Closing the dura: dural hitching versus surgicel and tisseel overlay graft in craniocervicaldecompression for Chiari 1 malformation

BACKGROUND

This study compares dural hitching to surgicel and tisseel overlay graft following craniocervical decompression and C1 laminectomy with simple durotomy for Chiari I malformation. Outcome measures were syrinx decompression, headache resolution and complication rates.

METHODS

A retrospective analysis of case notes was conducted. Patients who had undergone craniocervical decompression (CCD) were grouped by method of dural closure. Outcomes compared were rates of syrinx decompression, headache resolution, and post-operative complications. Statistical analysis was conducted using SPSS v20.

RESULTS

We identified 32 adult patients for inclusion in this study. 53.1% (n = 17) had asyrinx, and 78.1% (n = 25) had a pre-operative headache. All were treated with suboccipital craniectomy, C1 laminectomy (with or without C2 laminectomy), and durotomy. The dura was either left open by dural hitching (n = 23) or closed with surgicel and tisseel overlay graft (n = 9). We found a statistically significant association between the method of dural closure and the rate of syrinx resolution. Resolution occurred in 91.7% (n = 11) of the hitching group, compared to 20.0% (n = 1) of the overlay graft group: Χ²(1) = 5.6, p = .018. There were no statistically significant differences between the two groups in the rates of headache resolution or other complications.

CONCLUSIONS

In patients with symptomatic Chiari I malformation and associated syringomyelia, syrinx resolution is more likely if the dura is hitched open rather than closed bysurgicel and tisseel overlay graft after durotomy.

Charcot shoulder and elbow: a review of the literature and update on treatment

Charcot arthropathy of the shoulder and elbow is a rare disease process initially described in the 1700s; however, it was not until the 19th century that physicians understood its association with other disease processes such as cervical spine pathology and diabetes. A primary complaint is painful or painless joint dysfunction, meaning the orthopedic surgeon is regularly the first physician to evaluate the patient. Frequently, the condition of these patients is misdiagnosed. Although the pathogenesis of the disease is controversial, the etiology is commonly due to syringomyelia. The key to successful management is a thorough history and examination along with a workup including specific laboratory testing and imaging to rule out other disease processes. Most neuropathic shoulders and elbows have historically been managed conservatively because of poor outcomes with operative interventions. Newer data have emerged hinting that early neurosurgical intervention can stabilize this degenerative process. If clinical and radiographic stabilization occurs, recent studies have outlined surgical indications that can provide surgeons with a guide as to patients in whom successful operative outcomes can be achieved in the face of failed conservative management.

Neuropathic Knee Joint - A Complication of Syrinx Following Spinal Anesthesia: A Rare Case Report and Review of Literature

INTRODUCTION

Neurological complications due to spinal anesthesia are dysesthesia, paresthesia, cauda equina syndrome, and neuropathic joint. However, neurological complications are rare. We report a case of neuropathic joint of knee as a complication of syrinx following postspinal anesthesia.

CASE REPORT

A 33-year-old female came with complaints of pain and instability of her left knee and left foot drop. She had developed weakness of her left lower limb following her cesarean section surgery done under spinal anesthesia. Clinically and radiologically, she had features of the neuropathic left knee joint which had occurred as a complication of syrinx following spinal anesthesia.

CONCLUSION

The case is reported for its rarity and to highlight the possible neurological complications of spinal anesthesia.

Malignant Melanoma of the Syrinx and Liver in an African Grey Parrot ( Psittacus erithacus erithacus)

A 20-year-old, female African grey parrot ( Psittacus erithacus erithacus) was examined because of ataxia and weakness. Radiographs were unremarkable, and results of a complete blood cell count revealed leukocytosis and heterophilia. Because of poor response to therapy with meloxicam, doxycycline, and enrofloxacin; deteriorating condition; and poor prognosis, the parrot was euthanatized. Postmortem examination revealed 2 dark red nodules in the liver. No grossly visible mass was observed in the syrinx. Histologic examination of the liver and syrinx revealed similar foci of round, oval, and polygonal cells exhibiting severe pleomorphism, with poorly demarcated cytoplasmic borders and moderate amounts of eosinophilic cytoplasm containing brown to black granules (melanin). The mitotic index was 15. The presence of melanin pigment is consistent with a diagnosis of melanoma at both sites. The multifocal distribution and intravascular invasion indicate metastasis; however, the site of origin was unknown. To our knowledge, this is the first recorded case of melanoma in an avian syrinx.

