Mother-Daughter Ventricular Endoscopy: A Technical Note and 2-Dimensional Video.

BACKGROUND AND IMPORTANCE: While navigating the ventricles with a rigid endoscope provides excellent visualization and the ability to use endoscopic instruments for complex surgery, these endoscopes are often too large to navigate tight areas. We present a surgical video showing the technique of mother-daughter endoscopy, which consists of the introduction of a flexible 1-mm fiberoptic endoscope through the channel of a large rigid endoscope to allow visualization across small spaces or channels, in this case, the cerebral aqueduct. This combination of superior visualization and handling of rigid endoscopes and flexibility and small size of fiberoptic endoscopes enhances safety and broadens possibilities in ventricular surgery. CLINICAL PRESENTATION: A 64-year-old woman with prior endoscopic aqueductoplasty for triventricular hydrocephalus and a failed endoscopic third ventriculostomy presented with focal restenosis of the aqueduct. A repeat endoscopic aqueductoplasty with stent placement were performed. Mother-daughter endoscopy was used to explore the occluded aqueduct for improved safety before fenestration and to ensure proper stent placement after fenestration. CONCLUSION: Mother-daughter endoscopy can add safety to complex or high-risk endoscopic procedures, particularly those with tight spaces that the large mother endoscope cannot visualize.

Endoscopic transaqueductal stent placement for tumor-related aqueductal compression in pediatric patients: surgical consideration, technique, and results.

PURPOSE: Endoscopic transaqueductal stenting has become a well-accepted treatment option for a selected small subset of aqueductal stenosis-related obstructive hydrocephalus. However, transaqueductal stenting poses unique challenges and risks which requires critical consideration. This report discusses the clinical experiences with transaqueductal stenting for periaqueductal tumor-related aqueductal stenosis focusing on pediatric patients. METHODS: A retrospective analysis of all patients undergoing endoscopic TAS from 01/1993 to 01/2022 in the author's departments was performed. Demographic, clinical, radiological, and intraoperative endoscopic data were evaluated. All patients with AS-related occlusive hydrocephalus that was treated with TAS were analyzed and prospectively followed. Special attention has been given to providing insights into indications, surgical technique, and limitations. RESULTS: Out of 28 endoscopic transaqueductal endoscopis stenting procedures, five procedures were performed on periaqueductal tumor-related obstructive hydrocephalus, two children and three adult patients. CSF pathway was obstructed by tumor located in the aqueduct in 2, by tumor in the thalamus/mesencephalon in 1, by a tumor within the third ventricle in 1, and by a tumor of the lamina tecti in 1. Simultaneously with transaqueductal stenting, 2 endoscopic third ventriculostomies (ETV), 3 tumor biopsies, and 1 tumor resection were performed. Postoperative complications included the following: CSF fistula (1 case), and asymptomatic fornix contusion (1 case). A working aqueductal stent was achieved in all cases based on clinical follow-up evaluation. Postoperatively, all patients showed improvement or resolution of their symptoms. The mean follow-up period was 25.2 months (range, 1-108 months). One patient died due to tumor progression during early follow-up. No stent migration was seen. CONCLUSION: Endoscopic third ventriculostomy remains the gold standard for treatment of CSF circulation obstructions with lesions in the posterior third ventricle and aqueduct. Transaqueductal stenting for periaqueductal tumor-related aqueductal compression is technically feasible. However, because of the potential high risks and subtle advantages compared with ETV transaqueductal stenting, it might be indicated in a small subset of well-selected patients if alternative treatment options are not at hand.

The Extreme Anterior Interhemispheric Transcallosal Approach for Pure Aqueduct Tumors: An Anatomical Study.

AIM: To show the normal anatomy of the cerebral aqueduct, and the feasibility of the extreme anterior interhemispheric transcallosal approach to remove tumors within the aqueduct. MATERIAL AND METHODS: This human cadaveric brain research was composed of ten formalin-fixed human brains and one injected head. The dissection was performed under an operative microscope with 6x to 40x magnification. The cerebral aqueduct anatomy was delineated along with the relationship to nearby structures in the extreme anterior interhemispheric transcallosal approach. RESULTS: We described the anatomy of the cerebral aqueduct within the brain and showed that, with the proper angle for the extreme anterior interhemispheric transcallosal approach, lesions in the cerebral aqueduct can be reached in a single session without damaging periventricular structures. CONCLUSION: The extreme anterior interhemispheric transcallosal approach provides a direct corridor to the cerebral aqueduct and, thus, is feasible for resecting pure aqueduct tumors in an already dilated intraventricular foramen.

