Imaging characteristics of IIH: Are they reliable?

Idiopathic intracranial hypertension (IIH) is a disorder causing headache and visual loss due to papilledema and is seen predominantly in women of child bearing age. The diagnosis is usually based on the presence of clinical symptoms (headache, visual symptoms, tinnitus) and clinical signs (papilledema, sixth nerve palsy, visual field defect) of high intracranial pressure. Cerebrospinal fluid (CSF) pressure measures over 250 mm CSF in non-obese individuals measured in the lateral decubitus position. Imaging is used predominantly to assure that there is no mass lesion, venous thrombosis, or other cause for the intracranial pressure elevation. The study by Hoffmann et al. (1) in this journal is the first prospective, controlled study looking at signs on imaging that will reliably diagnose intracranial hypertension, and these authors supplied actual volumetric and morphometric measurements. They compared imaging characteristics of 25 patients with IIH using modern magnetic resonance imaging to sex, age, and weight matched controls and calculate the specificity and sensitivity of each of the signs. They also tried to correlate the clinical findings with the imaging findings.

Hoffman et al. (1) found that the most reliable findings of IIH were the diameter of the optic nerve sheath and the presence of an empty sella. The flattening of the globe by a distended nerve sheath was the most specific finding: 100% specific, but it was not sensitive (28%). Although the optic nerve diameters in the orbit were no different from the controls, the optic nerve sheath diameter was significantly enlarged compared with the controls. Using the cut off of 5.60 mm, this sign had a specificity of 96% and a sensitivity of 72–80%. The empty sella sign was evaluated by measuring the height of the pituitary in patients (the cut off was 4.80 mm); this sign was 76% specific and 88% sensitive. They also found that Meckel's cave was longer in IIH than controls, but the width was not helpful. The size of the ventricles and the CSF volume were not helpful signs, corroborating previous findings by other authors (2). Although they found some individuals with IIH had venous sinus narrowing, this finding did not reliably differentiate from normal controls. Venous sinus narrowing was not a universal finding and variations of the venous sinuses were found in normal controls. The authors also found that the severity of the headache, height of CSF pressure and body mass index (BMI) did NOT correlate with any imaging finding or with each other.

The importance of this study is that it shows us that imaging characteristics can be useful in making the diagnosis of IIH. Taken with other studies that have looked at imaging characteristics, we begin to understand imaging features that may assist us in making a diagnosis of IIH (Table 1)

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Table 1.

Imaging findings in the literature for IIH.

Papilla in globe		Flattening of globe		Nerve sheath tortuosity		Nerve sheath distention		Empty sella-partial or complete
Study	N	PRC	Sens (%)	Spec (%)	Sens (%)	Spec (%)	Sens (%)	Spec (%)	Sens (%)	Spec (%)	Sens (%)	Spec (%)
Brodsky 1998 (8)	20	RC	50	100	80	95	45	95	40	95	70	95
Yuh 2000 (9)	40	RC	NA	NA	NA	NA	NA	NA	NA	NA	80	92
Agid 2006 (10)	29	RC	3	100	43	100	67	82	40	91	27	95
Lim 2010 (children) (11)	23	C	17	100	61	60	65	65	30	95	26	95
Maralani 2012 (12)	43	RC	37	100	54	100	48	88	35	86	65	95
Butros 2012 (13)	44	RC	NA	NA	66	95	52	84	54	91	54	100
Hoffman 2013 (1), present study	25	Pc	NA	NA	28	100	72–80	96	NA	NA	88	70

Clinically the results of this study play a role in several situations. First, in IIH without papilledema, these imaging findings may assist us in making the diagnosis—symptoms of increased intracranial pressure like headache, but no papilledema; elevated intracranial pressure on a lumbar puncture, and these characteristic imaging findings could suggest the diagnosis. Sometimes individuals have anomalous discs or crowded discs and headache; these imaging characteristics may help us determine whether we should pursue a spinal tap to look for intracranial hypertension. These imaging findings for individuals with a new daily persistent headache or new chronic headache may lead practitioners to consider the diagnosis of IIH and check the optic discs more carefully. Another important feature of this study is that the venous sinus imaging was NOT a reliable indicator of high intracranial pressure and that many controls had venous anomalies. The authors comment that this finding is at odds with many who proposed venous stenting as a treatment in IIH (3).

The authors did not assess other imaging characteristics reported in other studies including: abnormal olfactory groove (4), enlarged foramen ovale, tortuosity of the nerve (5), Chiari-like formation (6), and extension of the papilla into the globe. There are also some unanswered questions that this study could have answered but did not. They asked each participant before the lumbar puncture to grade their headache on a visual analog scale, but they did not ask about the headache after the spinal tap. Did the headache resolve after normalization of the pressure for even a brief time? By IHS criteria (which the authors used in making the diagnosis of IIH, see Table 2) (7), criterion D was not met. Comparing the pain analog scale would have been an interesting finding; although many individuals with IIH report headache reduction after a lumbar puncture, it is not universal and has not been reported in a prospective study (1).

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Table 2.

International Classification of Headache Disorders (Second edition, 2004).

7.1.1 Idiopathic intracranial hypertension (7).

Diagnostic criteria:

A. Progressive headache with at least one of the following characteristic and fulfilling criteria C and D

1. Daily occurrence

2. Diffuse and/or constant (non-pulsating) pain

3. Aggravated by coughing or straining

B. Intracranial hypertension fulfilling the following criteria:

1. Alert patient with neurological examination that either is normal or demonstrates any of the following abnormalities:

a) Papilledema

b) Enlarged blind spot

c) Visual field defect (progressive if untreated)

d) Sixth nerve palsy

2. Increase CSF pressure (>200 mm H20 in the non-obese, >250 mm H20 in the obese) measured by lumbar puncture in the recumbent position or by epidural or intraventricular pressure monitoring

3. Normal CSF chemistry (low CSF protein is acceptable and cellularity

4. Intracranial diseases (including venous sinus thrombosis) ruled out by appropriate investigations

5. No metabolic, toxic or hormonal cause of intracranial hypertension

C. Headache develops in close temporal relation to increased intracranial pressure

D. Headache improves after withdrawal of CSF to reduce pressure to 120–170 mm H20 and resolves within 72 hours of persistent normalization of intracranial pressure

This study shows us we still have more to learn. How many criteria does it take to make an imaging diagnosis of IIH? How many individuals with IIH have these characteristic findings? Can we use the imaging findings to predict IIH without papilledema? Do the imaging characteristics resolve, for example the optic nerve sheath distention, when the pressure is normalized? How do the venous sinus findings reported by many fit into the diagnoses of IIH?

Imaging has become an integral part of our diagnostic armamentarium. The paper by Hoffman et al. (1) is an excellent start in understanding structural changes in IIH.