Syringomyelia in hereditary multiple exostosis

We describe five children with Hereditary Multiple Exostosis (HME) who also had syringomyelia. Of these, four had a tethered cord/fibrolipoma. No spinal osteochondromas were found in these patients. All had antecedent neurological signs or symptoms that prompted spinal imaging with MRI. Of all patients with HME seen in the Midwest Regional Bone Dysplasia Clinic from 1982 to present, 44% (17/39) of patients had signs or symptoms concerning for possible cord-related neurological findings. However, only 10 of 39 had spinal imaging. Assuming that all individuals with syringomyelia were identified, then 5/39 (13%) were in that way affected. This, of course, is a minimal estimate given that many were not imaged. The incidence of syringomyelia appears to be increased in this population, and seems to be unrelated to spinal osteochondromas. A low threshold for obtaining spinal MRI in patients with Hereditary Multiple Exostosis seems rational. © 2016 Wiley Periodicals, Inc.

Longitudinal measurements of syrinx size in a rat model of posttraumatic syringomyelia

OBJECTIVE Syringomyelia pathophysiology is commonly studied using rodent models. However, in vivo studies of posttraumatic syringomyelia have been limited by the size of animals and lack of reliable noninvasive evaluation techniques. Imaging the rat spinal cord is particularly challenging because the spinal cord diameter is approximately 1-3 mm, and pathological lesions within the spinal cord parenchyma are even smaller. The standard technique has been histological evaluation, but this has its limitations. The aim of the present study was to determine whether syrinx size could be reliably measured using a preclinical high-field MRI animal system in a rat model of posttraumatic syringomyelia. METHODS The authors used an existing rat model of posttraumatic syringomyelia, which was created using a controlled pneumatic compression device to produce the initial spinal cord injury, followed by a subarachnoid injection of kaolin to produce arachnoiditis. T2-weighted MRI was performed on each animal using a 9.4-T scanner at 7, 10, and 13 weeks after injury. Animals were killed and syrinx sizes were calculated from in vivo MRI and histological studies. RESULTS MRI measurements of syrinx volume and length were closely correlated to histological measurements across all time points (Pearson product moment correlation coefficient r = ± 0.93 and 0.79, respectively). CONCLUSIONS This study demonstrates that high-field T2-weighted MRI can be used to measure syrinx size, and data correlate well with syrinx size measured using histological methods. Preclinical MRI may be a valuable noninvasive technique for tracking syrinx formation and enlargement in animal models of syringomyelia.

Syringosubarachnoid shunting using a myringotomy tube

BACKGROUND

Syringomyelia results from obstruction of cerebrospinal fluid (CSF) flow due to a multitude of causes. Often symptoms of pain, weakness, and sensory disturbance are progressive and require surgical treatment. We present here a rare technique for syringosubarachnoid shunting.

CASE DESCRIPTION

We present the case of a 38-year-old male who suffered a traumatic cervical spinal cord injury due to a motor vehicle accident. With progressive pain and motor decline, a magnetic resonance imaging was obtained and showed a new syrinx extending cervical multiple segments. A unique surgical procedure using a myringotomy tube to shunt CSF into the subarachnoid space was employed in this case. The patient's examination stabilized postoperatively, and at 2 months and 6 months follow-up visits, his strength and sensation continued to improve.

CONCLUSION

We used a myringotomy tube for syringosubarachnoid shunting for the surgical management of a posttraumatic syrinx with good results. This technique minimizes suturing and may minimize shunt-related complications.

Inwardly rectifying potassium channel 4.1 expression in post-traumatic syringomyelia

