May 2014
Volume 55, Issue 5
Free
Glaucoma  |   May 2014
Optic Neuropathy Induced by Experimentally Reduced Cerebrospinal Fluid Pressure in Monkeys
Author Affiliations & Notes
  • Diya Yang
    Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology and Visual Sciences Key Laboratory, Beijing, China
    Beijing Institute of Ophthalmology, Beijing Tongren Hospital, Capital Medical University, Beijing, China
  • Jidi Fu
    Department of Neurosurgery, Beijing Tongren Hospital, Capital Medical University, Beijing, China
  • Ruowu Hou
    Department of Neurosurgery, Beijing Tongren Hospital, Capital Medical University, Beijing, China
  • Kegao Liu
    Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology and Visual Sciences Key Laboratory, Beijing, China
  • Jost B. Jonas
    Department of Ophthalmology, Medical Faculty Mannheim of the Ruprecht-Karls-University Heidelberg, Mannheim, Germany
  • Huaizhou Wang
    Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology and Visual Sciences Key Laboratory, Beijing, China
    Beijing Institute of Ophthalmology, Beijing Tongren Hospital, Capital Medical University, Beijing, China
  • Weiwei Chen
    Beijing Institute of Ophthalmology, Beijing Tongren Hospital, Capital Medical University, Beijing, China
  • Zhen Li
    Department of Ophthalmology, Xuanwu Hospital, Capital Medical University, Beijing, China
  • Jinghong Sang
    Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology and Visual Sciences Key Laboratory, Beijing, China
  • Zheng Zhang
    Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology and Visual Sciences Key Laboratory, Beijing, China
  • Sumeng Liu
    Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology and Visual Sciences Key Laboratory, Beijing, China
  • Yiwen Cao
    Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology and Visual Sciences Key Laboratory, Beijing, China
  • Xiaobin Xie
    Department of Ophthalmology, Eye Hospital of China Academy of Chinese Medical Sciences, Beijing, China
  • Ruojin Ren
    Beijing Institute of Ophthalmology, Beijing Tongren Hospital, Capital Medical University, Beijing, China
  • Qingjun Lu
    Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology and Visual Sciences Key Laboratory, Beijing, China
    Beijing Institute of Ophthalmology, Beijing Tongren Hospital, Capital Medical University, Beijing, China
  • Robert N. Weinreb
    Hamilton Glaucoma Center and Department of Ophthalmology, University of California-San Diego, La Jolla, California, United States
  • Ningli Wang
    Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology and Visual Sciences Key Laboratory, Beijing, China
    Beijing Institute of Ophthalmology, Beijing Tongren Hospital, Capital Medical University, Beijing, China
  • Correspondence: Ningli Wang, Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology and Visual Sciences Key Laboratory, No. 1 Dongjiaominxiang Street, Dongcheng District, Beijing, China, 100730; wningli@vip.163.com.  
Investigative Ophthalmology & Visual Science May 2014, Vol.55, 3067-3073. doi:10.1167/iovs.13-13657
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      Diya Yang, Jidi Fu, Ruowu Hou, Kegao Liu, Jost B. Jonas, Huaizhou Wang, Weiwei Chen, Zhen Li, Jinghong Sang, Zheng Zhang, Sumeng Liu, Yiwen Cao, Xiaobin Xie, Ruojin Ren, Qingjun Lu, Robert N. Weinreb, Ningli Wang; Optic Neuropathy Induced by Experimentally Reduced Cerebrospinal Fluid Pressure in Monkeys. Invest. Ophthalmol. Vis. Sci. 2014;55(5):3067-3073. doi: 10.1167/iovs.13-13657.

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Abstract

Purpose.: To examine the influence of experimentally reduced cerebrospinal fluid pressure (CSFP) on retinal nerve fiber layer (RNFL) thickness and neuroretinal rim area of the optic nerve head.

Methods.: This experimental study included nine monkeys that underwent implantation of a lumbar–peritoneal cerebrospinal fluid (CSF) shunt. In the study group (n = 4 monkeys), the shunt was opened to achieve a CSF of approximately 40 mm H2O, while the shunt remained closed in the control group (n = 5 monkeys). At baseline and in monthly intervals thereafter, optical coherence tomographic and photographic images of the optic nerve head and RNFL were taken of all monkeys.

Results.: Two out of four monkeys in the study group showed bilaterally a progressive reduction in RNFL thickness between 12% and 30%, reduction in neuroretinal rim area and volume, and increase in cup-to-disc area ratios. A third monkey developed a splinter-like disc hemorrhage in one eye. The fourth monkey in the study group did not develop morphologic changes during follow-up, nor did any monkey in the control group.

Conclusions.: Experimental and chronic reduction in CSF in monkeys was associated with the development of an optic neuropathy in some monkeys.