Neuronavigated endoscopic aqueductoplasty with panventricular stent plus septostomy for isolated fourth ventricle in complex hydrocephalus and syringomyelia associated with myelomeningocele: how I do it.

BACKGROUND: Isolated fourth ventricle (IFV) is a challenging entity to manage. In recent years, endoscopic treatment for aqueductoplasty has been on the rise. However, in patients with complex hydrocephalus and distorted ventricular system, its implementation can be complex. METHODS: We present a 3-year-old patient with myelomeningocele and postnatal hydrocephalus treated by ventriculoperitoneal shunt. In follow-up, a progressive IFV and isolated lateral ventricle with symptoms of the posterior fossa developed. An endoscopic aqueductoplasty (EA) with panventricular stent plus septostomy guided with neuronavigation was decided due to the complexity of the ventricular system. CONCLUSION: In IFV associated with complex hydrocephalus with distortion of the ventricular system, navigation can be of great help for planning and as a guide for performing EA.

Trapped Fourth Ventricle: Pathophysiology, History and Treatment Strategies.

Trapped fourth ventricle is a clinic-radiological entity characterised by progressive neurological symptoms due to enlargement and dilatation of fourth ventricle secondary to obstruction to its outflow. There are several causative mechanisms for the development of trapped fourth ventricle, including previous haemorrhage, infection or inflammatory processes. However, this condition is most commonly observed in ex preterm paediatric patients shunted for a post-haemorrhagic or post-infective hydrocephalus. Until the introduction of endoscopic aqueductoplasty and stent placement, treatment of trapped fourth ventricle was associated with high rates of reoperation and complications resulting in morbidity. With the advent of new endoscopic techniques, supratentorial and infratentorial approaches for aqueductoplasty and stent insertion have revolutionised the treatment of trapped fourth ventricle. Fourth ventricular fenestration and direct shunting remain viable options in cases where aqueduct anatomy and length of obstruction is not surgically favourable for endoscopic approaches. In this book chapter, we explore the background, historical developments,$ and surgical treatment strategies in the management of this challenging condition.

Spontaneous Third Ventriculostomy in Cases of Aqueductal Stenosis: A Retrospective Case Series.

BACKGROUND: Spontaneous third ventriculostomy (STV) is a rare occurrence in cases of obstructive hydrocephalus where the walls of the third ventricle rupture, communicating the ventricular system, and the subarachnoid space leading to arrest of active hydrocephalus. We aim to review our series of STVs while reviewing previous reports. METHODS: A retrospective review of cases undergoing cine phase-contrast magnetic resonance imaging (PC-MRI) from 2015 to 2022 of any age with imaging evidence of arrested obstructive hydrocephalus was performed. Patients in which aqueductal stenosis was radiologically evident and the presence of third ventriculostomy through which cerebrospinal fluid flow was detectable were included. Patients who previously underwent endoscopic third ventriculostomy were excluded. Data on patient demographics, presentation, and imaging details of STV and aqueductal stenosis were collected. We searched the PubMed database using the following keyword combination: ((("spontaneous ventriculostomy") OR ("spontaneous third ventriculostomy")) OR ("spontaneous ventriculocisternostomy")) including English reports of STV published between 2010 and 2022. RESULTS: Fourteen cases were included (7 adults, 7 pediatrics), all with history of hydrocephalus. STV occurred in the floor of the third ventricle in 57.1% of the cases, at the lamina terminalis in 35.7%, and at both sites in 1 case. Eleven publications reporting 38 cases of STV were identified from 2009 to date. Minimum follow-up period was 10 months and maximum follow-up is 77 months. CONCLUSIONS: In cases of chronic obstructive hydrocephalus, neurosurgeons should be minded with the possibility of the presence of an STV on cine phase-contrast magnetic resonance imaging leading to arrested hydrocephalus. The delayed flow at the aqueduct of Sylvius might not be the only determinant of the necessity of cerebrospinal fluid diversion and the presence of an STV should be factored into the neurosurgeon's decision considering the patient's clinical picture.

