry and no cells. Idiopathic intracranial hypertension (IIH) was diagnosed, and she was treated with 500 mg oral acetazolamide twice daily along with intravenous methylprednisolone 125mg every 6 hours.

Five days after starting therapy, the patient complained of worsening headaches and declining vision. Visual acuity remained 20/25 in the right eye but had worsened to 20/60 in the left eye. Color vision had declined in both eyes. Visual field testing showed worsening defects (Fig. 2).

FIG. 2
FIG. 2:
Five days after commencement of acetazolamide and corticosteroid therapy, visual fields show worsening defects in the left eye.

Repeat lumbar puncture revealed an opening pressure of 65 cm H2O (obtained by connecting 2 manometers). Bilateral optic nerve sheath fenestration (ONSF) was performed. The optic nerves were approached via a medial trans-conjunctival incision after securing and detaching the medial rectus muscles. The optic nerves were dissected from the surrounding connective tissue and a rectangular window was created in the optic nerve sheath. A large gush of cerebrospinal fluid (CSF) was noted to flow into the orbit immediately after incision of the optic nerve sheath on both sides. There were no intraoperative complications.

On the 4th postoperative day, visual acuities were 20/25 in the right eye and 20/40 in the left eye. Color vision had recovered to 14/14 Ishihara plates bilaterally. Visual fields had improved (Fig. 3). The optic discs were less swollen, and spontaneous venous pulsations were noted for the first time.

FIG. 3
FIG. 3:
Four days after bilateral optic nerve sheath fenestration, visual fields have improved markedly.

Fourteen days later, the patient complained of worsening headache and declining vision. Best-corrected visual acuities were 20/100 in both eyes. Color vision had also declined. Visual fields showed large bilateral central scotomas (Fig. 4). Ophthalmoscopy revealed complete resolution of papilledema without optic disc pallor in both eyes. Spontaneous venous pulsations were present in both eyes.

FIG. 4
FIG. 4:
Fourteen days after ONSF, visual fields have worsened markedly.

Results of brain MRI were normal. Lumbar puncture revealed an opening pressure of 65 cm H2O. The patient underwent ventriculoperitoneal (VP) shunting.

Two weeks after successful shunting, visual acuity had improved to 20/40 in the right eye and 20/80 in the left eye. Ophthalmoscopy showed no change in the appearance of the optic nerves. Visual fields showed a decrease in the size of the central scotomas in both eyes. Color vision had improved to 13/14 plates in the left eye.

Five weeks later the patient complained of a further decrease in vision. Headache was no longer present. Best-corrected visual acuities were 20/200 in the right eye and 20/200 in the left eye. Color vision was markedly reduced at 1/7 and 2/7 color plates in the right and left eyes, respectively. Visual field testing revealed bilateral central scotomas (Fig. 5). Ophthalmoscopy now revealed temporal optic disc pallor and nerve fiber layer dropout in the papillomacular bundle of both eyes.

FIG. 5
FIG. 5:
Five weeks after ventriculoperitoneal shunting, visual fields have slightly improved but still show profound defects.

At the last clinical visit, 10 months after the VP shunt, the patient's visual acuity was 20/150 in the right eye and 20/125 in the left eye. Visual fields were unchanged. Ophthalmoscopy showed moderate optic disc pallor bilaterally.

DISCUSSION

Our patient is reported because of her unusual clinical course after ONSF and VP shunting. After ONSF, visual function briefly improved initially, and optic disc edema resolved. But visual deterioration soon set in, and intracranial pressure (ICP), as measured by lumbar puncture, was very high. After VP shunting, headache resolved but vision continued to worsen, and optic disc pallor eventually appeared. Visual fields had high reliability indices and seemed to be consistent in confirming failing vision despite the interventions.

This phenomenon is unusual but has been reported previously. Mauriello et al (1) reported that 1 of 5 patients who had undergone ONSF for IIH had initial improvement in visual acuity but later required a lumboperitoneal (LP) shunt for worsening vision 3 days after the ONSF. In that report, however, there was no mention of the pre-shunt level of ICP or an explanation for the visual loss.

Previous studies have suggested that ONSF can lower the pressure within the optic nerve sheath and the intracranial space by allowing CSF flow through the fenestration (2,3). Long-term ICP lowering, however, would require a constant flow of CSF between the intracranial and optic sheath subarachnoid spaces (SASs), a phenomenon that may not occur in patients with IIH. For example, Killer et al (4) noted that CSF flow may not be bidirectional within the optic nerve sheath in individuals with IIH. Their work suggests a paucity of CSF flow from the intracranial SAS to the SAS of the optic nerve sheath in patients with IIH, evidenced by the presence of continued papilledema in some patients after successful VP shunting. This CSF flow impedance results in abnormal flow in the orbital segment of the nerve (4). These authors suggest that the anatomical narrowing of the intracanalicular optic nerve sheath SAS to a potential space could be a contributing factor. Perhaps the compartmentalization is due to changes within the dura and arachnoid related to inflammation and subsequent fibrosis in the SAS (4). There is also evidence that compartmentalization of the optic nerve SAS leads to the accumulation of biologically active molecules, which could lead to local cell apoptosis (5). It is unclear where the primary site of impedance lies, but it could be anywhere posterior to the site of ONSF. Although flow in the optic canal has not been reliably measured, the narrowed potential space there leads to the speculation that this is at least a contributing factor.

The compartmentalization described by Killer et al (4) may explain what happened in our patient. Notably, however, Killer et al (4) described persistent papilledema not after ONSF, but after VP shunt, indicating an impedance of CSF flow from the optic nerve sheath to the intracranial space. Our patient's persistently elevated ICP after ONSF suggests the possibility of a paucity of flow from the intracranial SAS to that of the optic nerve. Thus, ONSF may not always be able to decompress the elevated ICP affecting the intracranial and canalicular portions of the optic nerve. Shunting may not be able to reduce the elevated ICP on the optic nerves if this compartmentalization prevents fluid flow from the optic nerve SAS to the intracranial SAS.

The ICP in IIH has been reported to range from 20 to 50 cm H2O with a mean of 34.4 cm H2O (6). A smaller series did include a patient with an opening pressure of 65 cm H2O; however, this level of ICP is exceedingly rare (7). This persistently very high pressure after ONSF may explain why the patient continued to have visual loss despite adequate decompression of the optic nerve heads.

There are other explanations for this patient's progressive visual loss after surgical intervention. Poor outcomes in IIH are higher in pubescent individuals and in African Americans (8,9). The progressive visual loss may have been the result of a process well underway by the time ONSF occurred.

This case suggests that it is important to recognize that patients with IIH and highly elevated ICPs may not be adequately treated after ONSF even if it initially results in a reversal of visual loss and papilledema. In such instances, ICP must be measured, either by lumbar puncture or by direct intracranial monitoring. If it is still elevated, prompt shunting should be considered.

REFERENCES

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