Introduction
Anatomic investigations have shown that the retrolaminar tissue pressure and the orbital cerebrospinal fluid pressure (CSFP) form the counterpressure against the intraocular pressure (IOP) across the lamina cribrosa. 13 The so-called trans-lamina cribrosa pressure difference was described by Morgan and colleagues 1 as strongly correlated with the difference between IOP and CSFP in dogs when the CSFP was higher than zero. 1 It has been speculated that in addition to IOP, the trans-laminar cribrosa pressure difference may perhaps play a role in the pathogenesis of optic nerve diseases occurring at the optic nerve head. 36 To get hints regarding the dependence of the optic nerve on CSFP, we conducted an experimental study on monkeys in which lumbar–peritoneal shunting of the cerebrospinal fluid (CSF) resulted in a low CSFP. We compared study animals with low CSFP to control animals in which lumbar–peritoneal shunts were inserted but remained closed so that the CSF remained normal. We monitored the optic nerve head and retinal nerve fiber layer (RNFL) to identify optic nerve damage. The main target of the study was to assess whether an experimental lowering of CSFP was associated with a loss of optic nerve fibers and changes in the optic nerve head. 
Methods
The experimental study included nine Rhesus monkeys with an average age of 6 years and body weight of approximately 6 kg. All animals were treated in accordance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. The animals were randomly distributed between an experimental group (four monkeys) and a control group (five monkeys). At baseline of the study, none of the monkeys had undergone any intervention or had been involved in any study. For all ophthalmic examinations and procedures at baseline of the study and at the follow-up examinations, the animals were anesthetized with an intramuscular injection of ketamine HCl (20 mg/kg) and midazolam (0.2 mg/kg), with repeated injections of ketamine (10 mg/kg) as needed during the examinations and operations. The pupils were dilated by topical application of tropicamide (0.5%), and both eyes of all monkeys were examined. 
At baseline of the study and at regular intervals during the follow-up, all animals underwent an ophthalmologic examination including slit-lamp–assisted biomicroscopy of the anterior and posterior segment of the eye, rebound tonometry (Tonovet; iCare, Helsinki, Finland), photography of the optic nerve head and fundus (Digital Retinal Camera, Canon CR-DGi with Canon EOS 40D; Canon, Inc., Tokyo, Japan), and spectral-domain optical coherence tomography (OCT; RTVue-100 Fourier-domain OCT; Optovue, Inc., Fremont, CA, USA; software version A6.1.0.4) of the optic nerve head and RNFL. The fundus photographs were taken from two different angles. Three glaucoma specialists in a masked manner examined separately the photographs using a handheld stereo viewer (Screen-Vu Stereo Viewer; Berezin Stereo Photography Products, Mission Viejo, CA, USA). For all of the animals, no abnormality in the optic nerve head or retina was detected at baseline. The basic principles of spectral-domain OCT and the application of the OCT on measurements of monkey ocular parameters have been described in detail previously. 79 In brief, the RTVue (Optovue, Inc.) used a scanning laser diode for emission of a scan beam with a wavelength of 840 ± 10 nm. The RNFL3.45 mode assessed the RNFL thickness along a circle with a diameter of 3.45 mm centered on the optic disc. The RNFL thickness was measured in eight regions, and the mean RNFL thickness was calculated as the average of these eight measurements. Images with a signal strength index of less than 40 were excluded from further analysis. All images were acquired by the same trained examiner (DY). The boundary of the disc area defined as the innermost end of Bruch's membrane was automatically determined by the OCT device. The optic cup was defined based on a reference line located 150 μm above the level of Bruch's membrane opening. The neuroretinal rim area was calculated as difference of disc area minus cup area. The intra- and intersession repeatability of RTVue-100 (Optovue, Inc.) on Rhesus monkeys was assessed previously. 9  
At baseline of the study, a CSF-measuring microsensor was implanted into all monkeys. After shaving, preparation of the skin, and draping of the head and lumbar area, incisions were performed, and the scalp was retracted to expose the skull (Fig. 1). An electric drill (Spiggle & Theis, Medizintechnik Gmbh, Overath, Germany) was used to create the drill hole through which a microsensor (Codman Microsensor, Codman ICP Monitoring System; Codman Co., Tuttlingen, Germany) was placed. The drill hole was beveled on the side of the exit of the microsensor. A Touhy needle provided from the kit of the Codman microsensor was used to tunnel the scalp from the site of the transcranial hole to the exit site 5 cm away from the hole. The Touhy needle stylet was removed, and the microsensor was threaded from the tip of the needle to the appropriate length for placement exit from the hub. The tip of the microsensor was placed in the parenchyma through the puncture in the dura. The microsensor was secured to the scalp, and the catheter was connected to the monitoring system. After implantation of the lumbar–peritoneal shunt as described below, the microsensor for measuring the CSFP was removed at the end of the procedure. 
Figure 1
 
Photographs showing the preparation and insertion of a cerebrospinal fluid pressure–measuring sensor.
Figure 1
 
Photographs showing the preparation and insertion of a cerebrospinal fluid pressure–measuring sensor.
A second skin incision was made over the fourth lumbar vertebra for implanting the lumbar–peritoneal shunt. The incision was carried down using a blunt dissection technique until the bone was accessed. Electrocoagulation and an electrotome (Comermy Group, Inc., Nanjing, China) were used during the dissection procedure to avoid bleeding. A rongeur (Spiggle & Theis) was applied to remove the spines of the lumbar vertebra, and an electric drill was used to access the dura mater. A drainage catheter connected to a magnetically programmable shunt valve (Codman Hakim programmable valve; Codman Co.) was implanted into the lumbar subarachnoid space while the other end of the catheter was implanted into the abdomen. The adjustable valve accommodated 18 valve settings from 30 to 200 mm H2O in 10-mm H2O increments. Preoperatively, the valve was set to achieve a CSFP of 40 mm H2O. The CSFP, which was continuously measured by the cerebral microsensor during the surgery, was observed to guarantee that the CSFP was lowered to the target pressure. For the monkeys in the control group, the drainage catheter was closed so that no drainage could occur. 
After implantation of the lumbar–peritoneal CSF-drainage catheter at baseline of the study, all monkeys underwent follow-up ocular examinations at monthly intervals. At baseline, all ocular measurements were repeated five times; and at each examination during follow-up, all ocular measurements were repeated three times. The CSFP was measured by the cerebral CSFP microsensor during the surgery for implantation of the lumbar–peritoneal shunt. At weekly intervals, the function of the shunt was tested by pressing the test reservoir at the back of the monkey. The shunt was considered to be open if the reservoir could be emptied by slight compression. At 1 month and 6 months after baseline, the CSFP was measured by direct puncture. 
Statistical analysis was performed using a commercially available software system (SPSS for Windows, version 21.0; IBM-SPSS, Chicago, IL, USA). Parameters with a normal distribution are presented as mean ± standard deviation. The normal distribution of data was examined using the one-sample Kolmogorov-Smirnov test, and the homogeneity of the variance was examined using Levene's test. T-test for two independent samples was used for comparing the study group and control group. The paired-sample t-test was used for comparing between the baseline measurement values and the follow-up measurement values. The P values were based on two-sided tests and were considered to be statistically significant if P < 0.05. 
Results
At the baseline level of the study, the monkeys in the study group (n = 4) and the monkeys in the control group (n = 5) did not differ significantly in sex, age, body weight, IOP, thickness of the RNFL, areas of the optic nerve head, area and volume of the neuroretinal rim, and cup-to-disc area ratio (Table 1). Similarly, CSF did not differ significantly between the study group and the control group at baseline (7.4 ± 0.6 vs. 7.6 ± 0.9 mm Hg; P = 0.68). After implanting the drainage catheter, CSF was significantly lower in the study group than in the control group. The CSFP of the study group measured by the cerebral microsensor at the end of surgery was 1.25 ± 0.50 mm Hg (range, 1.0–2.0 mm Hg). The CSFP was significantly lower in the study group than in the control group at 1 month after baseline (1.6 ± 0.6 vs. 6.8 ± 1.1 mm Hg; P < 0.001) and at 6 months after baseline (2.0 ± 0.8 vs. 8.2 ± 2.6 mm Hg; P < 0.001). In all four monkeys of the study group, the CSF was reduced by at least 5 mm Hg. In the control group, the CSF did not differ significantly between the baseline values and measurements performed 1 month later (P = 0.10; Table 2). 
Table 1
 
