More than two centuries ago, Alexander Monro applied some of the principles of physics to the intracranial contents and for the first time hypothesized that the blood circulating in the cranium was of constant volume at all times. This hypothesis was supported by experiments by Kellie. In its original form, the hypothesis had shortcomings that prompted modification by others. What finally came to be known as the Monro-Kellie doctrine, or hypothesis, is that the sum of volumes of brain, CSF, and intracranial blood is constant. An increase in one should cause a decrease in one or both of the remaining two. This hypothesis has substantial theoretical implications in increased intracranial pressure and in decreased CSF volume. Many of the MRI abnormalities seen in intracranial hypotension or CSF volume depletion can be explained by the Monro-Kellie hypothesis. These abnormalities include meningeal enhancement, subdural fluid collections, engorgement of cerebral venous sinuses, prominence of the spinal epidural venous plexus, and enlargement of the pituitary gland.

Spinal cerebrospinal fluid (CSF) leaks are often implicated as the cause of the syndrome of spontaneous intracranial hypotension, but they have rarely been demonstrated radiographically or surgically. The authors reviewed their experience with documented cases of spinal CSF leaks of spontaneous onset in 11 patients including their surgical observations in four of the patients. The mean age of the six women and five men included in the study was 38 years (range 22-51 years). All patients presented with a postural headache; however, most had additional symptoms, including nausea, emesis, sixth cranial-nerve paresis, or local back pain at the level of the CSF leak. All patients underwent indium-111 radionucleotide cisternography or computerized tomographic (CT) myelography. The location of the spontaneous CSF leak was in the cervical spine in two patients, the cervicothoracic junction in three patients, the thoracic spine in five patients, and the lumbar spine in one patient. The false negative rate for radionucleotide cisternography was high (30%). Subdural fluid collections, meningeal enhancement, and downward displacement of the cerebellum, resembling a Chiari I malformation, were commonly found on cranial imaging studies. In most patients, the symptoms resolved in response to supportive measures or an epidural blood patch. Leaking meningeal diverticula were found to be the cause of the CSF leak in four patients who underwent surgery. In three patients these diverticula could be ligated with good result but in one patient an extensive complex of meningeal diverticula was found to be inoperable. Two patients had an unusual body habitus and joint hypermobility, and two other patients had suffered a spontaneous retinal detachment at a young age. In conclusion, spontaneous spinal CSF leaks are uncommon, but they are increasingly recognized as a cause of spontaneous intracranial hypotension. Most spinal CSF leaks are located at the cervicothoracic junction or in the thoracic spine, and they may be associated with meningeal diverticula. The radiographic study of choice is CT myelography. The disease is usually self-limiting, but in selected cases our experience with surgical ligation of leaking meningeal diverticula has been satisfactory. An underlying connective tissue disorder may be present in some patients with a spontaneous spinal CSF leak.

Background: Steroid-responsive encephalopathy associated with autoimmune thyroiditis (SREAT), often termed Hashimoto encephalopathy, is a poorly understood and often misdiagnosed entity. Objective: To characterize the clinical, laboratory, and radiologic findings in patients with SREAT to potentially improve recognition of this treatable entity. Design: Retrospective analysis of clinical features and diagnostic test data. Setting: Two affiliated tertiary care referral institutions. Patients: Twenty consecutive (6 male) patients diagnosed as having SREAT from 1995 to 2003. Main Outcome Measures: Clinical features and ancillary test findings associated with SREAT. Results: The median age at disease onset was 56 years (range, 27-84 years). The most frequent clinical features were tremor in 16 (80%), transient aphasia in 16 (80%), myoclonus in 13 (65%), gait ataxia in 13 (65%), seizures in 12 (60%), and sleep abnormalities in 11 (55%). All patients were assigned an alternative misdiagnosis at

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