Neurology India, Vol. 59, No. 4, July-August, 2011, pp. 506-512
Spontaneous intracranial hypo and hypertensions: An imaging review
Viratsinh Vaghela, Divyata Rajendra Hingwala, Tirur Raman Kapilamoorthy, Chandrasekharan Kesavadas, Bejoy Thomas
Department of Imaging Sciences and Interventional Radiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, Kerala, India
Cerebrospinal fluid (CSF) pressure changes can manifest as either intracranial hypertension or hypotension. The idiopathic forms are largely under or misdiagnosed. Spontaneous intracranial hypotension occurs due to reduced CSF pressure usually as a result of a spontaneous dural tear. Idiopathic intracranial hypertension (IIH) is a syndrome of elevated intracranial tension without hydrocephalus or mass lesions and with normal CSF composition. Neuroimaging plays an important role in excluding secondary causes of raised intracranial tension. As the clinical presentation is varied, imaging may also help the clinician in arriving at the diagnosis of IIH with the help of a few specific signs. In this review, we attempt to compile the salient magnetic resonance imaging findings in these two conditions. Careful observation of these findings may help in early accurate diagnosis and to provide appropriate early treatment.
Keywords: Idiopathic intracranial hypertension, optic nerve sheath distension, optic nerve tortuosity, orthostatic headache, posterior globe flattening, spontaneous intracranial hypotension
Two extremes of intracranial pressure (ICP) variations are intracranial hypertension and hypotension. Though not uncommon in clinical practice, both the entities are largely under or misdiagnosed. Both the disorders have distinct clinical and imaging characteristics with many overlapping features and present a diagnostic challenge for the physician. The aim of this article is to review the definitions, clinical and imaging characteristics of both idiopathic intracranial hypertension (IIH) and spontaneous intracranial hypotension (SIH). Pathophysiological basis of the clinical and imaging features has also been reviewed.
Spontaneous Intracranial Hypotension
SIH, also known as "CSF hypovolemia syndrome" or "spontaneous spinal CSF leak",  was first described by Schaltenbrand in 1938 who coined the term "Aliquorrhea".  It generally occurs due to spontaneous dural tear.  However, lumbar puncture, spinal surgery, trauma, intercourse, sneezing or even bending down can be the triggering events. , Connective tissue disorders such as Marfan syndrome and Ehlers-Danlos syndrome have been associated with SIH. ,
CSF hypovolemia plays a major part in the development of SIH as compared to CSF hypotension as many patients may have normal CSF pressure despite having typical symptoms of SIH. , Approximately 10% of the estimated total CSF volume has to be reduced to induce orthostatic headache.  CSF hypovolemia and hypotension (<60 mm H2O) results in the descent of brain. Headache occurs due to stretching of pain-sensitive intracranial structures due to brain descent which is more in the standing position and due to venous engorgement to compensate for the lost CSF as per Monro-Kellie doctrine.  Cascade of events and compensatory mechanisms leading to varied manifestations of SIH are illustrated in [Figure - 1].
Epidemiology and Clinical Presentation
The reported prevalence of SIH is 1 per 50 000, most commonly occurs in the 4 th to 5 th decade and is more frequent in females (M:F=1:2).  The typical clinical setting is of a young to middle-aged woman with new onset daily headache which is relived by lying down. Though postural headache is a hallmark, non-orthostatic headache, vomiting, neck stiffness and atypical presentations such as dementia, parkinsonism, and coma have been described [Table - 1]. ,,,
Imaging in SIH
Computed tomography (CT) myelography is considered the most reliable imaging technique for localizing the actual site of CSF leak.  Recently, intrathecal gadolinium-enhanced magnetic resonance (MR) myelography, has been shown to be comparable to radioisotope cisternography (RC) to detect the CSF leak. ,,
Most of the imaging features of SIH are secondary to increased intracranial blood volume as a compensation for the lost CSF [Figure - 1], [Figure - 2], [Figure - 3], and [Figure - 4]. Typical magnetic resonance imaging (MR) findings include linear, non-nodular and diffusely enhancing thickened pachymeninges with sparing of leptomeninges. , Subdural fluid collections may develop with approximately 10% of patients developing subdural hematoma (SDH). , Cerebral veins appear engorged with distended and rounded dural sinuses. Pituitary appears enlarged and may mimic a tumor.  Brain sagging is a very important imaging finding as often it may be the only abnormality present.  It may be due to loss of CSF buoyancy or as a final compensatory mechanism when others are exhausted. Following features can help to identify subtle brain sagging [Figure - 3] and [Figure - 4]: (1) effacement of the perisellar cisterns and interpeduncular cistern; (2) hyperintense midbrain and diencephalons; (3) reduction of angle between straight sinus and vein of Galen (VOG) (normal angle range 49°-80°);  (4) displaced midbrain tectum; (5) abnormally low position of iter [i.e. the entrance to the aqueduct from the third ventricle; normal position is within 1.8-2 mm below the incisural line connecting the tuberculum sellae with the confluence of the VOG into the straight sinus (SS);  and (6) increased sag ratio.  Chen et al. have described smaller superior ophthalmic veins' diameter (0.90 vs. 1.85 mm) in SIH patient as compared to controls.  Atypical findings include bilateral thalamic/basal ganglia/cerebellar hyperintensity on FLAIR, T2 and ADC which is associated with poor patient outcome. ,
Findings of spinal MRI are included in the new diagnostic criteria of SIH. , Extra-arachnoid fluid, meningeal diverticulae, meningeal enhancement and engorgement of epidural venous plexus can be seen. Gadolinium-enhanced MR myelography has been promising in detecting CSF leak but has low specificity as compared to CT myelography. , Radioisotope cisternography findings include presence of radioactivity outside the subarachnoid space and lack of ascent of the dye suggest leak. 