Demographic Data and Ocular Parameters (Mean ± Standard Deviation) of the Study Group and Control Group
Table 1
 
Demographic Data and Ocular Parameters (Mean ± Standard Deviation) of the Study Group and Control Group
Study Group Control Group P Value
Number of monkeys 4, 8 eyes 5, 10 eyes
Sex Male Male
Age, y 6.3 ± 1.3 6.2 ± 1.1 0.77
Weight, kg 6.5 ± 0.7 6.2 ± 0.9 0.58
Intraocular pressure, mm Hg 17.9 ± 0.7 17.2 ± 1.3 0.051
Retinal nerve fiber layer thickness, μm 103 ± 5.9 107 ± 7.8 0.21
Optic disc area, mm2 1.55 ± 0.41 1.54 ± 0.70 0.37
Neuroretinal rim area, mm2 0.80 ± 0.39 0.97 ± 0.46 0.87
Neuroretinal rim volume, mm3 0.05 ± 0.04 0.09 ± 0.06 0.19
Cup-to disc ratio 0.51 ± 0.14 0.31 ± 0.25 0.14
Table 2
 
Changes in Intracranial Pressure Before and After Shunt Surgery
Table 2
 
Changes in Intracranial Pressure Before and After Shunt Surgery
Before Surgery, Baseline After Surgery, 1 mo P Value
Study group 7.4 ± 0.55 1.6 ± 0.55 <0.001
Control group 7.6 ± 0.89 6.8 ± 1.10 0.1
P value 0.68 <0.001
During a follow-up of 1 year, two out of the four monkeys in the study group (monkeys 1 and 3) showed a progressive reduction in RNFL thickness in both of their eyes, accompanied by a significant reduction in the area and volume of the neuroretinal rim and a significant increase in the cup-to-disc area ratio (Figs. 2 1552 15525). As compared with the baseline values, the mean loss in RNFL thickness in monkey 1 was 24.5% in the right eye and 30% in the left eye (Table 3). The size of the optic nerve head did not change significantly between the baseline of the study and end of the follow-up in any monkey (all P > 0.10; Table 3). The two monkeys with loss of RNFL (monkeys 1 and 3; Table 3) showed mostly diffuse damage with a reduction in RNFL thickness across almost all sectors (Fig. 6). 
Figure 2
 
Diagram showing changes in the retinal nerve fiber layer thickness as measured by optical coherence tomography in monkeys with experimental reduction of cerebrospinal fluid pressure. (A) Monkey 1, (B) monkey 3.
Figure 2
 
Diagram showing changes in the retinal nerve fiber layer thickness as measured by optical coherence tomography in monkeys with experimental reduction of cerebrospinal fluid pressure. (A) Monkey 1, (B) monkey 3.
Figure 3
 
Fundus photograph at baseline (left column) and at 12 months after lowering of cerebrospinal fluid pressure (right column) of monkey 1. Note decreased visibility of the retinal nerve fiber layer. OD, right eye; OS, left eye.
Figure 3
 
Fundus photograph at baseline (left column) and at 12 months after lowering of cerebrospinal fluid pressure (right column) of monkey 1. Note decreased visibility of the retinal nerve fiber layer. OD, right eye; OS, left eye.
Figure 4
 
Optic disc photograph of monkey 2 of the study group at 4 months after lumbar–peritoneal shunting. Note splinter-like optic disc hemorrhage (blue arrow).
Figure 4
 
Optic disc photograph of monkey 2 of the study group at 4 months after lumbar–peritoneal shunting. Note splinter-like optic disc hemorrhage (blue arrow).
Figure 5
 
Optical coherence tomograms of the optic nerve head in monkey 1 (OD) and monkey 3 (OS) after lumbar–peritoneal shunting and reduction of cerebrospinal fluid pressure, taken at baseline (upper row) and at 12 months of follow-up (lower row). Note reduction in neuroretinal rim tissue.
Figure 5
 
Optical coherence tomograms of the optic nerve head in monkey 1 (OD) and monkey 3 (OS) after lumbar–peritoneal shunting and reduction of cerebrospinal fluid pressure, taken at baseline (upper row) and at 12 months of follow-up (lower row). Note reduction in neuroretinal rim tissue.
Figure 6
 
Illustration of the profile of retinal nerve fiber layer thickness in eight different regions and their comparison before and after lowering of CSF pressure. (A) Monkey 1, (B) monkey 2, (C) monkey 3. TL, temple lower; TU, temple upper; ST, superior temple; SN, superior nasal; NU, nasal upper; NL, nasal lower; IN, inferior nasal; IT, inferior temple. Red arrow: P > 0.05; blue arrow: P < 0.05.
Figure 6
 