Post-traumatic syringomyelia (PTS) is a serious neurological disorder characterized by fluid filled cavities that develop in the spinal cord. PTS is thought to be caused by an imbalance between fluid inflow and outflow in the spinal cord, but the underlying mechanisms are unknown. The ion channel Kir4.1 plays an important role in the uptake of K(+) ions from the extracellular space and release of K(+) ions into the microvasculature, generating an osmotic gradient that drives water movement. Changes in Kir4.1 expression may contribute to disturbances in K(+) homeostasis and subsequently fluid imbalance. Here we investigated whether changes in Kir4.1 protein expression occur in PTS. Western blotting and immunohistochemistry were used to evaluate Kir4.1 and glial fibrillary acidic protein (GFAP) expression in a rodent model of PTS at 3 days, 1, 6 or 12 weeks post-surgery. In Western blotting experiments, Kir4.1 expression increased 1 week post-surgery at the level of the cavity. Immunohistochemical analysis examined changes in the spinal parenchyma directly in contact with the syrinx cavity. In these experiments, there was a significant decrease in Kir4.1 expression in PTS animals compared to controls at 3 days and 6 weeks post-surgery, while an up-regulation of GFAP in PTS animals was observed at 1 and 12 weeks. This suggests that while overall Kir4.1 expression is unchanged at these time-points, there are many astrocytes surrounding the syrinx cavity that are not expressing Kir4.1. The results demonstrate a disturbance in the removal of K(+) ions in tissue surrounding a post-traumatic syrinx cavity. It is possible this contributes to water accumulation in the injured spinal cord leading to syrinx formation or exacerbation of the underlying pathology.

Delayed resolution of syrinx after posterior fossa decompression without dural opening in children with Chiari malformation Type I

OBJECT Chiari malformation Type I (CM-I) is associated with a syrinx in 25%-85% of patients. Although posterior fossa decompression (PFD) without dural opening is an accepted treatment option for children with symptomatic CM-I, many surgeons prefer to open the dura if a syrinx exists. The purpose of this study was to investigate the frequency and timing of syrinx resolution in children undergoing PFD without dural opening for CM-I. METHODS A retrospective review of 68 consecutive pediatric patients with CM-I and syringomyelia who underwent PFD without dural opening was conducted. Patient demographics, presenting symptoms and signs, radiographic findings, and intraoperative ultrasound and neuromonitoring findings were studied as well as the patients' clinical and radiographic follow-up. RESULTS During the mean radiographic follow-up period of 32 months, 70% of the syringes improved. Syrinx improvement occurred at a mean of 31 months postoperatively. All patients experienced symptom improvement within the 1st year, despite only 26% of patients showing radiographic improvement during that period. Patients presenting with sensory symptoms or motor weakness had a higher likelihood of having radiographic syrinx improvement postoperatively. CONCLUSIONS In children with CM-I and a syrinx undergoing PFD without dural opening, syrinx resolution occurs in approximately 70% of patients. Radiographic improvement of the syrinx is delayed, but this does not correlate temporally with symptom improvement. Sensory symptoms or motor weakness on presentation are associated with syrinx resolution after surgery.

Chiari I Malformation in Nephropathic Cystinosis

OBJECTIVE

To determine the relative incidence of Chiari I malformations in children with cystinosis compared with those in the general population.

STUDY DESIGN

Magnetic resonance imaging scans were performed on 53 patients with nephropathic cystinosis and 120 controls, age range 3-18 years.

RESULTS

Ten of 53 (18.9%) patients with cystinosis had Chiari I or tonsillar ectopia, and only 2 of 120 controls (1.6%) had a similar finding. At least 2 of the patients had symptoms or signs thought to be related to the malformation, and one had surgical decompression. Two had an associated cervical syrinx.

CONCLUSIONS

Children with cystinosis have a 12-fold higher prevalence of Chiari I malformations than the general pediatric population. Chiari I malformations should be high on the differential diagnosis when individuals with cystinosis develop neurologic signs and symptoms, and magnetic resonance imaging scans should be performed on children with cystinosis who present with new-onset headache, ataxia, incontinence, or other unexplained neurologic symptoms.

Syrinx location and size according to etiology: identification of Chiari-associated syrinx

