Myelography Using Energy-Integrating Detector CT Versus Photon-Counting Detector CT for Detection of CSF-Venous Fistulas in Patients With Spontaneous Intracranial Hypotension.

BACKGROUND. CSF-venous fistulas (CVFs), which are an increasingly recognized cause of spontaneous intracranial hypotension (SIH), are often diminutive in size and exceedingly difficult to detect by conventional imaging. OBJECTIVE. This purpose of this study was to compare energy-integrating detector (EID) CT myelography and photon-counting detector (PCD) CT myelography in terms of image quality and diagnostic performance for detecting CVFs in patients with SIH. METHODS. This retrospective study included 38 patients (15 men and 23 women; mean age, 55 ± 10 [SD] years) with SIH who underwent both clinically indicated EID CT myelography (slice thickness, 0.625 mm) and PCD CT myelography (slice thickness, 0.2 mm; performed in ultrahigh-resolution mode) to assess for CSF leak. Three blinded radiologists reviewed examinations in random order, assessing image noise, discernibility of spinal nerve root sleeves, and overall image quality (each assessed using a scale of 0-100, with 100 denoting highest quality) and recording locations of the CVFs. Definite CVFs were defined as CVFs described in CT myelography reports using unequivocal language and having an attenuation value greater than 70 HU. RESULTS. For all readers, PCD CT myelography, in comparison with EID CT myelography, showed higher mean image noise (reader 1: 69.9 ± 18.5 [SD] vs 37.6 ± 15.2; reader 2: 59.5 ± 8.7 vs 49.3 ± 12.7; and reader 3: 57.6 ± 13.2 vs 42.1 ± 16.6), higher mean nerve root sleeve discernibility (reader 1: 81.6 ± 21.7 [SD] vs 30.4 ± 13.6 reader 2: 83.6 ± 10 vs 70.1 ± 18.9; and reader 3: 59.6 ± 13.5 vs 50.5 ± 14.4), and higher mean overall image quality (reader 1: 83.2 ± 20.0 [SD] vs 38.1 ± 13.5; reader 2: 80.1 ± 10.1 vs 72.4 ± 19.8; and reader 3: 57.8 ± 11.2 vs 51.9 ± 13.6) (all p < .05). Eleven patients had a definite CVF. Sensitivity and specificity of EID CT myelography and PCD CT myelography for the detection of definite CVF were 45% and 96% versus 64% and 85%, respectively, for reader 1; 36% and 100% versus 55% and 96%, respectively, for reader 2; and 57% and 100% versus 55% and 93%, respectively, for reader 3. The sensitivity was significantly higher for PCD CT myelography than for EID CT myelography for reader 1 and reader 2 (both p < .05) and was not significantly different between the two techniques for reader 3 (p = .45); for all three readers, specificity was not significantly different between the two modalities (all p > .05). CONCLUSION. In comparison with EID CT myelography, PCD CT myelography yielded significantly improved image quality with significantly higher sensitivity for CVFs (for two of three readers), without significant loss of specificity. CLINICAL IMPACT. The findings support a potential role for PCD CT myelography in facilitating earlier diagnosis and targeted treatment of SIH, avoiding high morbidity during potentially prolonged diagnostic workups.

Severe CNS involvement in a subset of long-term treated children with infantile-onset Pompe disease.

