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Dr. Swati Phuljhele Aalok, P10477, Dr.Rohit Saxena, Dr.Pradeep Sharma

Abstract:

Aim: To quantify relationship between Optic nerve head tomography and functional parameters.

 Methods: Patients of idiopathic intracranial hypertension (IIH) were evaluated using Snellen’s chart, Spectral Domain Optical Coherence Tomography,standard automated perimetry,multifocal electroretinogram (mfERG). Tomographic parameters included RNFL(Retinal Nerve Fibre Layer),GCL (Ganglion Cell layer) and disc height. Visual acuity,mean deviation(MD),amplitude and latency of P wave within 2 degrees radius of mfERG field comprised functional parameters.

Results: Visual acuity did not show correlation with structural parameters. A strong negative correlation between optic disc height and MD(r=-0.51) was observed. Similar relation between RNFL and MD(r=-0.43) seen. A negative correlation between amplitude of P wave in mfERG and disc height(r=-0.47)and RNFL(r=-0.42)observed.

Conclusions:Disc height and mfERG are better indicators to assess the functional damage to optic nerve in IIH at presentation.

Introduction

Idiopathic intracranial hypertension (IIH), a diagnosis of exclusion  is characterised by raised intracranial hypertension  with a normal CSF composition and a normal MRI and MR venography  with normal neurological examination with presence of papilledema.  It usually affects obese women of child bearing age. Papilledema  with subsequent  visual  field loss is the most dreaded clinical consequence, and it mainly determines the therapy and outcome of the syndrome [1-4].Visual impairment is usually reversible if treated timely, however, 40 % cases  may show permanent visual loss, and 10 % may finally suffer from bilateral blindness[ 5][6]. There need for evidence based diagnostic tools and treatment strategies to monitor the progression of the disease.

Optical coherence tomography (OCT) is a relatively new non-invasive, noncontact trans-pupillary imaging technology, which provides high resolution, cross sectional images of ocular and biological structures to visualize and measure anatomic layers of the retina. It is the best modality for measuring retinal thickness and volume, and can precisely quantify changes in the retina through comparative imaging , and monitor the progression of a disease.

OCT in IIH may aid in diagnosis and monitoring the effect of the treatment in cases of IIH. The present study was undertaken to examine the relationship between the structural changes of optic nerve head on OCT and the visual functional parameters in cases of IIH.

Methodology

This was a prospective cross sectional study conducted at a tertiary eye care centre in India. Ethical clearance was obtained from the ethical committee of the Institute. The patients were enrolled, after taking written consent, from the Neuro-ophthalmic services and department of Neurology of our centre. The diagnosis of IIH was made on the basis of modified Dandy’s criteria, as follows; increased CSF opening pressure (ICP >25 cm H2O  for subjects with BMI > 30 kg/m2 and ICP > 20 cm H2O for subjects with BMI <=30 kg/m2), normal CSF composition, normal MRI and MR venography  and normal neurological examination except for  papilledema. The inclusion criteria were;    IIH diagnosed on basis of modified Dandy criteria, cases of early and established papilledema, age more than 18 years and patient willing to give consent. The exclusion criteria were; patients not giving consent voluntarily, patients with evidence of hydrocephalus, space occupying lesion, structural/vascular lesion, venous sinus thrombosis  on MRI  and MR  venography, patients with pre existing optic neuropathies including glaucoma and glaucoma suspects, patients with  dense  media opacification (e.g. Cataract ) that preclude precise ocular and OCT examinations, patient is unable to perform the tests, patient with chronic and atrophic papilledema.

Spectral domain OCT was used to assess the following parameters

  1. Peripapillary retinal nerve fibre thickness (RNFL – using   optic disc  cube (200 x 200 volume scans )
  2. Ganglinon- inner plexiform layer complex (GCL-IPL) – using Macular  cube  ( 512 x 128 volume scans ) centred on the fovea
  3. Optic disc height – using 5 line raster scan with 5 horizontally oriented line of length 9 mm, separated by 0.5 mm spread across the entire surface of optic disc.? Size of circle. Optic disc height was measured by measuring the ONH elevation by manually placing a vertical line from a line connecting the RPE layer temporal and nasal neural canal borders to the top of ONH. The average of the values from each of the 5 lines was considered to be the mean optic disc height.

The visual function was evaluated on following parameters:

  1. Best corrected visual acuity (BCVA) using Snellen’s chart
  2. Mean deviation (MD) in visual field  using Humphrey visual field 30-2 sita standard
  3. Visual evoked potential (VER) (monopack one, 2015, Metrovision, France)
  4. Multifocal electroretinogram (mfERG) (monopack one, 2015, Metrovision, France)

Results:

Twenty five eyes of 13 patients were included in the study. The mean age of patients was 29.4 years ± 6.4 years. All patients but two were females. The mean duration of presentation from onset of symptoms was 49.2 days ± 20.5 days.  The median BCVA in logMAR was 0.00( range, 0.0- 1.0)  The mean±SD of various structural and functional parameters is given in table 1.

