Accuracy of orbital MRI measurements of Optic sheath to Optic
nerve diameter ratio in the diagnosis of idiopathic intracranial
hypertension
Sattam S. Lingawi, MD, FRCPC, ABR (1)
Inas A. El-Ghoniemy, MD (2)
(1) Department of Radiology, King Abdulaziz University
(2) Department of Radiology, Cairo University
Abstract:
Aim:
To evaluate the usefulness of optic sheath: optic nerve (OS: ON) diameter ratio in the
diagnosis of Idiopathic Intracranial Hypertension.
Materials and Methods:
MRI of the brain and orbits were performed for 45 patients with the clinical diagnosis of
Idiopathic Intracranial Hypertension (IIH) and 19 normal volunteers. The brain MR
sequences included sagittal T1 spin echo, axial duel echo, axial fluid attenuated inversion
recovery (FLAIR), axial diffusion images and MR venography. The orbital MR images
included axial, coronal and oblique sagittal T1 and T2 sequences with and without fat
saturation at 3mm slice thickness. No contrast enhanced MR images were obtained.
Cases of secondary increased intracranial pressure were eliminated. Coronal orbital T2
sequence with fat saturation was used to measure the ratio between the optic nerve and
the optic sheath at the point of maximum optic sheath distension.
Results:
17 normal volunteer (88.6%) had a ratio of OS: ON diameters = 2:1. Only two normal
volunteers (11.4%) had higher OS: ON ratio measuring up to 3:1. On the other hand, all
45 patients of IIH (100%) had an OS: ON ratio greater than 2.5:1 and the lumbar
puncture performed for diagnosis revealed CSF opening pressure greater than 30mmHg.
Despite the overlap in the results between 11.4% of the normal volunteers and the IIH
patients, this should not be confusing since none of the normal volunteers is symptomatic
or fulfill the modified Dandy criteria for IIH diagnosis
Conclusion:
Measurement of optic sheath: optic nerve (OS: ON) diameter ratio is usefull in the
diagnosis of Idiopathic Intracranial Hypertension.
Introduction
Idiopathic intracranial hypertension (IIH), also known as pseudotumor cerebri, is a
disorder of elevated intracranial pressure of unknown cause.
Patients present with daily headache, pulse-synchronous tinnitus, transient visual
obscurations, papilledema with associated visual loss, and diplopia from sixth nerve
paresis. Clinical diagnosis of IIH is based a set of clinical and biochemical criteria
known as modified Dandy criteria (Table 1). 1 Potentially contributing causes of IIH
should be identified and excluded. The clinical diagnosis usually is made by subjecting
the patients to neurological examination and cross sectional imaging of the brain to
exclude any potential causes of intracranial hypertension. Biochemical analysis of the
CSF obtained via lumbar puncture should be normal in all the patients with IIH (2)
In our study we aim to evaluate the usefulness of optic sheath: optic nerve (OS: ON)
diameter ratio in the diagnosis of IIH and the follow up of the patients after treatment,
and to decrease the use of lumbar puncture as a tool of diagnosis and follow up in cases
of idiopathic intracranial hypertension (IIH).
Discussion
Idiopathic intracranial hypertension (IIH) is defined as a syndrome of signs and
symptoms of increased intracranial pressure without causative lesions on magnetic
resonance imaging (MRI) or computed tomography. The terms pseudotumor cerebri and
pseudotumor syndrome are also used, but the term benign intracranial hypertension is
now obsolete, reflecting current awareness of the major risks to vision from papilledema,
including severe visual loss may complicate IIH (1,2).
Several pathophysiological theories have been proposed as possible mechanisms for the
development of IIH. A defect in the CSF absorption mechanism at the arachnoid
granulations, increased CSF production, cerebral edema and increased intracranial
venous pressure are among the widely accepted mechanisms. 3. Similarly, associations
with different clinical conditions including obesity, obstructive sleep apnoea, Behcet's
disease, renal impairment and systemic lupus erythematosis were also documented in the
literature. 3
Treatment aims to arrest progressive visual loss. Medical therapies include alleviation of
associated systemic diseases, discontinuation of contributing medications, provision of
carbonic anhydrase inhibitors and the introduction of reduced sodium weight reduction
program. Patients who fail medical therapy may benefit from surgical fenestration of the
optic nerve sheath and / or CSF shunting procedures.3
The ophthalmic hallmark of raised intracranial pressure is papilloedema. Conventionally,
the term papilloedema is reserved for the cases of optic disc edema when the swelling is
due to raised intracranial pressure and does not arise from local optic nerve processes
such as inflammation, extrinsic compression or ischemia. 3
Raised CSF pressure is transmitted through the optic canal into the intra-orbital segment
of the optic nerve sheath. In addition to papilloedema, a number of retinal changes may
contribute to the visual deficit, including choroidal compression folds across the macula,
choroidal neovascularization and serous retinal elevation around the nerve head in acute
and severe cases. Flattening of the globe by a distended intra-orbital optic nerve sheath
may lead to refractive changes; however, diplopia may be a feature, usually as a result of
sixth cranial nerve paresis. 1
In our study we aim to evaluate the role of orbital MRI in the diagnosis of IIH
considering only one sign which is the distention of the optic nerve sheath due to
enlargement of the subaracnoid space around the optic nerve as a reflection of the
increased intracranial pressure. We introduce a ratio between the optic nerve sheath to the
optic nerve diameter that will help not only to diagnose IIH cases, but also for the followup of those cases after therapy.