Illustration of the profile of retinal nerve fiber layer thickness in eight different regions and their comparison before and after lowering of CSF pressure. (A) Monkey 1, (B) monkey 2, (C) monkey 3. TL, temple lower; TU, temple upper; ST, superior temple; SN, superior nasal; NU, nasal upper; NL, nasal lower; IN, inferior nasal; IT, inferior temple. Red arrow: P > 0.05; blue arrow: P < 0.05.
Table 3
 
Retinal Nerve Fiber Layer Thickness and Topometric Optic Disc Data of Monkeys With Experimental Reduction of Cerebrospinal Fluid Pressure at Baseline and at the End of Follow-up
Table 3
 
Retinal Nerve Fiber Layer Thickness and Topometric Optic Disc Data of Monkeys With Experimental Reduction of Cerebrospinal Fluid Pressure at Baseline and at the End of Follow-up
Monkey Number Group Parameters OD, OS Baseline Last P Value Paired Difference Percentage
1 Study group RNFLT, μm OD 93.0 ± 0.6 70.5 ± 1.3 <0.0001 22.9 ± 1.1 24.50
OS 95.4 ± 1.8 66.5 ± 1.1 <0.0001 28.9 ± 1.9 30.30
Disc area, mm2 OD 1.43 ± 0.01 1.43 ± 0.00 1 0.00 ± 0.01 0
OS 1.28 ± 0.01 1.29 ± 0.01 0.18 −0.01 ± 0.01 0.70
Rim area, mm2 OD 0.88 ± 0.04 0.65 ± 0.08 0.005 0.23 ± 0.09 26.10
OS 0.60 ± 0.02 0.38 ± 0.09 0.005 0.22 ± 0.09 36.70
Rim volume, mm3 OD 0.04 ± 0.01 0.02 ± 0.01 0.006 0.02 ± 0.01 50
OS 0.03 ± 0.00 0.01 ± 0.00 <0.0001 0.02 ± 0.003 66.70
Cup-to-disc ratio OD 0.39 ± 0.03 0.55 ± 0.06 0.006 −0.16 ± 0.07 41.00
OS 0.53 ± 0.01 0.70 ± 0.07 0.003 −0.18 ± 0.06 34.00
2 Study group RNFLT, μm OD 109.5 ± 1.4 106.1 ± 3.8 0.051 3.5 ± 2.8 3.20
OS 108.5 ± 1.0 105.3 ± 1.3 0.04 3.3 ± 1.2 3.00
Disc area, mm2 OD 2.24 ± 0.01 2.23 ± 0.00 0.14 0.01 ± 0.01 0.40
OS 2.09 ± 0.01 2.10 ± 0.01 0.34 −0.008 ± 0.02 0.40
Rim area, mm2 OD 1.32 ± 0.04 1.33 ± 0.02 0.54 −0.01 ± 0.02 0.80
OS 1.37 ± 0.1 1.50 ± 0.04 0.09 −0.13 ± 0.13 9.50
Rim volume, mm3 OD 0.10 ± 0.01 0.11 ± 0.01 0.1 −0.01 ± 0.01 10
OS 0.12 ± 0.03 0.16 ± 0.02 0.08 −0.03 ± 0.03 25
Cup-to-disc ratio OD 0.41 ± 0.01 0.40 ± 0.01 0.21 0.01 ± 0.01 2.40
OS 0.34 ± 0.05 0.29 ± 0.02 0.13 0.06 ± 0.07 17.60
3 Study group RNFLT, μm OD 102.8 ± 0.8 83.6 ± 0.8 <0.0001 19.3 ± 1.4 18.80
OS 101.6 ± 1.3 89.4 ± 0.7 <0.0001 12.2 ± 0.9 12.00
Disc area, mm2 OD 1.31 ± 0.00 1.31 ± 0.01 0.62 −0.00 ± 0.01 0
OS 1.11 ± 0.01 1.12 ± 0.00 0.7 −0.01 ± 0.01 0.90
Rim area, mm2 OD 0.43 ± 0.02 0.23 ± 0.03 <0.0001 0.20 ± 0.03 46.50
OS 0.30 ± 0.03 0.18 ± 0.05 0.02 0.12 ± 0.07 40
Rim volume, mm3 OD 0.02 ± 0.00 0.01 ± 0.00 <0.0001 0.02 ± 0.00 100
OS 0.01 ± 0.00 0.004 ± 0.001 0.007 0.01 ± 0.003 100
Cup-to-disc ratio OD 0.68 ± 0.01 0.82 ± 0.01 <0.0001 −0.15 ± 0.02 22
OS 0.72 ± 0.03 0.84 ± 0.04 0.014 −0.11 ± 0.06 15.30
4 Study group RNFLT, μm OD 105.6 ± 2.8 102.0 ± 1.6 0.1 3.6 ± 3.7 3.40
OS 105.5 ± 1.7 103.7 ± 1.4 0.07 1.8 ± 1.6 1.70
Disc area, mm2 OD 1.28 ± 0.01 1.28 ± 0.01 0.18 −0.00 ± 0.01 0
OS 1.69 ± 0.01 1.69 ± 0.01 0.75 0.002 ± 0.01 0.10
Rim area, mm2 OD 0.66 ± 0.04 0.65 ± 0.02 0.57 0.01 ± 0.02 1.50
OS 0.84 ± 0.02 0.82 ± 0.08 0.53 0.03 ± 0.09 3.60
Rim volume, mm3 OD 0.07 ± 0.00 0.06 ± 0.01 0.28 0.01 ± 0.01 14.30
OS 0.04 ± 0.002 0.04 ± 0.01 0.33 0.003 ± 0.006 7.50
Cup-to-disc ratio OD 0.48 ± 0.03 0.49 ± 0.02 0.43 −0.01 ± 0.02 2.10
OS 0.50 ± 0.02 0.50 ± 0.05 0.61 −0.012 ± 0.05 2.40
A splinter-like optic disc hemorrhage was detected at 4 months after baseline in the right eye of monkey 2 (Fig. 4). Monkeys 2 and 4 of the study group did not show any significant difference in RNFL thickness or in topography of the optic nerve head between baseline and the end of the follow-up (Table 3). 
In the control group, all optic nerve head measurements, including area and volume of the neuroretinal rim and optic cup, did not differ between baseline values and measurements obtained at the end of the follow-up. Comparison of the study group with the control group showed that the mean RNFL thickness was significantly less in the study group at the end of the follow-up (89.4 ± 15.0 vs. 100.9 ± 7.4 μm; P < 0.05). 
Discussion
In our experimental study, four eyes of two monkeys out of the whole study group of four monkeys with experimentally reduced CSFP developed morphological changes in their RNFL and of the optic nerve head. The changes included a statistically significant decrease in the area and volume of the neuroretinal rim, an increase in the volume of the optic cup, and a reduced thickness of the RNFL. Additionally, one eye showed a transient splinter-shaped optic disc hemorrhage. During the entire study period, IOP was in the normal range and did not differ between the study group and the control group or between the measurements taken at baseline and the measurements obtained during the follow-up. These findings suggest that some monkeys with experimentally reduced CSFP develop morphological changes of the optic nerve head and RNFL that are comparable with progressive optic neuropathy. 
Our study supported the concept that low CSFP alone may cause retinal ganglion cell injury and loss and thus that a low CSFP may be a risk factor in all forms of optic neuropathy including glaucoma. Our study did not present any evidence that low CSFP caused glaucoma at normal levels of IOP. Interestingly, previous clinical studies by Berdahl and colleagues 10 and others 6 found lower lumbar CSFP measurements in patients with normal-pressure glaucoma than in patients with high-pressure glaucoma. Another study demonstrated that patients with glaucomatous optic nerve damage and normal IOP had an abnormally narrow subarachnoid CSF space around the postlaminar optic nerve suggesting a low CSFP. 11  