OBJECT Syrinx size and location within the spinal cord may differ based on etiology or associated conditions of the brain and spine. These differences have not been clearly defined. METHODS All patients with a syrinx were identified from 14,118 patients undergoing brain or cervical spine imaging at a single institution over an 11-year interval. Syrinx width, length, and location in the spinal cord were recorded. Patients were grouped according to associated brain and spine conditions including Chiari malformation Type I (CM-I), secondary CM (2°CM), Chiari malformation Type 0 (CM-0), tethered cord, other closed dysraphism, and spinal tumors. Syringes not associated with any known brain or spinal cord condition were considered idiopathic. Syrinx characteristics were compared between groups. RESULTS A total of 271 patients with a syrinx were identified. The most common associated condition was CM-I (occurring in 117 patients [43.2%]), followed by spinal dysraphism (20 [7.4%]), tumor (15 [5.5%]), and tethered cord (13 [4.8%]). Eighty-three patients (30.6%) did not have any associated condition of the brain or spinal cord and their syringes were considered idiopathic. Syringes in patients with CM-I were wide (7.8 ± 3.9 mm) compared with idiopathic syringes (3.9 ± 1.0, p < 0.0001) and those associated with tethered cord (4.2 ± 0.9, p < 0.01). When considering CM-I-associated and idiopathic syringes, the authors found that CM-I-associated syringes were more likely to have their cranial extent in the cervical spine (88%), compared with idiopathic syringes (43%; p < 0.0001). The combination of syrinx width greater than 5 mm and cranial extent in the cervical spine had 99% specificity (95% CI 0.92-0.99) for CM-I-associated syrinx. CONCLUSIONS Syrinx morphology differs according to syrinx etiology. The combination of width greater than 5 mm and cranial extent in the cervical spine is highly specific for CM-I-associated syringes. This may have relevance when determining the clinical significance of syringes in patients with low cerebellar tonsil position.

Fast dynamic imaging technique to identify obstructive lesions in the CSF space: report of 2 cases

Disorders of CSF dynamics such as syringomyelia and obstructive hydrocephalus can be caused by thin mobile obstructive lesions not visible on traditional MRI sequences. New imaging techniques with balanced steady-state free precession (bSSFP) and dynamic imaging with bSSFP cine allow visualization of these pulsatile structures within the CSF space. The authors present 2 cases involving pediatric patients-one who developed presumed idiopathic syringomyelia and one with presumed communicating hydrocephalus in association with Pfeiffer syndrome-who harbored thin dynamic obstructive lesions seen on bSSFP cine studies using 1.5-T MRI. In combination with traditional CSF cine studies and bSSFP, bSSFP cine sequence was able to detect dynamic membranous adhesions not seen on traditional MRI sequences. These previously undetectable lesions on traditional MRI sequences were the etiology of CSF obstruction, and tailored surgical approaches were performed to avoid shunting in both patients. These reports demonstrate the clinical utility for using these novel imaging tools for the detection of thin adhesions and dynamic lesions in the central nervous system. Balanced SSFP cine sequences can supplement conventional MR modalities to identify these otherwise poorly visualized lesions responsible for presumed communicating hydrocephalus or idiopathic syringomyelia.

Impact of body mass index on cerebellar tonsil position in healthy subjects and patients with Chiari malformation

OBJECT

It is unclear if there is a relationship between Chiari malformation Type I (CM-I) and body mass index (BMI). The aim of this study was to identify the relationship between BMI and cerebellar tonsil position in a random sample of people.

METHODS

Cerebellar tonsil position in 2400 subjects from a cohort of patients undergoing MRI was measured. Three hundred patients were randomly selected from each of 8 age groups (from 0 to 80 years). A subject was then excluded if he or she had a posterior fossa mass or previous posterior fossa decompression or if height and weight information within 1 year of MRI was not recorded in the electronic medical record.

RESULTS

There were 1310 subjects (54.6%) with BMI records from within 1 year of the measured scan. Of these subjects, 534 (40.8%) were male and 776 (59.2%) were female. The average BMI of the group was 26.4 kg/m(2), and the average tonsil position was 0.87 mm above the level of the foramen magnum. There were 46 subjects (3.5%) with a tonsil position ≥ 5 mm below the level of the foramen magnum. In the group as a whole, there was no correlation (R(2) = 0.004) between BMI and cerebellar tonsil position.

CONCLUSIONS

In this examination of 1310 subjects undergoing MRI for any reason, there was no relationship between BMI and the level of the cerebellar tonsils or the diagnosis of CM-I on imaging.

Incidental findings on brain and spine imaging in children

In recent years, the utilization of diagnostic imaging of the brain and spine in children has increased dramatically, leading to a corresponding increase in the detection of incidental findings of the central nervous system. Patients with unexpected findings on imaging are often referred for subspecialty evaluation. Even with rational use of diagnostic imaging and subspecialty consultation, the diagnostic process will always generate unexpected findings that must be explained and managed. Familiarity with the most common findings that are discovered incidentally on diagnostic imaging of the brain and spine will assist the pediatrician in providing counseling to families and in making recommendations in conjunction with a neurosurgeon, when needed, regarding additional treatments and prognosis.