The Roof of the 4th Ventricle Seen From Inside: Endoscopic Anatomic Illustration-A Case Series.

BACKGROUND: The anatomy of the roof of the fourth ventricle has been illustrated in many laboratory investigations, but in vivo reports of the roof anatomy and its variants are still lacking. OBJECTIVE: To describe the topographical anatomy of the roof of the fourth ventricle explored through a transaqueductal approach that overcomes cerebrospinal fluid depletion, displaying in vivo anatomic images possibly quite close to normal physiological conditions. METHODS: We critically reviewed the intraoperative video recordings of our 838 neuroendoscopic procedures, selecting 27 cases of transaqueductal navigation that provided good quality image details of the anatomy of the roof of the fourth ventricle. Twenty-six patients affected by different forms of hydrocephalus were therefore categorized into three groups: Group A: blockage of the aqueduct-aqueductoplasty, Group B: communicating hydrocephalus, and Group C: tetraventricular obstructive hydrocephalus. RESULTS: Group A has shown what the roof of a normal fourth ventricle really looks like albeit the structures seemed overcrowded because of the narrow space. Images from groups B and C paradoxically allowed a more distinct identification of the roof structures flattened by ventricular dilation, making them more comparable with the topography traced on the laboratory microsurgical studies. CONCLUSION: Endoscopic in vivo videos and images provided a novel anatomic view and an in vivo redefinition of the real topography of the roof of the fourth ventricle. The relevant role of cerebrospinal fluid was defined and outlined, as well as the effects of hydrocephalic dilation on some structures on the roof of the fourth ventricle.

Flexible neuroendoscopy for endoscopic third ventriculostomy and fourth ventricular arachnoid cyst fenestration in an infant.

Arachnoid cysts of the fourth ventricle are rarely reported. Management options include CSF diversion, cyst fenestration, or cyst excision. Fenestration can be done via open microsurgical technique or endoscopically with or without simultaneous third ventriculostomy; and both rigid and flexible endoscopy have been used successfully. However, application of this treatment modality in pediatric patients is not well described. Therefore, to their knowledge, the authors report the first successful treatment of a fourth ventricular arachnoid cyst with a single frontal burr hole entry point for third ventriculostomy and fourth ventricular arachnoid cyst fenestration performed using flexible neuroendoscopy. The patient was a 13-month-old boy presenting with progressive macrocephaly. The authors review their technique, discuss special considerations when using this approach, and include an annotated intraoperative video for demonstration to help instruct and guide management. The authors demonstrate with an example that a single frontal burr hole entry point for flexible endoscopic third ventriculostomy and navigation through a dilated cerebral aqueduct for fourth ventricular arachnoid cyst fenestration is a viable treatment for symptomatic fourth ventricular arachnoid cysts in children.

Endoscopic fenestration of the superior medullary velum for the treatment of a trapped fourth ventricle-technical note.

A trapped fourth ventricle is a clinic-radiological entity characterised by progressive neurological symptoms due to an enlargement of the fourth ventricle secondary to obstruction to its outflow. This condition is most commonly observed in ex-preterm patients shunted for a post-haemorrhagic or post-infective hydrocephalus. Until the introduction of endoscopic aqueductoplasty and stent placement, through a supratentorial or an infratentorial approach, treatment of trapped fourth ventricle entailed high rates of complications, repeated procedures and consequent morbidity. We describe the first case of successful treatment of trapped fourth ventricle by fenestration of superior medullary velum through an infratentorial approach in a 20-month-old child with a functional supratentorial ventriculoperitoneal shunt and an aqueductal anatomy not favourable for stenting. To the best of our knowledge, this is the first reported case of utilisation of this technique in a patient with a trapped fourth ventricle, and we wish to highlight this new alternative approach in cases where conventional aqueductoplasty and stenting may not be feasible.

A rare case of intralesional haemorrhage of a benign aqueductal cyst after CSF diversion treatment. A case report and review of literature.

Benign cysts within the pure aqueductal region are a rare entity. Their critical location within the ventricular system presents a risk of potentially catastrophic outcomes. We present a case of a 68-year-old female who was transferred to our unit with an acute obstructive triventricular hydrocephalus caused by a benign cyst within the cerebral aqueduct. She became unconscious and had an urgent endoscopic third ventriculostomy (ETV). Post-operatively, the patient was recovering well but then developed a sudden onset severe headache accompanied by vomiting. Imaging revealed intracystic haemorrhage with expansion of lesion but there was no obstructive hydrocephalus due to CSF diversion performed 9 d prior. She was treated conservatively and continued to improve.

