Accuracy of orbital MRI measurements ofOptic sheath to Optic nerve diameter ratio in the diagnosis of idiopathic intracranial hypertension
Sattam S. Lingawi, MD, FRCPC, ABR (1)
InasA. El-Ghoniemy, MD (2)
(1)Department of Radiology, KingAbdulazizUniversity
(2)Department of Radiology, CairoUniversity
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 sheathat 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 anOS: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 pseudotumorcerebri, 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 syndromeof signs and symptoms of increased intracranial pressure withoutcausative lesions on magnetic resonance imaging (MRI) or computedtomography. The terms pseudotumor cerebri and pseudotumorsyndrome are also used, but the term benign intracranial hypertensionis now obsolete, reflecting current awareness ofthe major risksto vision from papilledema, including severe visual loss may complicateIIH (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 surgicalfenestrationof 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 casesofoptic disc edema when the swelling is due to raised intracranialpressure and does not arise from local optic nerve processessuch as inflammation, extrinsic compression or ischemia. 3
RaisedCSF pressure is transmitted through the optic canal into theintra-orbital segment of the optic nerve sheath. In addition to papilloedema, a number of retinal changes maycontribute to the visual deficit, including choroidal compressionfolds across the macula, choroidal neovascularization and serousretinal elevation around the nerve head in acute andsevere cases.Flattening ofthe globe by a distended intra-orbital optic nervesheath may lead to refractive changes; however, diplopiamay be a feature, usually as a result of sixth cranial nerveparesis. 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 follow-up 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 fat-saturation pulse sequence. The OSand 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 anOS: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 authorin a different publication 5examined 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 studies4-5 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 6-7evaluated 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 reportedthat 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 thesesigns werealso 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 DandycriteriafordiagnosisofIIH include the following: 9
- High-pressureheadache and papilloedema;
- CSF opening pressure of >25cmwater;
- Awake and alert patient;
- No localizing signs otherthan lateral rectus paresis;
- Normal CSF constituents;
- Normalbrain imaging with no evidence of venous obstruction;
- Benignclinical course apart from visual deterioration;
- No othercauseof raised intracranial pressure.
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