British _ournal of Ophthalmology 1995; 79: 182-185

Histopathological and ultrast-ructural examinationof optic nerve sheath decompression

James C Tsai, Margaret S Petrovich, Alfredo A Sadun

Doheny Eye Instituteand the Department ofOphthalmology,University of SouthernCalifornia School ofMedicine, Los Angeles,USAJ C TsaiM S PetrovichA A Sadun

Correspondence to:Dr Alfredo Sadun,Doheny Eye Institute,1450 San Pablo Street,Los Angeles, CA 90033,USA.

Accepted for publication5 September 1994

AbstractBoth optic nerves were obtained atautopsy from a 28-year-old man with a 2year history of idiopathic intracranialhypertension who had undergone bilateraloptic nerve sheath decompression 14 daysbefore death. Histopathological and ultra-structural examination of the tissuerevealed fibroblasts localised to the sitesof fenestration. Adipose tissue was alsoadherent to the optic nerve pia in areas ofincised dura. No patent fistula site wasobserved. It was concluded that filtrationof cerebrospinal fluid after optic nervesheath decompression may occur throughan enclosed bleb of fibrosis rather thanthrough an open fistula.(BrJ Ophthalmol 1995; 79: 182-185)

Figure I Optic disc photographs of the right eye (A) and left eye (B) immediately beforesurgery. Note presence ofsecondary optic atrophy in both discs.

Optic nerve sheath decompression (ONSD)has been reported to be effective in preservingvision in patients with idiopathic intracranialhypertension (that is, pseudotumour cere-bri) 1-3 and progressive anterior ischaemicneuropathy.4 At present, its exact mechanismof action has not been clearly defined, andfew histopathological studies of optic nervespecimens after sheath decompression havebeen described.5-9 Keltner and colleagues5 6reported the presence of intact fistulas at thesites of fenestration, and concluded that themechanism of action is the successful egress ofcerebrospinal fluid. Davidson7 8 observedsecondary fibrosis surrounding the incisionsites, and suggested that this scarring phenom-enon ultimately protects the optic discfrom the more proximal elevated intracranialpressure.We report here the histopathological and

ultrastructural finding in both optic nervesfrom a patient with chronic idiopathic intra-cranial hypertension who died 14 days afterundergoing bilateral ONSD.

Case reportA 28-year-old right handed man was initiallyreferred to our institution with a 3 week historyof transient obscurations of vision, bifrontaland bitemporal headaches, and dimming ofvision. The patient had otherwise been inexcellent health except for morbid obesity of atleast 10 years' duration. Visual acuity was20/200 in the right eye and 5/200 in the lefteye. Colour vision was markedly diminished inthe left eye, but he could discern 7 5 of 8pseudoisochromatic plates (American Optical)with the right eye. Brightness sense in the lefteye was 28% of that in the right eye. There wasa marked afferent pupillary defect of the lefteye. Tangent field testing, performed with a10 mm white target, showed marked constric-tion of the peripheral field in the right eye. Theleft eye showed a central altitudinal field defectin addition to peripheral field constriction.Dilated fundus examination revealed floridbilateral disc oedema, worse in the left eye, andmarked oedema of the peripapillary retina.The patient was diagnosed as having

papilloedema with severe impairment of visionmost probably secondary to idiopathicintracranial hypertension. A computed tomo-graphy scan of the brain with contrast wasnormal. A lumbar puncture revealed anopening pressure of 420 mm Hg. Results oflaboratory studies including cerebrospinal fluidstudies were all within normal limits except foran elevated erythrocyte sedimentation rate of65 mm/h (Westergren).

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Histopathological and ultrastructural examination of optic nerve sheath decompression

Figure 2 Light micrographs ofone of the fenestration sites (trichrome stain). (A) Lowmagnification view (X 2) of the optic nerve in cross section. Note that the fenestration sitetraverses the dura and arachnoid into the subarachnoid space. The pia (small arrow) isintact. Adipose tissue (large arrow) is adherent to pia in areas of incised dura. (B) Highmagnification view (X4-75) demonstrates proliferation ofgranulation tissue and theadherence of adipose tissue to the pia (arrow) in areas offenestration. A peripheral ciliarynerve is located centrally. No discerniblefiltration bleb is seen. (C) Highest magnificationview (X 75) reveals no obvious fistula site between the pia and the overlyingfibrosis(see arrows).