INTRODUCTION: The standard of care for patients with infantile-onset Pompe disease (IOPD) is enzyme replacement therapy (ERT), which does not cross the blood brain barrier. While neuromuscular manifestations of IOPD are well-described, central nervous system (CNS) manifestations of this disorder are far less characterized. Here we describe severe CNS-related neurological manifestations including seizures and encephalopathy in six individuals with IOPD. METHOD: We identified six children with IOPD who developed CNS manifestations such as seizures and/or encephalopathy. We studied their brain magnetic resonance imaging scans (MRIs) and graded the severity of white matter hyperintensities (WMHI) using the Fazekas scale scoring system as previously published. Longitudinal cognitive measures were available from 4/6 children. RESULTS: All six IOPD patients (4 males/2 females) had been treated with ERT for 12-15 years. Seizures and/or encephalopathy were noted at a median age at onset of 11.9 years (range 9-15 years). All were noted to have extensive WMHI in the brain MRIs and very high Fazekas scores which preceded the onset of neurological symptoms. Longitudinal IQ scores from four of these children suggested developmental plateauing. DISCUSSION: Among a subset of IOPD patients on long-term ERT, CNS manifestations including hyperreflexia, encephalopathy and seizures may become prominent, and there is likely an association between these symptoms and significant WMHI on MRI. Further study is needed to identify risk factors for CNS deterioration among children with IOPD and develop interventions to prevent neurological decline.

Sudden-onset, non-traumatic large volume pneumocephalus following presentation of acute bacterial meningitis.

A man in his 30s, with sinonasal undifferentiated carcinoma status post resection 6 years prior, presented with acute onset of fever, headache and altered mentation. The patient was diagnosed with bacteremia and meningitis due to Streptococcus pneumoniae A standard antibiotic and corticosteroid regimen was started. Brain MRI showed an encephalocele abutting the superolateral nasopharynx mucosa. After several days of clinical improvement, the patient's mental status and headache acutely relapsed. A CT head venogram showed a large volume pneumocephalus originating from the region of a surgical defect. Management included external ventricular drain placement followed by right pterional craniotomy with skull base packing. Skull base defects increase the risk of life-threatening conditions such as bacterial meningitis and pneumocephalus. It is crucial for clinicians to be aware of the possibility of cranial surgical defects developing years after surgery.

Patient experience of spontaneous intracranial hypotension (SIH): qualitative interviews for concept elicitation.

BACKGROUND & OBJECTIVES: Spontaneous intracranial hypotension (SIH) is an underdiagnosed and debilitating condition caused by a spinal cerebrospinal fluid (CSF) leak. Although SIH can lead to substantial morbidity and disability, little data exists about patients' perspectives. Without hearing directly from patients, our understanding of the full experience of having SIH is limited, as is our ability to identify and use appropriate patient-reported outcome measures (PROMs) within clinical care and research. The purpose of this study was to conduct qualitative interviews with confirmed SIH patients to fully describe their experiences and identify relevant concepts to measure. METHODS: Patients were recruited from an SIH specialty clinic at a large, U.S.-based healthcare center. Patients undergoing an initial consultation who were ≥ 18 years old, English-speaking, met the International Classification of Headache Disorders-3 criteria for SIH, and had a brain MRI with contrast that was positive for SIH were eligible to participate. During semi-structured qualitative interviews with a trained facilitator, participants were asked to describe their current SIH symptoms, how their experiences with SIH had changed over time, and the aspects of SIH that they found most bothersome. Analysts reviewed the data, created text summaries, and wrote analytic reports. RESULTS: Fifteen participants completed interviews. Common symptoms reported by patients included headache, tinnitus, ear fullness/pressure/pain, and neck or interscapular pain. Patients reported that their symptoms worsened over the course of their day and with activity. The most bothersome aspect of SIH was disruption to daily activities and limits to physical activities/exercise, which were severe. With regard to symptoms, the most bothersome and impactful included physical pain and discomfort (including headache), as well as fatigue. CONCLUSIONS: Patients reported a diverse set of symptoms that were attributed to SIH, with devastating impacts on functioning and high levels of disability. Researchers considering use of PROMs for SIH should consider inclusion of both symptom scales and aspects of functioning, and future work should focus on evaluating the validity of existing measures for this patient population using rigorous qualitative and quantitative methods in diverse samples. Additionally, these data can be used to assist clinicians in understanding the impacts of SIH on patients.

Imaging of pediatric spine and spinal cord tumors: A COG Diagnostic Imaging Committee/SPR Oncology Committee/ASPNR White Paper.