When interpreting the measurements of our study, one must clearly keep in mind that we did not examine any morphological change in the lamina cribrosa of the monkeys. Previous landmark studies have shown that experimental glaucoma in monkeys is characterized by a deformation of the lamina cribrosa, including a lamina cribrosa thickening at the earliest stages of the disease, and that the late stage of glaucoma is associated with a thinning of the lamina cribrosa. 1223 The findings were based on series of investigations that included studies with unilateral experimental glaucoma. These earlier investigations applied histology, three-dimensional histomorphometry, and longitudinal spectral-domain OCT imaging, and examined the transition from normal monkey optic nerve heads to optic nerve heads with early glaucomatous damage. Using histomorphometry or spectral-domain OCT, future studies may address whether monkeys with experimental lowering of their CSFP show a change in the configuration of the lamina cribrosa, such as a posterior deformation as well as loss of optic nerve fibers and thinning of the neuroretinal rim. Control groups consisting of monkeys with a unilateral optic nerve transection (with a presumably unchanged configuration of the lamina cribrosa) and monkeys with a unilateral mild to moderate IOP elevation (with a deformation of the lamina cribrosa) also should be studied. 1821  
One has also to take into account that the laminar scleral dynamics suggest that scleral tensile forces generated by IOP at all levels of IOP likely are much higher than the translaminar pressure difference, and that they powerfully determine the levels of stress and strain within the laminar tissues. These findings were reported in studies by Bellezza and colleagues 12 using serial histologic sections of the optic nerve head of monkeys with experimental glaucoma, in investigations by Yang and coworkers 13,18 applying three-dimensional histomorphometric reconstructions of the optic nerve head, and in studies by Sigal, 14,15 Girard, 22 and others with modeling of the forces in the optic nerve head region. 1223 These studies suggested that primary changes in CSFP may influence the translaminar pressure difference and may damage the optic nerve fibers but that they are not likely to lead to laminar deformation or to produce a “glaucomatous” optic neuropathy in which the lamina deforms. It is possible that even at physiologic levels of IOP, the IOP-related tensile forces within the sclera will pull the laminar taut within the canal more effectively than the translaminar pressure difference will “push” the lamina cribrosa outward. 
Potential limitations of the current study should be mentioned. First, two monkeys with artificially low CSF did not develop the optic neuropathy observed in the other two monkeys. One may argue that the reduction in CSF by the lumbar–peritoneal shunt was not sufficient or that intracranial CSFP did not reflect the orbital CSFP in the monkeys without a change in the RNFL or optic nerve head. Another explanation may be that the absence of optic neuropathy in these animals was similar to the clinical observation of ocular hypertension, in which optic nerve damage does not develop despite elevated IOP. 23  
Second, several investigators have shown that monkey eyes with an elevated trans-lamina cribrosa pressure difference due to an increase in IOP have lateral tension on the lamina cribrosa due to the IOP elevation-related increase in the diameter of the optic nerve scleral canal opening. 16,2023 It is unlikely that such a phenomenon occurs in a monkey with decreased CSF and normal IOP. This suggests that there are differences in the influence of an elevated IOP versus the influence of a reduced CSF on the optic nerve head in monkeys. 
Third, as a corollary, the magnitude of the trans-laminar cribrosa pressure difference is considerably higher in monkeys with experimental glaucoma induced by elevation of the IOP than in our model with a reduction in CSFP. Differences between these two models in optic nerve head changes, and also differences in the time course of these optic nerve head changes, may be related to differences in the trans-lamina cribrosa pressure difference. 
Fourth, the extent to which results from experimental studies in monkeys are relevant to patients has remained unclear. Besides differences in the anatomy of the optic nerve head, including differences in the thickness of the lamina cribrosa and sclera and the length of the peripapillary scleral flange, monkeys and humans differ in the posture of the head in relationship to the spine. While humans are upright during daytime, monkeys keep their head more or less on the same level as the upper part of their spine. Also in contrast, humans sleep in a lying position, while monkeys usually sleep in a sitting position. It has remained inconclusive whether these differences influence the physiologic requirements on the CSF. 
Fifth, to summarize all these limitations, it is not possible to use the results of our study to draw conclusions on the questionable role of a low CSFP or of an altered composition of the CSF in the pathogenesis of glaucoma in some patients normal IOP. 36,10,11,2427 It has also remained unclear whether the model of low CSFP as used in our study is contrary to the clinical situation or experimental model of an elevated CSFP. 28  
In conclusion, our study is the first to demonstrate that primary CSF lowering in the nonhuman primate produces an optic neuropathy in a subset of eyes. The implications of our findings on the search for experimental models of normal-pressure glaucoma and the mechanisms of retinal ganglion cell axon insult in human glaucoma remain to be determined. 
Acknowledgments
Supported by National Natural Science Foundation of China (81271005, 81300767), Beijing Natural Science Foundation (7122038), and two separate donations by China Health and Medical Development Foundation. 
Disclosure: D. Yang, None; J. Fu, None; R. Hou, None; K. Liu, None; J.B. Jonas, None; H. Wang, None; W. Chen, None; Z. Li, None; J. Sang, None; Z. Zhang, None; S. Liu, None; Y. Cao, None; X. Xie, None; R. Ren, None; Q. Lu, None; R.N. Weinreb, None; N. Wang, None 
