Atypical Presentations of Intracranial Hypotension: Comparison with Classic Spontaneous Intracranial Hypotension

Discussion

Patients with atypical SIH were older than those with classic SIH and had a trend toward higher representation of men, while women are more commonly affected than men in classic SIH.² Patients with atypical SIH had a longer clinical history and lower odds of becoming symptom-free compared with those with classic SIH. The degree of brain sagging at the tentorial incisura was more severe in patients with atypical-versus-classic SIH without significant differences in other radiographic signs of intracranial hypotension, such as dural enhancement, fluid collections, or cerebellar tonsillar herniation. A Sagging Index higher than 10 should prompt consideration of intracranial hypotension in subjects with atypical clinical presentations.

Case 3 (On-line Table) exemplifies the challenges associated with atypical SIH. A 57-year-old man presented with behavioral changes, anterograde amnesia, headache, hypersomnia, dysphagia, ataxia, dysarthria, and gait instability and was diagnosed with behavioral variant frontotemporal dementia. He had a history of mitral valve prolapse, suggesting connective tissue laxity. Brain MR imaging showed severe brain sagging but no cortical atrophy. On CT myelography, 2 small CSF leaks at T10–T11 and T11–T12 were seen and treated with fluoroscopically guided autologous blood patches (3 times), followed by transient clinical improvement. Neuropsychological evaluation showed mild deficits in processing speed, attention, and concentration, with improvement after blood patches. His symptoms relapsed. Therefore, he was referred to another institution, where on dynamic CT myelogram, multiple CSF leaks at the cervicothoracic neuroforamina were detected and patched under CT guidance first with blood and later with glue; and he also received artificial CSF infusions. The patient's gait and speech improved after the spinal procedures, but symptoms eventually relapsed.

Three years after presentation, he underwent endoscopic third ventriculostomy. Although intracranial hypotension is not an indication for endoscopic third ventriculostomy, the rationale was the presence of a gradient between low intracranial pressure (as measured in an outside ventriculostomy) and normal spinal pressure measured at spinal procedures. Because there was no evidence of a blockade at the foramen magnum, the neurosurgeons hypothesized that the endoscopic third ventriculostomy would equalize pressures between the ventricular system and the subarachnoid spaces of the basal cisterns and spine. His gait and speech improved for 2 weeks, but full relapse followed. Brain MR imaging showed stable sagging and undetectable CSF flow in the aqueduct (Fig 3A–C). Technetium Tc99m SPECT spinal CSF leak study findings were negative. Four months after ventriculostomy, he underwent T9 through T12 laminoplasties, levels where small CSF leaks had been seen on myelography, but these were not found at the operation. Exposure of the arachnoid revealed adhesions, which were partially released, and arachnoid biopsy reported fibrosis without inflammation or granulomas. Fibrin sealant (TISSEEL; Baxter Healthcare, Deerfield, Illinois) was applied to seal the dura. Six months after the operation, he had continued clinical improvement with normalization of gait and speech, recovery of the gag reflex, and behavioral and cognitive recovery to baseline. Brain MR imaging showed resolution of brain sagging and recovery of aqueductal CSF flow for the first time at the 4-year follow-up (Fig 3D–F).

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Fig 3.

Preoperative (A–C) and postoperative (D–F) sagittal T1 (A and D), axial T2 (B and E), and sagittal CISS (C and F) brain MRI in case 3. Note postoperative resolution of brain sagging and recovery of aqueductal CSF lumen.

The etiology of SIH has been linked to weakness of the spinal dura, resulting in spinal CSF leak, in some cases associated with hereditary connective tissue disorders.¹³ CSF venous fistulas have also recently been implicated in a few cases.¹⁴ Management includes medical treatment (rest, hydration, caffeine) with escalation to spinal epidural patching when symptoms persist. The success rate of the epidural blood patch for SIH, however, is about 30%, far lower than that for post–lumbar puncture headache,¹⁵ and patients with SIH may require repeat spinal procedures. Surgical repair of the spinal dural defect by using different techniques¹⁶ is reserved for cases that fail to respond to conservative measures. As in case 3, a CSF leak was not seen on surgery in 6 of 10 patients with SIH in whom epidural patching with muscle and Gelfoam (Pfizer, New York, New York) was, nevertheless, 100% successful in treating headache.¹⁶ Neuroradiologists make key contributions in the diagnosis and management of classic and particularly atypical SIH. The latter has a longer clinical history probably because the atypical clinical features distract clinicians from the consideration of intracranial hypotension. Furthermore, clinical response is less favorable in patients with atypical compared with those with classic SIH. This could result from structural weakness of the dura or insufficient physiologic adaptation to chronic CSF hypovolemia.¹

