A 270-Degree Decompression of Optic Nerve in Refractory Idiopathic Intracranial Hypertension Using an Ultrasonic Aspirator - A Prospective Institutional Study
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.310080
Source of Support: None, Conflict of Interest: None
Keywords: IIH, modified Dandy's criteria, ONSF, visual acuity, visual fields
Idiopathic intracranial hypertension (IIH), is a disorder of unknown cause presenting with increased intracranial pressure (ICP) and the symptoms associated resulting from defective cerebrospinal fluid (CSF) absorption and is typically seen in obese women of childbearing age, with a worldwide incidence of 12–20 per 100000 people per year in this group, while its incidence in the general population is 0·5–2 per 100 000 people per year. IIH is diagnosed by using modified Dandy criteria, which includes: (1) signs and symptoms due to raised ICP, (2) other than unilateral or bilateral lateral rectus palsy, no other neurological signs are seen, (3) increased CSF opening pressure (≥25 cm of water) with normal CSF analysis, and (4) no abnormality on brain imaging. Patients with malignant IIH present with the following symptoms: (i) rapid onset of the disease, (ii) severe visual loss with a duration of <4 weeks after the onset of disease, and (iii) a recent rapid worsening of visual symptoms. Nonsurgical management includes weight loss, diuretics, steroids, and multiple lumbar punctures. Surgery in the form of either optic nerve sheath decompression (ONSD) or CSF diversion procedure is indicated in case of failure to respond to medical management. Optic nerve decompression was earlier performed through the transconjunctival route. The success in visual improvement was associated with high rates of complications which included diplopia, anisocoria, orbital apex syndrome, traumatic optic neuropathy, and orbital hematoma. The endoscopic approach for optic nerve decompression had complications of CSF leak, cavernous sinus hemorrhage, epistaxis, and the degree of optic canal decompression was inadequate. A cadaveric study by Gogela et al. demonstrated a significantly higher degree of optic canal decompression through a transcranial approach as compared to the endonasal approach. Injury to carotids/visual apparatus limits the full potential of such surgeries., Recently, the use of an ultrasonic bone aspirator for clinoidectomy is safe, requiring less expertise than the standard drilling. There is a lack of evidence-based studies highlighting the risk and benefits of different surgical methods over one another, which was also confirmed by a recent Cochrane collaboration.
To determine the efficacy and extent of bony decompression of optic canal in ONSF using ultrasonic aspirator as an adjunct in the management of medically refractory IIH.
This prospective study was carried out in the Department of Neurosurgery, Nizam's Institute of Medical Sciences, Hyderabad, India from November 2017 to June 2019. The study protocol was approved by the Institutional Ethical Board.
All patients of IIH (diagnosed using modified Dandy criteria) were referred to the neurosurgery following failed medical management and a recent onset visual deterioration (≥2 lines on Snellen's chart) was included in the study.
Patients who 1) responded to medical management, 2) patients who did not consent for surgery, and 3) those lost to follow-up before evaluation in the postoperative period.
A detailed history was elicited to exclude any medical disorder or drug usage as an etiologic factor for IIH. The neurological examination included visual acuity, perimetry, and fundoscopy. Magnetic resonance imaging (MRI) along with magnetic resonance venography (MRV) of the brain was performed to rule out the secondary cause of IIH. CSF opening pressures were measured along with biochemical analysis. The study also included four patients who continued to have visual deterioration despite undergoing a CSF diversion procedure.
Basis of surgery
The cisternal spaces are all in communication with each other and not just exchange the CSF across but the pressure as well which is manifested distantly in the closed cranial cavity in raised ICP as in IIH, wherein the absorption is defective. The principle goal is to create an alternative CSF pathway, thus reducing the ICP and its effects.
Following a fronto-temporo-sphenoidotomy with head in mild hyperextension and neutral rotation, extradural clinoidectomy was performed after separating the cavernous sinus dura with a sharp dissector followed by ultrasonic aspirator (SONOCA 300/Soring, Germany) under high magnification as a standard clinoidectomy described by Dolenc. The decompression was extended over the foraminal roof superiorly till the medial margin of the foramen visible and then the bony decompression was continued along the optic strut till the sphenoid making the optic nerve free on superior, medial, and inferior aspect. The medial part of the sphenoid which is not accessible was left undisturbed. This when measured in the postoperative computed tomography (CT) scans measured 270-degrees freedoms for the optic nerve. The dura over the optic nerve was opened sharply along the length of the optic nerve followed by the opening of cisterns around the optic nerve to release CSF. The dural opening thus made was not sealed and along with the bone so drilled, created a pathway for egress of CSF, and a fistula so created was left to be communicating with the overlying muscle and subgalea for systemic absorption. [Figure 1], [See Video 1]
Headache was assessed by visual analog scale (VAS) scoring. Neuro-ophthalmological evaluation in the follow-up was done at 1 month, 3 months, and 6 months. Visual acuity was the primary outcome modality. Improvement in visual acuity was defined as a change of at least one line on the Snellen's chart. Improvement in visual fields was not quantified and was defined as contraction of field defect noted on a grayscale representation in Humphrey's visual field analyzer chart. Statistics were analyzed using statistical package for the social sciences (SPSS) 17.0 software (SPSS Inc., Chicago, IL). Univariate analysis using the Chi-square test and Fisher's exact test was performed to compare the postoperative visual improvement rate among different groups. Results were considered significant at P < 0.05.