References
Morgan WH Yu DY Cooper RL The influence of cerebrospinal fluid pressure on the lamina cribrosa tissue pressure gradient. Invest Ophthalmol Vis Sci . 1995; 36: 1163–1172. [PubMed]
Morgan WH Chauhan BC Yu DY Optic disc movement with variations in intraocular and cerebrospinal fluid pressure. Invest Ophthalmol Vis Sci . 2002; 43: 3236–3242. [PubMed]
Jonas JB Berenshtein E Holbach L. Anatomic relationship between lamina cribrosa, intraocular space, and cerebrospinal fluid space. Invest Ophthalmol Vis Sci . 2003; 44: 5189–5195. [CrossRef] [PubMed]
Morgan WH Yu DY Balaratnasingam C. The role of cerebrospinal fluid pressure in glaucoma pathophysiology: the dark side of the optic disc. J Glaucoma . 2008; 17: 408–413. [CrossRef] [PubMed]
Berdahl JP Allingham RR Johnson DH. Cerebrospinal fluid pressure is decreased in primary open-angle glaucoma. Ophthalmology . 2008; 115: 763–768. [CrossRef] [PubMed]
Ren R Jonas JB Tian G Cerebrospinal fluid pressure in glaucoma: a prospective study. Ophthalmology . 2010; 117: 259–266. [CrossRef] [PubMed]
Kim JS Ishikawa H Sung KR Retinal nerve fibre layer thickness measurement reproducibility improved with spectral domain optical coherence tomography. Br J Ophthalmol . 2009; 93: 1057–1063. [CrossRef] [PubMed]
Shin CJ Sung KR Um TW Comparison of retinal nerve fibre layer thickness measurements calculated by the optic nerve head map (NHM4) and RNFL3.45 modes of spectral-domain optical coherence tomography (RTVue-100). Br J Ophthalmol . 2010; 94: 763–767. [CrossRef] [PubMed]
Zhang Z Yang DY Sang JH Reproducibility of macular, retinal nerve fiber layer, and ONH measurements by OCT in Rhesus monkeys: the Beijing Intracranial and Intraocular Pressure (iCOP) Study. Invest Ophthalmol Vis Sci . 2012; 53: 4505–4509. [CrossRef] [PubMed]
Berdahl JP Allingham RR Johnson DH. Cerebrospinal fluid pressure is decreased in primary open-angle glaucoma. Ophthalmology . 2008; 115: 763–768. [CrossRef] [PubMed]
Wang NL Xie XB Yang DY Orbital cerebrospinal fluid space in glaucoma: the Beijing iCOP Study. Ophthalmology . 2012; 119: 2065–2073. [CrossRef] [PubMed]
Bellezza AJ Rintalan CJ Thompson HW Downs JC Hart RT Burgoyne CF. Anterior scleral canal geometry in pressurised (IOP 10) and non-pressurised (IOP 0) normal monkey eyes. Br J Ophthalmol . 2003; 87: 1284–1290. [CrossRef] [PubMed]
Yang H Downs JC Sigal IA Roberts MD Thompson H Burgoyne CF. Deformation of the normal monkey optic nerve head connective tissue after acute IOP elevation within 3-D histomorphometric reconstructions. Invest Ophthalmol Vis Sci . 2009; 50: 5785–5799. [CrossRef] [PubMed]
Sigal IA. Interactions between geometry and mechanical properties on the optic nerve head. Invest Ophthalmol Vis Sci . 2009; 50: 2785–2795. [CrossRef] [PubMed]
Sigal IA Yang H Roberts MD IOP-induced lamina cribrosa deformation and scleral canal expansion: independent or related? Invest Ophthalmol Vis Sci . 2011; 52: 9023–9032. [CrossRef] [PubMed]
Burgoyne CF. A biomechanical paradigm for axonal insult within the optic nerve head in aging and glaucoma. Exp Eye Res . 2011; 93: 120–132. [CrossRef] [PubMed]
He L Yang H Gardiner SK Longitudinal detection of optic nerve head changes by spectral domain optical coherence tomography in early experimental glaucoma. Invest Ophthalmol Vis Sci . 2014; 55: 574–586. [CrossRef] [PubMed]
Yang H Downs JC Bellezza A Thompson H Burgoyne CF. 3-D histomorphometry of the normal and early glaucomatous monkey optic nerve head: prelaminar neural tissues and cupping. Invest Ophthalmol Vis Sci . 2007; 48: 5068–5084. [CrossRef] [PubMed]
Burgoyne CF Downs JC Bellezza AJ Suh JK Hart RT. The optic nerve head as a biomechanical structure: a new paradigm for understanding the role of IOP-related stress and strain in the pathophysiology of glaucomatous optic nerve head damage. Prog Retin Eye Res . 2005; 24: 39–73. [CrossRef] [PubMed]
Bellezza AJ Rintalan CJ Thompson HW Downs JC Hart RT Burgoyne CF. Deformation of the lamina cribrosa and anterior scleral canal wall in early experimental glaucoma. Invest Ophthalmol Vis Sci . 2003; 44: 623–637. [CrossRef] [PubMed]
Yang H Williams G Downs JC Posterior (outward) migration of the lamina cribrosa and early cupping in monkey experimental glaucoma. Invest Ophthalmol Vis Sci . 2011; 52: 7109–7121. [CrossRef] [PubMed]
Girard MJ Suh JK Bottlang M Burgoyne CF Downs JC. Biomechanical changes in the sclera of monkey eyes exposed to chronic IOP elevations. Invest Ophthalmol Vis Sci . 2011; 52: 5656–5669. [CrossRef] [PubMed]
Weinreb RN Khaw PT. Primary open-angle glaucoma. Lancet . 2004; 363: 1711–1720. [CrossRef] [PubMed]
Killer HE Miller NR Flammer J Cerebrospinal fluid exchange in the optic nerve in normal-tension glaucoma. Br J Ophthalmol . 2012; 96: 544–548. [CrossRef] [PubMed]
Killer HE Jaggi GP Miller NR Cerebrospinal fluid dynamics between the basal cisterns and the subarachnoid space of the optic nerve in patients with papilloedema. Br J Ophthalmol . 2011; 95: 822–827. [CrossRef] [PubMed]
Jaggi GP Harlev M Ziegler U Dotan S Miller NR Killer HE. Cerebrospinal fluid segregation optic neuropathy: an experimental model and a hypothesis. Br J Ophthalmol . 2010; 94: 1088–1093. [CrossRef] [PubMed]
Marek B Harris A Kanakamedala P Cerebrospinal fluid pressure and glaucoma: regulation of trans-lamina cribrosa pressure [ published online ahead of print December 4, 2013]. Br J Ophthalmol. doi: 10.1136/bjophthalmol-2013-303884 .
Hayreh SS. Pathogenesis of oedema of the optic disc (papilloedema): a preliminary report. Br J Ophthalmol . 1964; 48: 522–543. [CrossRef] [PubMed]
Footnotes
 DY and JF are joint first authors.  DY and JF contributed equally to the work presented here and should therefore be regarded as equivalent authors.
Figure 1
 
