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 250mm 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 5.60mm, 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 4.80mm); 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) 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