The initial treatment regimen included oralacetazolamide and steroids, weekly lumbarpunctures, and an intensive diet programme.Visual acuity returned to 20/20 in both eyesbut with residual peripheral field constriction

(15 degrees of central vision in the right eyeand 10 degrees in the left eye by static visualfield testing). Fundus examination revealedsecondary optic atrophy with characteristicgliosis of the optic discs.Two years later the patient was referred

again to our neuro-ophthalmology clinic. Hehad 20/25 vision in both eyes with constrictedvisual fields, moderate disc oedema withsecondary optic atrophy (Fig 1), and markedCushingoid features. A relative afferentpupillary defect was not present. The patientidentified 6 of 8 pseudoisochromatic plateswith his right eye and 5 of 8 plates with his lefteye. In view of progressive visual field loss, thecontinued increased intracranial pressures,and the dangers of prolonged corticosteroiduse, we recommended bilateral ONSD inorder to begin immediately tapering off thecorticosteroids.The patient underwent bilateral ONSD by a

medial approach. Five separate longitudinalincisions were made into the nerve sheathsbilaterally with either a ruby knife or super-sharp blade. Each incision was made from apoint 2-3 mm posterior to the globe andextended 5 mm distally along the nerve sheath.Clear cerebrospinal fluid (upon which floatedsmall yellow oil drops) emanated from theexposed subarachnoid space.Four days postoperatively, the patient

immediately reported seeing more clearly andhad resolution of his headaches but now noteddiplopia. Visual acuity was 20/25 in the righteye and 20/30 in the left eye. He identified 7 of8 pseudoischromatic plates with each eye, andhis colour vision was subjectively much better.The pupils were briskly reactive to light andnear target, without evidence of an afferentdefect. Funduscopic examination revealedmild disc oedema. The patient was begun onan acute taper of his oral corticosteroids.

Approximately 2 weeks later, he hadgastrointestinal haemorrhaging that proved tobe fatal. An autopsy was performed, duringwhich both eyes and optic nerves wereremoved for microscopic and ultrastructuralstudy.

ResultsEach eye measured 24-5X23'5X24-0 mm,appeared normal externally, and had approxi-mately 15 mm of optic nerve attached. Grossexamination of the sectioned globes revealeddiffuse secondary atrophy of the optic nervesbilaterally. There was no evidence of discoedema. No discernible filtration blebs werenoted along the fenestration sites. The opticnerves were adjacently sectioned and thenstained with haematoxylin and eosin, luxol fastblue, trichrome, and Bodian stains.

Light microscopy with multiple coronaland sagittal sections revealed severe opticatrophy with only a small percentage of axonsremaining near the centre of each optic nerve.Localised proliferation of connective tissue wasseen overlying the fenestration sites through-out the entire nerve and were present in alllevels of the sections (Fig 2). Adipose tissue

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Tsai, Petrovich, Sadun

Figure 3 Transmission electron micrograph of the edge ofthe peripheral ciliary nerve depicted in Figure 2. Fibroblast

cells are seen; one of them is seen entering the surrounding

collagen matrix (magnification X 3900).

was noted to be adherent to the optic nerve pia

in areas of incised dura. No patent fistula site

was observed in any of the specimens of either

optic nerve. Regions of the nerve other than

the fenestration sites did not reveal any

fibrovascular proliferation or adherent fat.

There was no histological evidence of inflam-

mation in the orbital fat, muscles, or blood

vessels.

Ultrastructural examination clearly demon-

strated the extent of this fibrotic response

(Fig 3). Endothelial cells lined the edge of theperipheral ciliary nerve. Many fibroblasts were

seen within the surrounding collagen matrix.

DiscussionEven though ONSD has been performed since

the late nineteenth century for relief of papil-loedema,10 its exact mechanism of action

remains obscure. Davidson78 proposed that

the relief of papilloedema after ONSD was

attributable to an obliteration of the subarach-

noid space by fibroblasts, which preventedtransmission of the increased intracranial pres-

sure along the subarachnoid space. The patientdescribed by Davidson7 in 1969 survived for

21 days after surgery, and histopathological

examination of postmortem tissues revealed

occlusion of the dural incision sites with

'young granulation tissue' and the presence of

'organised granulation tissue with some fat

necrosis.' Another patient reported in 19728

underwent ONSD approximately 12 days

before death. Histological examination showedthat the incised dura was plugged by fibro-blasts that arose from the subarachnoid space.It is important to note that Davidson believed

that occlusive fibrosis was responsible for the

initial reduction in disc oedema in this patient,though the optic nerve specimen was taken 1

week after this patient's vision had decreasedfrom preoperative levels.