Childhood spinal tumors are rare. Tumors can involve the spinal cord, the meninges, bony spine, and the paraspinal tissue. Optimized imaging should be utilized to evaluate tumors arising from specific spinal compartments. This paper provides consensus-based recommendations for optimized imaging of tumors arising from specific spinal compartments at diagnosis, follow-up during and after therapy, and response assessment.

MRI Radiogenomics of Pediatric Medulloblastoma: A Multicenter Study.

Background Radiogenomics of pediatric medulloblastoma (MB) offers an opportunity for MB risk stratification, which may aid therapeutic decision making, family counseling, and selection of patient groups suitable for targeted genetic analysis. Purpose To develop machine learning strategies that identify the four clinically significant MB molecular subgroups. Materials and Methods In this retrospective study, consecutive pediatric patients with newly diagnosed MB at MRI at 12 international pediatric sites between July 1997 and May 2020 were identified. There were 1800 features extracted from T2- and contrast-enhanced T1-weighted preoperative MRI scans. A two-stage sequential classifier was designed-one that first identifies non-wingless (WNT) and non-sonic hedgehog (SHH) MB and then differentiates therapeutically relevant WNT from SHH. Further, a classifier that distinguishes high-risk group 3 from group 4 MB was developed. An independent, binary subgroup analysis was conducted to uncover radiomics features unique to infantile versus childhood SHH subgroups. The best-performing models from six candidate classifiers were selected, and performance was measured on holdout test sets. CIs were obtained by bootstrapping the test sets for 2000 random samples. Model accuracy score was compared with the no-information rate using the Wald test. Results The study cohort comprised 263 patients (mean age ± SD at diagnosis, 87 months ± 60; 166 boys). A two-stage classifier outperformed a single-stage multiclass classifier. The combined, sequential classifier achieved a microaveraged F1 score of 88% and a binary F1 score of 95% specifically for WNT. A group 3 versus group 4 classifier achieved an area under the receiver operating characteristic curve of 98%. Of the Image Biomarker Standardization Initiative features, texture and first-order intensity features were most contributory across the molecular subgroups. Conclusion An MRI-based machine learning decision path allowed identification of the four clinically relevant molecular pediatric medulloblastoma subgroups. © RSNA, 2022 Online supplemental material is available for this article. See also the editorial by Chaudhary and Bapuraj in this issue.

Dual energy CT for the identification of CSF-Venous Fistulas and CSF leaks in spontaneous intracranial hypotension: Report of four cases.

Spontaneous intracranial hypotension (SIH) is a debilitating condition caused by spinal CSF leaks or CSF-venous fistulas (CVFs). Localizing the causative CSF leak or CVF is critical for definitive treatment but can be difficult using conventional myelographic techniques because these lesions are often low contrast compared to background, diminutive, and in some cases may be mistaken for calcified structures. Dual energy CT (DECT) can increase the conspicuity of iodinated contrast compared to background and can provide the ability to distinguish materials based on differing anatomic properties, making it well suited to address the shortcomings of conventional myelography in SIH. The purpose of this report is to illustrate the potential benefits of using DECT as an adjunct to traditional myelographic techniques in order to increase the conspicuity of these often-subtle CVFs and CSF leaks. This retrospective case series included 4 adult patients with SIH who demonstrated findings equivocal for either CVF or CSF leak using our institution's standard initial CT myelogram and in whom subsequent evaluation with DECT ultimately helped to identify the CVF or CSF leak. DECT demonstrated utility by increasing the conspicuity of two subtle CVFs compared to background and also helped to differentiate between calcified osteophytes and extradural contrast in 2 CSF leaks, confirming their presence and identifying the causative pathology. Our observations demonstrate the benefit of DECT as a problem-solving tool in the accurate diagnosis and localization of CVFs and CSF leaks.

Pediatric GAD-65 Autoimmune Encephalitis: Assessing Clinical Characteristics and Response to Therapy With a Novel Assessment Scale.