Photographs showing the preparation and insertion of a cerebrospinal fluid pressure–measuring sensor.
Figure 1
 
Photographs showing the preparation and insertion of a cerebrospinal fluid pressure–measuring sensor.
Figure 2
 
Diagram showing changes in the retinal nerve fiber layer thickness as measured by optical coherence tomography in monkeys with experimental reduction of cerebrospinal fluid pressure. (A) Monkey 1, (B) monkey 3.
Figure 2
 
Diagram showing changes in the retinal nerve fiber layer thickness as measured by optical coherence tomography in monkeys with experimental reduction of cerebrospinal fluid pressure. (A) Monkey 1, (B) monkey 3.
Figure 3
 
Fundus photograph at baseline (left column) and at 12 months after lowering of cerebrospinal fluid pressure (right column) of monkey 1. Note decreased visibility of the retinal nerve fiber layer. OD, right eye; OS, left eye.
Figure 3
 
Fundus photograph at baseline (left column) and at 12 months after lowering of cerebrospinal fluid pressure (right column) of monkey 1. Note decreased visibility of the retinal nerve fiber layer. OD, right eye; OS, left eye.
Figure 4
 
Optic disc photograph of monkey 2 of the study group at 4 months after lumbar–peritoneal shunting. Note splinter-like optic disc hemorrhage (blue arrow).
Figure 4
 
Optic disc photograph of monkey 2 of the study group at 4 months after lumbar–peritoneal shunting. Note splinter-like optic disc hemorrhage (blue arrow).
Figure 5
 
Optical coherence tomograms of the optic nerve head in monkey 1 (OD) and monkey 3 (OS) after lumbar–peritoneal shunting and reduction of cerebrospinal fluid pressure, taken at baseline (upper row) and at 12 months of follow-up (lower row). Note reduction in neuroretinal rim tissue.
Figure 5
 
Optical coherence tomograms of the optic nerve head in monkey 1 (OD) and monkey 3 (OS) after lumbar–peritoneal shunting and reduction of cerebrospinal fluid pressure, taken at baseline (upper row) and at 12 months of follow-up (lower row). Note reduction in neuroretinal rim tissue.
Figure 6
 
Illustration of the profile of retinal nerve fiber layer thickness in eight different regions and their comparison before and after lowering of CSF pressure. (A) Monkey 1, (B) monkey 2, (C) monkey 3. TL, temple lower; TU, temple upper; ST, superior temple; SN, superior nasal; NU, nasal upper; NL, nasal lower; IN, inferior nasal; IT, inferior temple. Red arrow: P > 0.05; blue arrow: P < 0.05.
Figure 6
 
Illustration of the profile of retinal nerve fiber layer thickness in eight different regions and their comparison before and after lowering of CSF pressure. (A) Monkey 1, (B) monkey 2, (C) monkey 3. TL, temple lower; TU, temple upper; ST, superior temple; SN, superior nasal; NU, nasal upper; NL, nasal lower; IN, inferior nasal; IT, inferior temple. Red arrow: P > 0.05; blue arrow: P < 0.05.
Table 1
 
Demographic Data and Ocular Parameters (Mean ± Standard Deviation) of the Study Group and Control Group
Table 1
 
Demographic Data and Ocular Parameters (Mean ± Standard Deviation) of the Study Group and Control Group
Study Group Control Group P Value
Number of monkeys 4, 8 eyes 5, 10 eyes
Sex Male Male
Age, y 6.3 ± 1.3 6.2 ± 1.1 0.77
Weight, kg 6.5 ± 0.7 6.2 ± 0.9 0.58
Intraocular pressure, mm Hg 17.9 ± 0.7 17.2 ± 1.3 0.051
Retinal nerve fiber layer thickness, μm 103 ± 5.9 107 ± 7.8 0.21
Optic disc area, mm2 1.55 ± 0.41 1.54 ± 0.70 0.37
Neuroretinal rim area, mm2 0.80 ± 0.39 0.97 ± 0.46 0.87
Neuroretinal rim volume, mm3 0.05 ± 0.04 0.09 ± 0.06 0.19
Cup-to disc ratio 0.51 ± 0.14 0.31 ± 0.25 0.14
Table 2
 