There were a total of 21 patients who met the defined criteria and were included in the study after being diagnosed to have IIH based on modified Dandy criteria. There were two males and 19 females (M: F = 1:9.5). The mean age of the study population was 27.47 years (range 16–44 years). The mean BMI was 26.80 (range 19.4–35.4 kg/m2).
The presenting complaints were recent-onset visual disturbance (n = 21, 100%), headache (n = 18, 85.71%), diplopia (n = 8, 38.09%), and pulsatile tinnitus (n = 4, 19.04%). Papilloedema of Frisen's grade = III was observed in 19 patients (90.47%). Active symptoms of headache for more than 6 months were recorded in 14 patients (66.67%) while four patients had a duration of less than 6 months. The mean CSF opening pressure was 38.9 cm H2O (range 30–48 cm H2O). About four patients had theco-peritoneal (TP) shunt placement done before ONSF [Table 1]a.
A significant improvement in vision described as best-corrected visual acuity (BCVA) was noted in (78.51%) 31 of 39 eyes (three eyes had 6/6 vision), more so on the side of surgery (19 of 21, 90.47%) [Table 1]b. Improvement in visual fields measured as improvement in contraction in the visual field defects was noted in 17 patients (80.95%) over the follow-up period [Figure 3]. Headache improved in 12 out of 18 patients (66.67%) [Table 2]. Complications included surgical site infection which required reexploration of the wound for debridement in one patient while another patient with prior TP shunt, developed subdural hygroma. The peritoneal end of the TP shunt had to be tied for symptom relief [Table 3]. When the improvement in visual outcome was compared between the group of patients with symptom less than 6 months (seven patients, 100% improvement) and those more than 6 months (14 patients, 71.42% improvement), no statistically significant difference was found (P = 0.2550). Besides, relief of headache in a patient undergoing ONSF as primary intervention (8 of 13, 61.54%) against those undergoing ONSF as secondary intervention (4 of 5, 80%) did not show any statistical significance (P = 0.6148). Papilloedema improved in all except two patients who had Frisen's grade V edema [Figure 2]. None of the patients had deterioration of the vision after the ONSF and patients who did not improve had stable vision until the last maximum follow-up period.
Optic nerve sheath fenestration was first described in 1872 by DeWecker. However, the use of ONSF for the treatment of IIH was first described by Hayreh in 1964. The pathophysiologic mechanisms by which ONSF improves visual function are still debated. The suggested mechanisms include 1) fibroblast proliferation around the optic nerve causing obliteration of subarachnoid spaces, preventing transmission of raised ICP distal to the operative site and 2) uninterrupted outflow of CSF from the dural fistula formed at the operative site. Relief of headache due to a generalized decrease in ICP and bilateral improvement in visual function after surgery support this hypothesis.
The decision for treatment of IIH depends upon the severity of IIH-related visual loss and headache. Patients who fail conservative measures (weight loss) and maximal medical therapy for lowering raised ICP and headache might benefit from surgical intervention. A “frank visual loss” at presentation suggests a poor visual prognosis and is an indication for surgical intervention rather than weeks of medical management which takes long to assess its response. The diminution of visual acuity in IIH indicates 1) involvement of macula by edema or 2) setting in of optic atrophy. ONSF is often the preferred surgical option in patients with predominant acute onset visual loss, although headache has been found to improve in up to 50% cases.
The approach for ONSF can be either transconjunctival, endoscopic endonasal, or transcranial. ONSF, through the medial transconjunctival approach the improvement of visual acuity was seen in 67–94% while visual field improvement was observed in 64–88%. The fundus picture improved in the range of 71–95%.,,,
ONSF through the endoscopic approach in patients with IIH had improvement in visual field deficits up to 93.8%, visual acuity up to 85.3%, papilloedema up to 81.4%, and headaches up to 81.8%.
Patients who underwent unilateral ONSF had improvement in visual function in nonoperated eye as well. Our study had a 66.67% improvement in acuity and a 50% improvement in visual fields in the nonoperated eye. This was hypothesized to be due to decreased mean intrasheath CSF pressures of both optic nerves, better compliance to medical management of persistence of patent fistula at autopsy study.,, It was suggested that patients with a shorter duration of symptoms had better responses after ONSF. Headache improved in 66.67% of patients in our study, which was better than previously reported studies (41–50%)., Remaining patients had a stable headache which did not worsen after surgery. As the fundus picture in these patients too improved after surgery, the persistence of headache suggests a nonspecific cause unrelated to IIH.