BACKGROUND: Glutamic acid decarboxylase (GAD) encephalitis is a neuroinflammatory disease characterized by a broad range of symptoms including cognitive deficits, behavioral changes, and seizures. Children with this disorder have heterogeneous presentations, and little is known about symptom progression over time and response to immunotherapy. METHODS: This study reports 10 pediatric GAD encephalitis cases and symptoms found at presentation and follow-up. In addition, symptom severity was reported utilizing a novel scale evaluating functional outcomes across the domains affected by autoimmune encephalitis including cognition, language, seizures, psychiatric symptoms, sleep, and movement. Retrospective chart review was conducted for 10 patients aged <18 years, diagnosed with GAD encephalitis, and followed for one year or more. Chart review included clinical, imaging, and laboratory findings at time of diagnosis and at six- and 12-month follow-ups. RESULTS: At presentation, cognitive deficits were found in all patients, seizures in six of 10, and language decline in seven of 10. Psychiatric symptoms were prominent for all but one patient with three of nine patients presenting with psychosis. Fatigue, sleep disruption, and movement disorders were less prominent symptoms, occurring in approximately half of the cohort. Cognition and fatigue improved significantly over time when compared with symptom severity, whereas seizures, psychiatric symptoms, and sleep did not. Language and sleep showed improvement only in early stages. Analysis of seizure frequency and type noted variability mirroring trends noted in adult studies of GAD encephalitis. CONCLUSIONS: This study demonstrated the variability of symptom profiles of pediatric GAD encephalitis and benefits of symptom severity scales. Symptom profiles and progression vary in this population.

Differentiation of Chiari malformation type 1 and spontaneous intracranial hypotension using objective measurements of midbrain sagging.

OBJECTIVE: Chiari malformation type 1 (CM-1) and spontaneous intracranial hypotension (SIH) are causes of headache in which cerebellar tonsillar ectopia (TE) may be present. An accurate method for differentiating these conditions on imaging is needed to avoid diagnostic confusion. Here, the authors sought to determine whether objective measurements of midbrain morphology could distinguish CM-1 from SIH on brain MRI. METHODS: This is a retrospective case-control series comparing neuroimaging in consecutive adult subjects with CM-1 and SIH. Measurements obtained from brain MRI included previously reported measures of brain sagging: TE, slope of the third ventricular floor (3VF), pontomesencephalic angle (PMA), mamillopontine distance, lateral ventricular angle, internal cerebral vein-vein of Galen angle, and displacement of iter (DOI). Clivus length (CL), an indicator of posterior fossa size, was also measured. Measurements for the CM-1 group were compared to those for the entire SIH population (SIHall) as well as a subgroup of SIH patients with > 5 mm of TE (SIHTE subgroup). RESULTS: Highly significant differences were observed between SIHall and CM-1 groups in the following measures: TE (mean ± standard deviation, 3.1 ± 5.7 vs 9.3 ± 3.5 mm), 3VF (-16.8° ± 11.2° vs -2.1° ± 4.6°), PMA (44.8° ± 13.1° vs 62.7° ± 9.8°), DOI (0.2 ± 4.1 vs 3.8 ± 1.6 mm), and CL (38.3 ± 4.5 vs 44.0 ± 3.3 mm; all p < 0.0001). Eight (16%) of 50 SIH subjects had TE > 5 mm; in this subgroup (SIHTE), a cutoff value of < -15° for 3VF and < 45° for PMA perfectly discriminated SIH from CM-1 (sensitivity and specificity = 1.0). DOI showed perfect specificity (1.0) in detecting SIH among both groups. No subjects with SIH had isolated TE without other concurrent findings of midbrain sagging. CONCLUSIONS: Measures of midbrain sagging, including cutoff values for 3VF and PMA, discriminate CM-1 from SIH and may help to prevent misdiagnosis and unnecessary surgery.

Machine Assist for Pediatric Posterior Fossa Tumor Diagnosis: A Multinational Study.