This study included 45 patients and 19 normal volunteers. Modified Dandy criteria were
satisfied in all the patients. MRI of the orbits were performed in all patients and
volunteers using slandered non enhanced MRI orbital protocol including a 3mm axial,
sagittal and coronal T1 and T2-weighted images with and without application of fatsaturation pulse sequence. The OS and ON diameters were measured using the outer
diameter of the subarachnoid space and nerve at the point of maximum OS distention as
depicted from the fat saturated T2 weighted images. All measurements were performed
by the main author on the coronal T2 fat saturated sequence. (Fig. 2) Correlation with the
respective lumbar puncture opening pressure was made in the patient population.
We found that 88.6% of the normal population had a ratio of OS: ON diameters = 2:1.
Only 11.4% of the normal population had higher OS: ON ratio measuring up to 3:1. On
the other hand, all cases of IIH had an OS: ON ratio greater than 2.5:1 and the lumbar
puncture performed for diagnosis revealed CSF opening pressure greater than 30mmHg.
Despite the overlap in the results between 11.4% of the normal volunteers and the IIH
cases, this should not be confusing since none of the normal volunteers is symptomatic or
fulfill the modified Dandy criteria for IIH diagnosis.
Although the English literature has many publications that agree with our observation in
the main concept of using the OS: ON ratio as a marker for the diagnosis and follow up
of patients with IIH, however, there is no mention of specific ratio in these publications.
Watanabe et al., 4 stated that dilated OS is associated with IIH and is believed to reflect
the increased intracranial pressure (ICP). The study was conducted on 12 patients with
chronic subdural hematoma. The patients were subjected to burr hole craniotomy with
continuous drainage for the chronic subdural hematoma. Orbital thin-slice fat-saturated
MR images were obtained before and after surgery, and the OS diameters were measured
just behind the optic globe. Subdural pressure was measured using a manometer before
opening of the dura mater. A significant correlation was found between the OS diameter
and the subdural pressure. The OS diameter before surgery was significantly reduced
after surgery.
The same author in a different publication 5 examined 3 patients with CSF hypovolemia
using coronal thin-slice-fat-saturated T2-weighted MR images before and after treatment
and found that the subarachnoid space is decreased in patients with hypovolemia.
The author in both studies 4-5 had a small patient population sample and did not present
any ratio between the optic sheath and the optic nerve diameters.
Agid et al, in two different studies 6-7 evaluated the accuracy of previously reported
neuroimaging signs and MR venography in establishing or excluding the diagnosis of
idiopathic intracranial hypertension (IIH). All examinations were evaluated for the
presence or absence of the 'traditional' signs of IIH including: empty sella turcica,
deformation of the pituitary, slit-like ventricles, 'tight' subarachnoid spaces, flattening of
the posterior globe, enhancement of the optic nerve head, distension of the optic nerve
sheath and vertical tortuosity of the optic nerve. The study concluded that: (1) Optic
nerve enhancement, slit-like ventricles and tight cerebrospinal fluid spaces were not
significantly associated with IIH. (2) Optic nerve sheath distension, optic nerve
tortuosity, pituitary deformity and empty sella turcica were significantly associated with
IIH. However, most of these are not helpful in a clinical setting, (3) Posterior globe
flattening was the only sign that, if present, strongly suggests the diagnosis of IIH.
Brodsky et al, 8 evaluated 20 patients with IIH and 20 control subjects. The study
reported that the MR imaging disclosed flattening of the posterior sclera in 80% of
patients with pseudotumor cerebri, empty sella in 70%, distension of the perioptic
subarachnoid space in 45%, enhancement of the prelaminar optic nerve in 50%, vertical
tortuosity of the orbital optic nerve in 40%, and intraocular protrusion of the prelaminar
optic nerve in 30%. Most of these signs were also detected in 5% of control subjects.
Based on these MR imaging signs, the author was able to predict the presence of elevated
intracranial pressure in 90% of cases with pseudotumor cerebri and the absence of
elevated intracranial pressure in all control subjects.
Table 1: Modified Dandy criteria for diagnosis of IIH include the following:
i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
9
High-pressure headache and papilloedema;
CSF opening pressure of >25 cm water;
Awake and alert patient;
No localizing signs other than lateral rectus paresis;
Normal CSF constituents;
Normal brain imaging with no evidence of venous obstruction;
Benign clinical course apart from visual deterioration;
No other cause of raised intracranial pressure.
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