Tetraventricular hydrocephalus with aqueductal flow void: an overlooked entity having consistent improvement following endoscopic third ventriculostomy.

BACKGROUND: Tetraventricular Hydrocephalus (TetHCP) is a heterogeneous group of cerebrospinal fluid (CSF) flow disorders having varying success rates with Endoscopic third ventriculostomy (ETV). This is report on the efficacy and rationale of ETV in a specific subset of primary TetHCP with aqueductal CSF flow voids. METHODS: Patients of primary acquired TetHCP presenting with increasing head size and/or headache having aqueductal CSF flow void on sagittal Magnetic Resonance Imaging (MRI) were included in this study. All of them underwent ETV. All patients were evaluated for clinical improvement & MRI at 3 months, and need for any additional procedure, in contrast to those without CSF flow void. The pathophysiology of hyperdynamic CSF circulation and its correlation to ETV was further reviewed. RESULTS: Eleven patients had tetraventricular hydrocephalus and aqueductal flow void, with age ranging from 10 months to 59 years. Two patients who could undergo quantitative flow study confirmed the hyperdynamic flow across the aqueduct. Following ETV, all showed clinical improvement. MRI at 3 months showed CSF flow void across the third ventricular stoma in addition to across the aqueduct. None of these patients required any redo procedures for a mean follow-up of 39.2 months. In contrast, there was 30% failure rate after ETV among 10 patients of tetraventricular hydrocephalus without aqueductal flow void. CONCLUSION: Tetraventricular hydrocephalus with aqueductal CSF flow void may be a unique entity with hyperdynamic CSF circulation and relative resistance at fourth ventricular outlets. ETV is highly efficacious in these patients, resulting in consistent clinico-radiological improvement.

Late-onset obstructive hydrocephalus associated with occipital encephalocele with large skull defect successfully treated by endoscopic third ventriculostomy.

BACKGROUND: Hydrocephalus is one of the most common presentations of occipital encephaloceles and usually develops within the first year of life. This case report presents a rare case of late-onset obstructive hydrocephalus associated with occipital encephalocele with an extraordinarily large occipital skull defect. CASE REPORT: At birth, a newborn girl presented with an absence of a vast amount of occipital cranium and skin and was diagnosed with occipital hydroencephalomeningocele. Under meticulous sterile management, the affected area was successfully epithelialized, and the patient was discharged without infectious complication. Despite an obstructed cerebral aqueduct, she grew without any signs of hydrocephalus until the age of 7 years. Her gait gradually worsened, and imaging tests at the age of 8 years revealed markedly enlarged lateral and third ventricles but not the fourth ventricle. Endoscopic third ventriculostomy successfully relieved her symptoms with improvement of hydrocephalus. CONCLUSION: This is the first case of late-onset obstructive hydrocephalus associated with an occipital encephalocele characterized by large-scale cranial bony defects. Although further investigation is required to elucidate the mechanism of hydrocephalus, this rare phenomenon should be noted during neurological and radiological follow-up.

Diagnosis and Surgical Management of Neonatal Hydrocephalus.

Neonatal hydrocephalus represents an important pathological condition with significant impact on medical care and neurocognitive development. This condition requires early recognition, appropriate medical and surgical management, and long-term surveillance by clinicians and pediatric neurosurgeons. Common etiologies of neonatal and infant hydrocephalus include intraventricular hemorrhage related to prematurity with subsequent post-hemorrhagic hydrocephalus, myelomeningocele, and obstructive hydrocephalus due to aqueductal stenosis. Clinical markers of elevated intracranial pressure include rapid increases in head circumference across percentiles, elevation and firmness of the anterior fontanelle, splitting or splaying of cranial sutures, upgaze palsy, lethargy, frequent emesis, or episodic bradycardia (unrelated to other comorbidities). Complementing these clinical markers, imaging modalities used for the diagnosis of neonatal hydrocephalus include head ultrasonography, brain magnetic resonance imaging, and head computed tomography in urgent or emergent situations. Following diagnosis, temporizing measures may be employed prior to definitive treatment and include ventricular access device or ventriculo-subgaleal shunt insertion. Definitive surgical management involves permanent cerebrospinal fluid (CSF) diversion via CSF shunt insertion, or endoscopic third ventriculostomy with or without choroid plexus cauterization. Surgical decision-making and approaches vary based on patient age, hydrocephalus etiology, neuroanatomy, imaging findings, and medical comorbidities. Indications, surgical techniques, and clinical outcomes of these procedures continue to evolve and elicit significant attention in the research environment. In this review we describe the epidemiology, pathophysiology, clinical markers, imaging findings, early management, definitive surgical management, and clinical outcomes of pediatric patients with neonatal hydrocephalus.