Changes in Intracranial Pressure Before and After Shunt Surgery
Table 2
 
Changes in Intracranial Pressure Before and After Shunt Surgery
Before Surgery, Baseline After Surgery, 1 mo P Value
Study group 7.4 ± 0.55 1.6 ± 0.55 <0.001
Control group 7.6 ± 0.89 6.8 ± 1.10 0.1
P value 0.68 <0.001
Table 3
 
Retinal Nerve Fiber Layer Thickness and Topometric Optic Disc Data of Monkeys With Experimental Reduction of Cerebrospinal Fluid Pressure at Baseline and at the End of Follow-up
Table 3
 
Retinal Nerve Fiber Layer Thickness and Topometric Optic Disc Data of Monkeys With Experimental Reduction of Cerebrospinal Fluid Pressure at Baseline and at the End of Follow-up
Monkey Number Group Parameters OD, OS Baseline Last P Value Paired Difference Percentage
1 Study group RNFLT, μm OD 93.0 ± 0.6 70.5 ± 1.3 <0.0001 22.9 ± 1.1 24.50
OS 95.4 ± 1.8 66.5 ± 1.1 <0.0001 28.9 ± 1.9 30.30
Disc area, mm2 OD 1.43 ± 0.01 1.43 ± 0.00 1 0.00 ± 0.01 0
OS 1.28 ± 0.01 1.29 ± 0.01 0.18 −0.01 ± 0.01 0.70
Rim area, mm2 OD 0.88 ± 0.04 0.65 ± 0.08 0.005 0.23 ± 0.09 26.10
OS 0.60 ± 0.02 0.38 ± 0.09 0.005 0.22 ± 0.09 36.70
Rim volume, mm3 OD 0.04 ± 0.01 0.02 ± 0.01 0.006 0.02 ± 0.01 50
OS 0.03 ± 0.00 0.01 ± 0.00 <0.0001 0.02 ± 0.003 66.70
Cup-to-disc ratio OD 0.39 ± 0.03 0.55 ± 0.06 0.006 −0.16 ± 0.07 41.00
OS 0.53 ± 0.01 0.70 ± 0.07 0.003 −0.18 ± 0.06 34.00
2 Study group RNFLT, μm OD 109.5 ± 1.4 106.1 ± 3.8 0.051 3.5 ± 2.8 3.20
OS 108.5 ± 1.0 105.3 ± 1.3 0.04 3.3 ± 1.2 3.00
Disc area, mm2 OD 2.24 ± 0.01 2.23 ± 0.00 0.14 0.01 ± 0.01 0.40
OS 2.09 ± 0.01 2.10 ± 0.01 0.34 −0.008 ± 0.02 0.40
Rim area, mm2 OD 1.32 ± 0.04 1.33 ± 0.02 0.54 −0.01 ± 0.02 0.80
OS 1.37 ± 0.1 1.50 ± 0.04 0.09 −0.13 ± 0.13 9.50
Rim volume, mm3 OD 0.10 ± 0.01 0.11 ± 0.01 0.1 −0.01 ± 0.01 10
OS 0.12 ± 0.03 0.16 ± 0.02 0.08 −0.03 ± 0.03 25
Cup-to-disc ratio OD 0.41 ± 0.01 0.40 ± 0.01 0.21 0.01 ± 0.01 2.40
OS 0.34 ± 0.05 0.29 ± 0.02 0.13 0.06 ± 0.07 17.60
3 Study group RNFLT, μm OD 102.8 ± 0.8 83.6 ± 0.8 <0.0001 19.3 ± 1.4 18.80
OS 101.6 ± 1.3 89.4 ± 0.7 <0.0001 12.2 ± 0.9 12.00
Disc area, mm2 OD 1.31 ± 0.00 1.31 ± 0.01 0.62 −0.00 ± 0.01 0
OS 1.11 ± 0.01 1.12 ± 0.00 0.7 −0.01 ± 0.01 0.90
Rim area, mm2 OD 0.43 ± 0.02 0.23 ± 0.03 <0.0001 0.20 ± 0.03 46.50
OS 0.30 ± 0.03 0.18 ± 0.05 0.02 0.12 ± 0.07 40
Rim volume, mm3 OD 0.02 ± 0.00 0.01 ± 0.00 <0.0001 0.02 ± 0.00 100
OS 0.01 ± 0.00 0.004 ± 0.001 0.007 0.01 ± 0.003 100
Cup-to-disc ratio OD 0.68 ± 0.01 0.82 ± 0.01 <0.0001 −0.15 ± 0.02 22
OS 0.72 ± 0.03 0.84 ± 0.04 0.014 −0.11 ± 0.06 15.30
4 Study group RNFLT, μm OD 105.6 ± 2.8 102.0 ± 1.6 0.1 3.6 ± 3.7 3.40
OS 105.5 ± 1.7 103.7 ± 1.4 0.07 1.8 ± 1.6 1.70
Disc area, mm2 OD 1.28 ± 0.01 1.28 ± 0.01 0.18 −0.00 ± 0.01 0
OS 1.69 ± 0.01 1.69 ± 0.01 0.75 0.002 ± 0.01 0.10
Rim area, mm2 OD 0.66 ± 0.04 0.65 ± 0.02 0.57 0.01 ± 0.02 1.50
OS 0.84 ± 0.02 0.82 ± 0.08 0.53 0.03 ± 0.09 3.60
Rim volume, mm3 OD 0.07 ± 0.00 0.06 ± 0.01 0.28 0.01 ± 0.01 14.30
OS 0.04 ± 0.002 0.04 ± 0.01 0.33 0.003 ± 0.006 7.50
Cup-to-disc ratio OD 0.48 ± 0.03 0.49 ± 0.02 0.43 −0.01 ± 0.02 2.10
OS 0.50 ± 0.02 0.50 ± 0.05 0.61 −0.012 ± 0.05 2.40
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