If the CSF diversion fails, there can be a rapid and severe deterioration in vision. Kelman et al. and Sergott et al. demonstrated that in patients with functional CSF shunts with a persistent progressive visual deterioration which is suggestive of shunt failure, prophylactic ONSF may be beneficial. All the four patients who had an unsuccessful CSF diversion improved after ONSF.
Complication rates associated with the transconjunctival ONSF approach varies from 4.8–45%, which included orbital hemorrhage, retinal artery occlusion, increased intraocular pressure, pupillary palsy, accommodation failure, and diplopia. Diplopia was found to be the most common complication in this approach.,
Endoscopic procedures are associated with CSF leak, meningitis, subcutaneous orbital emphysema, epistaxis. The inability to cut the falciform ligament without risk of CSF leak forms another limitation of the endonasal approach. [Table 3]
The use of high-speed drills for decompression of bony optic canal in conventional transcranial ONSF has its share of operative risks. Firstly, a rapidly rotating drill can tear and damage the nerve or lacerate the frontal lobe. Secondly, the rotating tip can cause contact injury to the optic apparatus and internal carotid artery. Thirdly, the heat generation due to the rotating drill can cause direct thermal injury to the nerve. The use of an ultrasonic bone aspirator, which works on the principle of to and fro movements and ultrasonic waves for bony decompression with inbuilt continuous irrigation poses limited risk of injury to adjacent structures. Further being extradural, a layer of subarachnoid CSF acts as a water bath cushion avoiding direct compression of the nerve.
The degree of optic nerve decompression achieved in our study was 270 degrees. Bony optic nerve decompression in earlier studies with transcranial than endonasal technique were 245.2 degrees and 114.8 degrees, respectively. The maximum degree of 180 of optic nerve decompression was achieved in the endoscopic endonasal approach in a review by Tarrats et al. [Table 4] The dural incision on the basifrontal optic nerve is devoid of any major vessel, especially the ophthalmic artery which is encountered consistently in both endonasal endoscopic and transconjunctival route.
The other major surgical procedure in current practice is the CSF diversion procedure. Initially, a VP shunt was used, however, had a limitation of tapping the small ventricles. Lumboperitoneal shunting for IIH was first described by Vander Ark et al. in 1971. Spetzler developed a percutaneous technique for lumbar peritoneal (LP) shunt placement. An LP shunt works by draining the CSF and thus reducing the overall decrease in ICP and improvement in visual symptoms. Shunt surgeries though were effective in reducing the persistent headaches were found wanting in overall visual improvement as compared to ONSF. Feldon observed 47%, 44.6%, and 80% visual improvement following VP shunt, LP shunt, and ONSD, respectively. The complications of ONSF are mainly related to the trajectory and injury to adjacent neurovascular structures. The trajectory which we have chosen is devoid of major vessels and nerves and hence is not comparable. The trans-conjunctival route has all whole orbit at risk while the endoscopic procedure has a major risk of injuring the carotids or inferiorly located ophthalmic artery. However, the majority of complications of ONSF were temporary. We had four patients with no improvement following LP shunt, who required ONSF. Subdural hygroma was another complication seen in our patient. The reported revision rates for LP shunts range from 38–64% (overall 52%). The number of revisions per patient has been reported as high as 6.6% (mean 3.9%). Low-pressure headaches, shunt dependency, sudden visual loss due to shunt block, infections, lumbar radiculopathy, CSF leaks, and acquired cerebellar tonsillar herniation are few other complications reported in the literature.
Both ONSF and LP/VP shunt have their pros and cons and the debate on the topic has been inconclusive in the absence of head-on randomized control trials. The Cochrane review in 2005 “Interventions for idiopathic intracranial hypertension” was guarded in its recommendations of one surgical procedure over others. It, however, favored a surgical procedure over medical management in the event of visual loss. We need to wait for the results of the ongoing SIGHT (Surgical idiopathic intracranial hypertension treatment) trial.
In nutshell, the advantages are as follows: 1) extradural, 2) CSF cushion around the optic nerve till the clionectomy, 3) lack of thermal injury by ultrasonic aspirator as compared to high-speed drill, 4) lack of mechanical entanglement of adjacent structures (high-speed drill), 5) maximal bony decompression (270 degrees), and 6) avoids potential infected space (trans-nasal, endoscopic approach).
Transcranial optic nerve sheath decompression with a bone ultrasonic aspirator is a safe and direct decompression of the optic nerve in malignant/refractory cases of IIH.
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Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4]