Tectal Plate Gliomas Masquerading as Idiopathic Aqueduct Stenosis.

Aqueduct stenosis is a recognized cause of obstructive hydrocephalus in children and can be treated effectively with endoscopic third ventriculostomy. Preoperative magnetic resonance imaging is often diagnostic of the cause of aqueduct stenosis. We describe 2 pediatric cases with obstructive hydrocephalus secondary to a working diagnosis of idiopathic aqueduct stenosis. Following successful endoscopic third ventriculostomy, repeat magnetic resonance brain imaging revealed tectal plate glioma as the primary cause of obstruction. We believe these 2 reported cases demonstrate a previously unreported phenomenon whereby concealed tectal gliomas presenting with hydrocephalus are only unmasked following relief of hydrocephalus and decompression and normalization of the ventricular system. We aim to raise awareness about this unusual phenomenon and recommend routine postoperative interval imaging following endoscopic third ventriculostomy to avoid missing underlying pathology masquerading as aqueduct stenosis.

Agenesis of the right internal carotid artery and aqueductal stenosis in a child with hydrocephalus, the role of endoscopic third ventriculostomy: a case report.

Agenesis of the internal carotid artery (aICA) is a rare congenital vascular condition that can affect one or both sides of the patient. Most patients remain asymptomatic, but ischemic/hemorrhagic stroke, intracranial aneurysm, and other neurologic findings can occur. CT scan can demonstrate the absence of the bony carotid canal and helps to differentiate a complete aICA from aplasia or hypoplasia. The association of aICA and aqueductal stenosis (AS) has never been reported in the literature. We report the case of a 9-year-old with agenesis of the right ICA associated with AS and hydrocephalus, which was treated successfully with an endoscopic third ventriculostomy (ETV). We review the literature looking for the association of the clinical findings and the evolution of the patient.

The extreme anterior interhemispheric transcallosal approach for pure aqueduct tumors: surgical technique and case series.

Purely aqueductal tumors represent a rare but distinct entity of neoplasms with characteristic morphology and clinical presentation. This study aims to describe the extreme anterior interhemispheric transcallosal approach as a surgical option for purely aqueductal tumors in the upper part of the cerebral aqueduct and present the surgical results. Prospectively collected data of 4 patients undergoing the extreme anterior interhemispheric transcallosal approach for purely aqueductal tumors in the upper cerebral aqueduct was analyzed. The technique is a variation of the anterior interhemispheric transcallosal approach. The callosotomy is placed at the transition between the body and genu of the corpus callosum, allowing an approach steep enough to reach through the foramen of Monro to the upper cerebral aqueduct without opening the choroidal fissure. All patients had preoperative, and intraoperative or immediate postoperative 3-T magnetic resonance imaging, and underwent examination at admission, after surgery, at discharge, and 3 months postoperatively. Patient data are reported according to common descriptive statistics. All patients harbored low-grade gliomas causing hydrocephalus. Complete resection was achieved without mortality or morbidity. All patients recovered and presented neurologically intact at the 3-month postoperative follow-up. None had recurrence or needed adjuvant therapy. The extreme anterior interhemispheric transcallosal approach proved to be effective and safe. This approach does not require manipulation of the choroidal fissure or disrupt healthy brain parenchyma (except for a small callosotomy). We propose it as an option for removing a purely aqueductal tumor in the upper cerebral aqueduct with associated hydrocephalus.

Case report: Flattening of the tectal plate in obstructive hydrocephalus with auto-ventriculostomy.