BACKGROUND: Clinicians and machine classifiers reliably diagnose pilocytic astrocytoma (PA) on magnetic resonance imaging (MRI) but less accurately distinguish medulloblastoma (MB) from ependymoma (EP). One strategy is to first rule out the most identifiable diagnosis. OBJECTIVE: To hypothesize a sequential machine-learning classifier could improve diagnostic performance by mimicking a clinician's strategy of excluding PA before distinguishing MB from EP. METHODS: We extracted 1800 total Image Biomarker Standardization Initiative (IBSI)-based features from T2- and gadolinium-enhanced T1-weighted images in a multinational cohort of 274 MB, 156 PA, and 97 EP. We designed a 2-step sequential classifier - first ruling out PA, and next distinguishing MB from EP. For each step, we selected the best performing model from 6-candidate classifier using a reduced feature set, and measured performance on a holdout test set with the microaveraged F1 score. RESULTS: Optimal diagnostic performance was achieved using 2 decision steps, each with its own distinct imaging features and classifier method. A 3-way logistic regression classifier first distinguished PA from non-PA, with T2 uniformity and T1 contrast as the most relevant IBSI features (F1 score 0.8809). A 2-way neural net classifier next distinguished MB from EP, with T2 sphericity and T1 flatness as most relevant (F1 score 0.9189). The combined, sequential classifier was with F1 score 0.9179. CONCLUSION: An MRI-based sequential machine-learning classifiers offer high-performance prediction of pediatric posterior fossa tumors across a large, multinational cohort. Optimization of this model with demographic, clinical, imaging, and molecular predictors could provide significant advantages for family counseling and surgical planning.

CSF-Venous Fistulas: Anatomy and Diagnostic Imaging.

CSF-venous fistulas (CVFs), first described in 2014, are an important cause of spontaneous intracranial hypotension. CVFs can be challenging to detect on conventional anatomic imaging because, unlike other types of spinal CSF leak, they do not typically result in pooling of fluid in the epidural space, and imaging signs of CVF may be subtle. Specialized myelographic techniques have been developed to help with CVF identification, but these techniques are not yet widely disseminated. This article reviews the current understanding of CVFs, emphasizing correlations between venous anatomy and imaging findings as well as potential mechanisms for pathogenesis, and describes current imaging techniques used for CVF diagnosis and localization. These techniques are broadly classified into fluoroscopy-based methods, including digital subtraction myelography and dynamic myelography, and cross-sectional methods, including decubitus CT myelography and MR myelography with intrathecal injection of gadolinium. Knowledge of these various options, including their relative advantages and disadvantages, is critical in the care of patients with spontaneous intracranial hypotension. Investigation is ongoing, and continued advances in knowledge about CVFs as well as in optimal imaging detection are anticipated.

Image quality of EOS low-dose radiography in comparison with conventional radiography for assessment of ventriculoperitoneal shunt integrity.

OBJECTIVE: Patients with shunted hydrocephalus often accumulate high levels of radiation over their lifetimes during evaluation of hardware integrity. Current practice involves the use of a series of conventional radiographs for this purpose. Newer low-dose EOS radiography is currently used to evaluate scoliosis but has not been explored to evaluate shunt integrity on a large scale. The goal of this study was to compare the quality of imaging using EOS low-dose radiography to conventional radiography to evaluate shunt tubing. METHODS: A retrospective chart review was performed on 57 patients who previously had both conventional radiographs and low-dose EOS images of their cerebral shunt tubing from 2000 to 2018. Patient demographics (age, sex, type of shunt tubing, primary diagnosis) were collected. Conventional radiographic images and low-dose EOS images were independently analyzed by a neurosurgeon and neuroradiologist in three categories: image quality, delineation of shunt, and distinction of shunt compared to adjacent anatomy. RESULTS: All patients had shunted hydrocephalus due to spina bifida and Chiari type II malformation. Ratings of EOS and conventional radiographic images by both raters did not differ significantly in terms of image quality (rater 1, p = 0.499; rater 2, p = 0.578) or delineation of shunt (p = 0.107 and p = 0.256). Conventional radiographic images received significantly higher ratings than EOS on the ability to distinguish the shunt versus adjacent anatomy by rater 1 (p = 0.039), but not by rater 2 (p = 0.149). The overall score of the three categories combined was not significantly different between EOS and conventional radiography (rater 1, p = 0.818; rater 2, p = 0.186). In terms of cost, an EOS image was less costly than a conventional radiography shunt series ($236-$366 and $1300-$1547, respectively). The radiation dose was also lower for EOS images, with an effective dose of 0.086-0.140 mSv compared to approximately 1.6 mSv for a similar field of view with conventional radiography. CONCLUSIONS: The image quality of low-dose EOS radiography does not significantly differ from conventional radiography for the evaluation of cerebral shunts. In addition, EOS affords a much lower radiation dose and a lower cost.