In 1977 Keltner et al6 reported a histologicalstudy of the optic nerves of a patient who died39 days after nerve sheath decompressionfor intractable chronic papilloedema. Thepresence of intact fistulas in the dura and an

open subarachnoid space around the optic

nerve led them to propose that the mechanismof action of ONSD was primarily that itallowed egress of cerebrospinal fluid (CSF).They concluded that the flow of CSF throughthe loose arachnoid-like connective tissue thatproliferated into the fistulas accounted for res-olution of the papilloedema. Moreover, thisincreased resistance, open shunt mechanismcould account for the clinically observed rapid-ity of onset of symptomatic improvement andbilateral diminution of disc oedema followingunilateral ONSD.

Hayreh9 performed a histological study ofthe optic nerve in nine monkeys with inducedpapilloedema that had undergone ONSD.Examination revealed proliferation of connec-tive tissue with adherence to the pia at the sitesof decompression. Closure of the incised nervesheath was seen in only two animals, andresulted from invasion of the deeper layers ofoptic nerve by the proliferative connectivetissue. No temporal relation could be found forthis exaggerated fibrosis response since opensheath sites were seen in some animals at 31 to63 days after surgery, while in the other twoanimals closed sheath sites were noted after 27and 40 days, respectively.

In our patient there were localised regions ofproliferative fibrosis overlying the fenestrationsites throughout the nerve, and adipose tissuewas noted to be adherent to the optic nerve piain areas of incised dura, consistent with theobservations of Davidson8 and Hayreh.9 Nopatent fistula was identified. In areas otherthan the sites of fenestration, we did not noteeither adherent fat or fibrovascular prolifera-tion. We conclude that the histopathologicalpicture of fat adhesion and fibrosis followingONSD is consistent with a mechanism ofclosed filtration of CSF (that is, analogousto aqueous filtration through an enclosedtrabeculectomy bleb) rather than filtrationthrough an open fistula.We believe, as did Keltner and colleagues,5 6

that egress of CSF is the initial mode of actionof ONSD. However, we hypothesise that thisfiltration of fluid occurs through an enclosedbleb of proliferative fibrosis, rather thanthrough an open fistula. The fibrotic responseseen in our optic nerve specimens may besimilar to the proliferation of connective tissueat the fenestration sites observed by Hayreh.9The high dose corticosteroids received by ourpatient in the immediate postoperative periodmay also have diminished the extent of theproliferative fibrosis. Closure of the inciseddura sites noted by Hayreh9 in two of hismonkeys, and seen also by Davidson7 8 intwo human patients may represent excessivefibrosis around the incision site, such as seenwhen a bleb becomes flattened and fibrosedafter glaucoma filtering surgery. In fact,Davidson's histopathological description offibrotic scarring at the incision sites in onepatient8 may have represented failure of a pre-viously functioning filtering bleb that had beenresponsible for the initial reduction in discoedema noted in the first 5 postoperative days.

In conclusion, our histopathological andultrastructural study may further elucidate the

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Histopathological and ultrastructural examination of optic nerve sheath decompression

mechanism of action of ONSD. We believethat the majority of successful cases of ONSDresult from the continual egress of CSFthrough an enclosed bleb, rather than throughan obvious fistula. Excessive fibrosis may

eventually undermine the function of thisfiltration bleb and lead to total closure of theincised dura and surrounding subarachnoidspace.

This study was supported in part by Core grant EY03040 fromthe National Eye Institute, Bethesda, MD, USA.Dr Tsai is currently affiliated with Moorfields Eye Hospital,

London, England.Presented in part at the Annual Meeting of the Association

for Research in Vision and Ophthalmology, Sarasota, FL,7 May, 1993.We thank Ann Dawson, MLA, for editorial assistance and

Narsing A Rao, MD, for his assistance in interpreting the lightand transmission electron micrographs.

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4 Sergott RC, Cohen MS, Bosley TM, Savino PJ. Optic nervedecompression may improve the progressive form ofnonarteritic ischemic optic neuropathy. Arch Ophthalmol1989; 107: 1743-54.

5 Keltner JL. Optic nerve sheath decompression. How does itwork? Has its time come? Arch Ophthalmol 1988; 106:1365-9.

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