Parameter Mean ± SD
Average RNFL 275.59±102.93 microns
Average GCL 58.42±27.80 microns
Disc Height 1083.48 ±246.1033 microns
Amplitude on VER 8.74±4.01 micro volts
Latency on VER 122.10± 9.76 milliseconds
Amplitude on mfERG (central 2 degrees) 1012.60 ± 563.52
Latency on mfERG (central 2 degrees) 47.54 ±4.36
MD -15.46 ±9.80

 

A correlation between the structural and functional parameters was sought. At presentation visual acuity did not show any correlation with any on the parameter on OCT. A strong negative correlation between optic disc height and MD(r=-0.51) was observed.  And a moderate negative correlation (r = -0.43) between RNFL values and mean deviation were observed. A negative correlation was observed between amplitude of P wave in mfERG  and disc height (r = -0.47) and RNFL(r = -0.42).

Discussion

Dalia M et al7 had studied the relationship between OCT changes, grade of papilledema and severity of visual dysfunction measured by automated perimetry and CSF opening pressure .They concluded that grade of papilledema significantly correlated to CSF opening pressure (r = 0.494, P < 0.001) and to the mean initial average RNFL thickness (r = 0.68, P = 0.03 ).The initial GCC showed significant linear correlation with final MD (r = 0.710, P = 0.000) and final PSD (r = −0.450, P = 0.000) .

Moss et al8 evaluate the photopic negative response (PhNR) as an index of retinal ganglion cell (RGC) function in idiopathic intracranial hypertension (IIH). He concluded that photopic negative response amplitude, which can be decreased in IIH subjects, correlates well with a clinical measure of visual function ( standard automated perimetry –mean deviation) and can be used a parameter to evaluate retinal ganglion cell function in IIH patients.

There present study was done with aim of understanding the relation between the structural changes in optic nerve and the visual functions in cases of early and established IIH. We found that there was a negative correlation between disc height and mean deviation suggesting that as the oedema increased the visual field would worsen. A similar relationship was found between RNFL and visual field.  There was negative correlation between P wave amplitude on mfERG with disc height and with RNFL, which again indicates that with increase in oedema the inner retinal function is also compromised. The fact that was no correlation of BCVA with any of the structural parameters suggests that visual acuity may not be a reliable measure to assess the amount of injury to the visual function at the presentation in cases of early IIH. The GCL-IPL complex may also not reveal the actual functional damage to the optic nerve in early cases. A good correlation between disc height and RNFL with mfERG and mean deviation suggests that these patients can be followed up on OCT and visual field to see the response to the treatment.

Major limitations of the study were the small sample size and difficulty in measurements of optic disc height in patients with severe papilledema where identifying different retinal layer is challenging. Moreover, further follow up of the patients is required to comment on the changes in the structural and functional parameters of optic nerve in IIH with time and treatment.Knowledge regarding changes in these parameters and correlation between these parameters will help us to prognosticate each case separately and plan management accordingly.

References

  1. Ball AK, Clarke CE. Idiopathic intracranial hypertension. The Lancet Neurology 2006; 5: 433–442.
  2. Friedman DI, Rausch EA (2002) Headache diagnoses in patients with treated idiopathic Intracranial hypertension.Neurology 58: 1551–1553.
  3. Wong GK, Poon WS Pseudotumor Cerebri: Time to Reflect on Treatment. World Neurosurgery 75: 592–593.
  4. Shah VA, Kardon RH, Lee AG, Corbett JJ, Wall M () Long-term follow-up of Idiopathic intracranial hypertension , Neurology. 2008; 70: 634–640
  5. Wall M, George D. Idiopathic intracranial hypertension: a prospective study of 50   1991;114(pt 1A):155-180.
  6. Corbett JJ, Savino PJ, Thompson HS, et al. Visual loss in pseudotumor cerebri: follow-up of 57 patients from five to 41 years and a profile of 14 patients with permanent severe visual loss. Arch Neurol. 1982;39(8):461-474.
  7. Dalia M et al , Diagnostic value of optical coherence tomography in patients with idiopathic intracranial hypertension, The Egyptian Journal of Neurology, Psychiatry and Neurosurgery 2015, 52(4):249–253.
  8. Moss HE, Park J C, McAnany The Photopic Negative Response in Idiopathic Intracranial Hypertension. Invest Ophthalmol Vis Sci. 2015 Jun; 56(6): 3709–3714.

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