Magnetic Resonance Imaging Abnormalities of the Optic Nerve Sheath and Intracranial Internal Carotid Artery in Giant Cell Arteritis.

BACKGROUND: Giant cell arteritis (GCA) is an important diagnostic consideration in elderly patients with vision changes. Superficial temporal artery biopsy (TAB) has long been considered the gold standard diagnostic approach for GCA, but MRI has gained interest as an alternative diagnostic modality. Although most of the literature has focused on imaging abnormalities of branches of the external carotid artery, there have been a few reports of GCA-related inflammatory involvement of the orbit and internal carotid arteries (ICAs) on MRI. METHODS: This was a retrospective cross-sectional study of patients undergoing TAB at a single tertiary referral center over a 5-year period. Patients who had undergone contrast-enhanced MRI of the brain and orbits within 1 month of biopsy were included. Fifty-four TAB-positive and 78 TAB-negative patients were reviewed, with the MRI studies of 7 TAB-positive and 6 TAB-negative patients deemed adequate for interpretation. MRI studies were reviewed by 2 masked neuroradiologists, and the findings were correlated with biopsy results and clinical findings. RESULTS: Intracranial ICA vessel wall enhancement was identified in 6 of 7 TAB-positive patients (sensitivity 86%), compared with 2 of 6 TAB-negative patients (specificity 67%). Optic nerve sheath enhancement was identified in 5 of 7 TAB-positive patients (sensitivity 71%) and in 2 of 6 TAB-negative patients (specificity 67%), bilateral in all such cases. The combination of both abnormal imaging findings was observed in 5 of 7 TAB-positive patients (sensitivity 71%) and in none of the 6 TAB-negative patients (specificity 100%). CONCLUSIONS: Intracranial ICA and optic nerve sheath enhancement were observed in a majority of patients with TAB-proven GCA, and the combination of these findings was highly specific for GCA. Identification of these abnormalities on MRI should raise concern for GCA and prompt a thorough review of systems, laboratory testing, and consideration of TAB in patients with ocular complaints potentially consistent with ischemia.

Novel approaches to quantify CNS involvement in children with Pompe disease.

OBJECTIVE: To characterize the extent of CNS involvement in children with Pompe disease using brain MRI and developmental assessments. METHODS: The study included 14 children (ages 6-18 years) with infantile Pompe disease (IPD) (n = 12) or late-onset Pompe disease (LOPD) (n = 2) receiving enzyme replacement therapy. White matter (WM) hyperintense foci seen in the brain MRIs were systematically quantified using the Fazekas scale (FS) grading system with a novel approach: the individual FS scores from 10 anatomical areas were summed to yield a total FS score (range absent [0] to severe [30]) for each child. The FS scores were compared to developmental assessments of cognition and language obtained during the same time period. RESULTS: Mild to severe WM hyperintense foci were seen in 10/12 children with IPD (median age 10.6 years) with total FS scores ranging from 2 to 23. Periventricular, subcortical, and deep WM were involved. WM hyperintense foci were seen throughout the path of the corticospinal tracts in the brain in children with IPD. Two children with IPD had no WM hyperintense foci. Children with IPD had relative weaknesses in processing speed, fluid reasoning, visual perception, and receptive vocabulary. The 2 children with LOPD had no WM hyperintense foci, and high scores on most developmental assessments. CONCLUSION: This study systematically characterized WM hyperintense foci in children with IPD, which could serve as a benchmark for longitudinal follow-up of WM abnormalities in patients with Pompe disease and other known neurodegenerative disorders or leukodystrophies in children.