BACKGROUND: Obstructive hydrocephalus in adulthood can be caused by stenosis in the aqueductal area. Chronic changes lead to a dilatation of the lateral ventricles and ballooning of infratentorial recesses. In rare cases a rupture of the floor of the third ventricle (so-called spontaneous ventriculostomy) has been described in the literature.Case presentation: We present two cases of chronic obstructive hydrocephalus due to aqueductal stenosis in adult patients. Magnetic resonance imaging included phase-contrast-imaging and revealed significant flow through the floor of the third ventricle in keeping with spontaneous ventriculostomy. In addition to other typical changes associated with chronic hydrocephalus, a distinct flattening of the tectal plate could be identified in one case. CONCLUSION: We present two cases of spontaneous ventriculostomy in patients with chronic hydrocephalus. To our knowledge, flattening of the tectal plate has not yet been described in the literature and may be caused by continuous cerebrospinal fluid-pulsation.

Where the central canal begins: endoscopic in vivo description.

OBJECTIVE: Although evidence and descriptions of the central canal (CC) along the medulla oblongata and the spinal cord have been provided by several anatomical and radiological studies, a clear picture and assessment of the opening of the CC, or apertura canalis centralis (ACC), into the fourth ventricle is lacking, due to its submillimetric size and hidden position in the calamus scriptorius. METHODS: The authors reviewed all of their cases in which patients underwent ventricular transaqueductal flexible endoscopic procedures and selected 44 cases in which an inspection of the region of the calamus scriptorius had been performed and was suitable for study inclusion. Patients were divided into different groups, based on the presence or absence of a chronic pathological process involving the fourth ventricle. In each case, the visual appearance of the opening of the CC of the ACC was classified as no evidence (A0), indirect evidence (A1), or clear evidence (A2). Morphometric measurements were inferred from surrounding structures and the size of surgical tools visible in the field. RESULTS: The opening of the CC could be clearly observed in all cases (A1 4.5%, A2 95.5%). In normal cases, a lanceolate shape along the median sulcus was most frequently found, with an average size of 600 × 250 µm that became rounded and smaller in size in cases of hydrocephalus. The distance between the caudal margin of the ACC and the obex was about 1.8 mm in normal cases, 2.1 mm in cases of obstructive hydrocephalus, and 1 mm in cases of normal pressure hydrocephalus. The two wings of the area postrema, variable in size and shape, were sited just caudal to the opening. CONCLUSIONS: A flexible scope inserted through the cerebral aqueduct can approach the hidden calamus scriptorius like a pen fits into an inkpot. With this privileged viewpoint, the authors provide for the first time, to their knowledge, a clear and novel vision of the opening of the CC in the fourth ventricle, along with the precise location of this tiny structure compared to other anatomical landmarks in the inferior triangle.

Parkinsonism and cerebrospinal fluid disorders.

BACKGROUND: Although various motor manifestations can be seen in patients with cerebrospinal fluid (CSF) disorders, such as hydrocephalus or intracranial hypotension, the clinical presentation with parkinsonism is not clearly elucidated. METHODS: We searched the literature for studies describing the occurrence of parkinsonism in subjects with normal pressure hydrocephalus (NPH), obstructive hydrocephalus, and intracranial hypotension. We analyzed the clinical presentation (particularly with respect to bradykinesia, rigidity, rest tremor, and gait disturbance/postural instability) as well as the response to treatment. RESULTS: Parkinsonism was most commonly reported in NPH patients. Although gait disturbance/postural instability is a well-known motor symptom of NPH, other cardinal signs include upper limb involvement or asymmetric presentation. As for obstructive hydrocephalus, parkinsonism was mainly observed in subjects with aqueductal stenosis and more often after shunt surgery. Patients with NPH or obstructive hydrocephalus rarely improved with levodopa therapy, while most subjects only improved with shunt surgery. Although the mechanism is still controversial, a functional involvement of nigrostriatal pathway has been hypothesized based on imaging studies and case reports. Brain imaging is also helpful for atypical cases of intracranial hypotension presenting with parkinsonism. Parkinsonism improved after treatment in such cases as well. CONCLUSIONS: Studies exploring the relationship between CSF disorders and parkinsonism are mainly descriptive and their quality is generally poor. However, considering that these disorders can be treated, clinicians' awareness of the differential diagnosis is important and future studies better exploring the underlying pathophysiological mechanisms are warranted. This article is part of the Special Issue "Parkinsonism across the spectrum of movement disorders and beyond" edited by Joseph Jankovic, Daniel D. Truong and Matteo Bologna.