Increased density in the basal cisterns has been termed as pseudo-subarachnoid hemorrhage (pseodu-SAH) appearance on CT.  However, CT is generally less optimum than MRI for imaging SIH. CT myelography is the most reliable technique to localize the site of CSF leak. ,
Follow-up imaging: Pituitary and dural hyperemia normalize earlier than SDH and meningeal thickening. Some authors have recommended 6 monthly follow-up as some patients may develop repeated CSF leaks. 
Patients with rapidly evolving brain sagging, basal ganglia and thalamic hyperintensities and normal MRI in presence of highly suggestive symptoms require close follow-up as they may have poor prognosis. 
With advances in imaging, our understanding of SIH is increasing with addition of many new imaging signs of SIH in the past few decades. However, many questions still remain unanswered, such as why some patients develop predominant brain sagging and is there an imaging point of transition which helps us to identify the patients who are going for a more serious outcome such as coma or dementia. A sound knowledge of various imaging appearances of SIH and their relation with patients' symptoms is essential to diagnose this highly subtle but potentially curable condition.
Idiopathic Intracranial Hypertension
IIH is a syndrome of elevated intracranial tension without hydrocephalus or mass lesions and with normal CSF composition. It was formerly referred to as pseudotumor cerebri or benign intracranial hypertension and was first described by Quinke as "meningitis serosa" in 1893.  Dandy first described the diagnostic features of this condition in 1937,  which were formulated into a set of diagnostic criteria (Modified Dandy criteria) by Smith in 1985 [Table - 2].  IIH is a condition with unknown pathophysiology and its diagnosis requires elimination of secondary causes of raised intracranial tension like mass lesions, meningitis, venous sinus thrombosis and other medical conditions like Addison's disease, renal failure and medications like Tetracycline, vitamin A and a few others. 
Pressure dynamics of the cranial cavity are given by the formula ICP = CSF formation × CSF outflow resistance + outflow pressure in the superior sagittal sinus. Thus, intracranial hypertension is caused by any condition that causes increase in the outflow resistance or increased pressure in the venous sinuses. 
The typical patient is an obese woman of childbearing age,  though this condition may also occur in children and patients who are elderly or thin. They usually present only with symptoms of raised ICP like headache, diplopia, visual loss, papilledema  without lateralizing signs, except VI cranial nerve palsy. Atypical symptoms like nausea, vomiting, seizures or altered consciousness should raise the suspicion of venous sinus thrombosis or meningitis.  About one-third of patients with venous sinus thrombosis may present only with symptoms suggestive of raised ICP. 
Imaging in IIH
Advances in neuroimaging can detect many of the secondary causes of intracranial hypertension and help the clinician in arriving at the diagnosis of IIH with the help of a few specific signs described below. These non-invasive techniques (compared to lumbar puncture) help to make an early diagnosis and help in symptom control and preservation of vision.  Cross-sectional imaging is now incorporated in the newer diagnostic criteria for IIH.  This includes absence of mass lesion, hydrocephalus or vascular lesions on MRI or CT for patients with typical symptoms and MRI with MR venography for all other patients.  For patients with suspected IIH and atypical symptoms, venous sinus thrombosis or meningitis must be excluded by MRI and MR venography. ,
The traditional imaging findings of IIH include empty sella turcica, altered shape of the pituitary gland, slit-like ventricles, "tight" subarachnoid spaces, flattening of the posterior sclera, vertical tortuosity of the orbital optic nerve, enhancement of the optic nerve head, distension of the optic nerve sheath (more than 2 mm) and intraocular protrusion of the optic nerve head (reversal of the optic nerve head)  [Figure - 5]a-d. These changes are reversible on reduction of the intracranial tension.  Agid et al. have shown that the above findings were significantly associated with IIH and posterior globe flattening, optic nerve protrusion and slit-like ventricles had maximum specificity (100%). However, none of the signs taken separately or in combination were sensitive. Posterior globe flattening was the only finding which, if present, was suggestive of the diagnosis of IIH. Lim et al. found a similar high specificity for optic nerve protrusion but lower specificity for flattening of the posterior sclera (60%). They found statistical significance only for optic nerve tortuosity.
Other findings of IIH include altered flow velocity in the venous sinuses  and narrowing of the sinuses [Figure - 5]e and f. These findings may be reversible or irreversible , [Figure - 6]. Fixed stenoses may obstruct the venous outflow and increase the ICP which can be treated with endovascular stenting. , However, if venous narrowing is a result of raised ICP, endovascular treatment may lead to re-stenosis and should not be considered a therapeutic approach. These cases are more appropriately treated with measures like lumbar puncture.
Differentiation of narrowed venous sinuses from venous sinus thrombosis is especially important in the subgroup of patients who present only with features of raised ICP.  Few clinical or laboratory findings help to distinguish between these two conditions , and sinus thrombosis is often missed on CT.  MR venography or conventional angiography may be required to make the correct diagnosis in some cases.
A patient presenting with headache can have a spectrum of intracranial tension disturbances including that of SIH or IIH. Although it has been more than six decades since the identification of both these conditions, further insights into their natural history are increasing. Advances in imaging have revolutionized our understanding of intracranial tension disturbances with improved diagnostic accuracy and patient management. Awareness of these often subtle imaging findings is important to make an early accurate diagnosis.
Copyright 2011 - Neurology India
The following images related to this document are available:
Photo images[ni11161t2.jpg] [ni11161f5.jpg] [ni11161t1.jpg] [ni11161f3.jpg] [ni11161f4.jpg] [ni11161f2.jpg] [ni11161f1.jpg] [ni11161f6.jpg]