Management of glioblastoma: State of the art and future directions.

Glioblastoma is the most common malignant primary brain tumor. Overall, the prognosis for patients with this disease is poor, with a median survival of <2 years. There is a slight predominance in males, and incidence increases with age. The standard approach to therapy in the newly diagnosed setting includes surgery followed by concurrent radiotherapy with temozolomide and further adjuvant temozolomide. Tumor-treating fields, delivering low-intensity alternating electric fields, can also be given concurrently with adjuvant temozolomide. At recurrence, there is no standard of care; however, surgery, radiotherapy, and systemic therapy with chemotherapy or bevacizumab are all potential options, depending on the patient's circumstances. Supportive and palliative care remain important considerations throughout the disease course in the multimodality approach to management. The recently revised classification of glioblastoma based on molecular profiling, notably isocitrate dehydrogenase (IDH) mutation status, is a result of enhanced understanding of the underlying pathogenesis of disease. There is a clear need for better therapeutic options, and there have been substantial efforts exploring immunotherapy and precision oncology approaches. In contrast to other solid tumors, however, biological factors, such as the blood-brain barrier and the unique tumor and immune microenvironment, represent significant challenges in the development of novel therapies. Innovative clinical trial designs with biomarker-enrichment strategies are needed to ultimately improve the outcome of patients with glioblastoma.

Spontaneous Intracranial Hypotension: Pathogenesis, Diagnosis, and Treatment.

Spontaneous intracranial hypotension (SIH) is a clinical syndrome that is increasingly recognized as an important and treatable secondary cause of headaches. Insight into the condition has evolved significantly over the past decade, resulting in a greater understanding of the underlying pathophysiology, development of new diagnostic imaging tools, and a broadening array of targeted treatment options. This article reviews the clinical presentation and pathogenesis of SIH, discusses the important role of imaging in diagnosis, and describes how imaging guides treatment.

Intraspecific variation in semicircular canal morphology-A missing element in adaptive scenarios?

OBJECTIVES: Recent evidence suggests that the amount of intraspecific variation in semicircular canal morphology may, itself, be evidence for varying levels of selection related to locomotor demands. To determine the extent of this phenomenon across taxa, we expand upon previous work by examining intraspecific variation in canal radii and canal orthogonality in a broad sample of strepsirrhine and platyrrhine primates. Patterns of interspecific variation are re-examined in light of intraspecific variation to better understand the resolution at which locomotion can be reconstructed from single individuals. MATERIALS AND METHODS: Data was collected from high-resolution CT scans of 14 size-matched, related species. Six of these taxa have existing data on rotational head speeds. RESULTS: The level of intraspecific variation was found to differ in strepsirrhine and in platyrrhine species pairs, with larger ranges of variation generally observed for the slower moving taxon than the faster moving one. Taxa that are classified as relatively agile can to some extent be separated from those who are slower-moving, but only when comparing similarly sized, closely related species with more extreme forms of locomotion. DISCUSSION: Our findings agree with previous research showing that canal intraspecific variation can fluctuate according to species-specific locomotor behavior and extends this further by identifying behaviors that may be under unusual selective pressure. It also demonstrates the complexity of interpreting inner ear morphology in the context of broadly applicable locomotor "categories" of the kind commonly used in behavioral studies. We suspect that simplified models predicting vestibular sensitivity may be unable to differentiate behaviors when only a single specimen is available.