Atypical clinical presentations of SIH can be grouped into 4 domains: cognitive-behavioral, lower cranial nerve, imbalance-gait, and arousal deficits. Typically patients present with a combination of these symptom complexes. Neuropsychologically, patients with atypical SIH present with a not-otherwise-specified profile suggestive of a subcortical type of cognitive impairment¹⁷ with slowed information processing, preserved language, and anterograde amnesia. FBSS is a rare, recently recognized syndrome in which behavioral variant frontotemporal dementia-like symptoms present in the context of intracranial hypotension, though an unidentified etiology can not be ruled out,¹⁰ and therefore should be included among the causes of potentially reversible cognitive impairment. Age of onset and male predominance in FBSS coincide with those in behavioral variant frontotemporal dementia. On imaging, FBSS lacks the typical cortical atrophy seen in frontotemporal dementia, presenting with brain sagging instead.¹⁰ In the few reported cases of neuropathologic examination,¹⁰ including our case 2, no specific neurodegenerative disease was found. We propose that FBSS, with its predominant neurocognitive presentation, should be included in the more inclusive category of atypical SIH.

Apart from cognitive and behavioral deficits, patients with atypical SIH frequently present with gait disturbance, imbalance, dysarthria, dysphagia, daytime somnolence, and movement disorders, which are not typical features of behavioral variant frontotemporal dementia. These clinical manifestations suggest midbrain and/or brain stem involvement, corresponding to the level of brain sagging and anatomic distortion. The frequent association of daytime somnolence in our sample of patients with atypical SIH and in the published series of FBSS¹⁰ points to involvement of the brain stem reticular activation system. In agreement with this notion, reversibility of obtundation from SIH has been highly time-correlated to recovery of a normal intracranial pressure waveform with reactive pupils after intrathecal saline infusion, supporting a mesencephalic level.³ Ataxia and dysarthria likely result from stretching of the superior cerebellar peduncles. Some of the symptoms associated with atypical SIH respond promptly to intrathecal saline infusion, such as the transient gait improvements noted in our case 3. Half of our patients with atypical SIH presented with involuntary movement disorders including a case of upper limb chorea with negative neuropathologic findings for Huntington disease or other neurodegenerative diseases. Parkinsonism and ataxia with recovery after blood patch were reported in a case of SIH.⁹ Finally, oculomotor abnormalities were also reported in FBSS,¹⁰ though these were not observed in our sample.

The cardinal radiologic feature of SIH is brain sagging at the tentorial incisura, resulting in the following morphologic distortions: approximation of both thalami, caudal displacement of the mammillary bodies with shortened pontomammillary distance, and anteroposterior elongation from lateral compression of the midbrain at the tentorial incisura. The Monro-Kellie doctrine¹⁸ explains the imaging findings of pachymeningeal enhancement (from venous engorgement) and subdural fluid collections as compensatory mechanisms to preserve the homeostasis of intracranial volume in the setting of a CSF volume deficit.¹⁹ Atypical SIH is a more chronic and severe brain sagging syndrome compared with classic SIH, as evidenced by the significantly higher Sagging Index (Fig 2) in the absence of other neuroradiologic differences. Compensatory mechanisms to CSF hypovolemia might become insufficient in such chronic conditions to prevent the observed parenchymal stretching across the tentorial incisura. On the basis of imaging and clinical features, we propose that the anatomic site of involvement in atypical SIH corresponds to the structures adjacent to the tentorial incisura: the medial thalamus, hypothalamus, midbrain tegmentum, and upper brain stem.

The main limitation of this study is the low number of patients with atypical SIH, inherent in the rarity of this syndrome and/or its underdiagnosis. Another limitation is the incomplete clinical and imaging follow-up related to the retrospective nature of this study. One patient with atypical SIH was treated with Chiari decompression based on neurosurgical indications, with no clinical improvement. Finally, no systematic information was available on connective tissue anomalies, which could underlie the chronicity and frequent relapses of patients with atypical SIH.