Syringomyelia in an older patient

We describe the case of an 80-year-old man who presented with lower limb upper motor neurone weakness and spinothalamic tract sensory deficit secondary to previously undiagnosed syringomyelia. The case highlights the need for methodical history, examination and investigation in elderly patients to achieve diagnostic accuracy.

Does a Syrinx Matter for Return to Play in Contact Sports? A Case Report and Evidence-Based Review of Return-to-Play Criteria After Transient Quadriplegia

Transient quadriplegia is a rare injury that can change the course of an athlete’s career if misdiagnosed or managed inappropriately. The clinician should be well versed in the return-to-play criteria for this type of injury. Unfortunately, when an unknown preexisting syrinx is present in the athlete, there is less guidance on their ability to return to play. This case report and review of the current literature illustrates a National Collegiate Athletic Association (NCAA) Division I football player who suffered a transient quadriplegic event during a kickoff return that subsequently was found to have an incidental cervical syrinx on magnetic resonance imaging. The player was able to have a full neurologic recovery, but ultimately he was withheld from football.

Subarachnoid-subarachnoid bypass for spinal adhesive arachnoiditis

The authors report a case of adhesive arachnoiditis (AA) and arachnoid cyst successfully treated by subarachnoid to subarachnoid bypass (S-S bypass). Arachnoid cysts or syringes sometimes compress the spinal cord and cause compressive myelopathy that requires surgical treatment. However, surgical treatment for AA is challenging. A 57-year-old woman developed leg pain and gait disturbance. A dorsal arachnoid cyst compressed the spinal cord at T7-9, the spinal cord was swollen, and a small syrinx was present at T9-10. An S-S bypass was performed from T6-7 to T11-12. The patient's gait disturbance resolved immediately after surgery. Two years later, a small arachnoid cyst developed. However, there was no neurological deterioration. The myelopathy associated with thoracic spinal AA, subarachnoid cyst, and syrinx improved after S-S bypass.

Peripheral androgen action helps modulate vocal production in a suboscine passerine

Androgenic activation of intracellular androgen receptors (AR) influences avian vocal production, though this has largely been investigated at the level of the brain. We investigated the influence of predominantly peripheral AR on vocal output in wild Golden-collared Manakins (Manacus vitellinus). In this suboscine species, males court females by performing acrobatic displays and by producing relatively simple chee-poo vocalizations. To assess whether peripheral AR influences the acoustic structure of these vocal signals, we treated reproductively active adult males with the peripherally selective antiandrogen bicalutamide and then measured phonation performance. Inhibiting AR outside of the central nervous system increased the duration of the chee note and decreased the fundamental frequency of the poo note. This treatment caused no discernable change to chee-poo frequency modulation or entropy. Our results show that activation of peripheral AR mediates note-specific changes to temporal and pitch characteristics of the Golden-collared Manakin's main sexual call. Thus, our study provides one of the first demonstrations that androgenic action originating outside of the brain and likely on musculoskeletal targets can modulate avian vocal production.

Detection of syringomyelia in a pediatric patient with mild scoliosis: a case report

It can be challenging to detect syringomyelia in patients with scoliosis, as some cases are mildly symptomatic with little to no neurological deficits. However, a timely diagnosis of syringomyelia is needed to facilitate important treatment considerations. This case report details an 11-year-old female with mild scoliosis and a two-year history of spinal pain that had short-term symptomatic relief from chiropractic treatment. Subtle neurological signs were detected only at re-evaluation, which prompted further investigation with radiographs and subsequent magnetic resonance imaging (MRI). MRI revealed a non-expansile syrinx measuring 3 mm at its widest diameter that extended from C5 to the conus medullaris. The aim of this case is to heighten awareness of the potential diagnostic challenges in patients with syringomyelia and scoliosis. The incidence, pathogenesis, clinical presentation, and management of syringomyelia will be presented to help primary contact providers with appropriate referral and co-management of these patients.

A rare case of giant terminal lipomyelocystocele with partial sacral agenesis

Terminal myelocystoceles (TMCs) are a rare form of occult spinal dysraphism. They constitute approximately 5% of skin covered lumbosacral masses. The TMC is composed of a low-lying conus medullaris with cystic dilatation of caudal central canal, a surrounding meningocele and a lipoma that extends from the conus to a subcutaneous fat collection. A 6-month-old female child presented with a large lumbosacral mass (14 cm × 12 cm × 10 cm) and weakness at the ankle joints since birth. Magnetic resonance imaging and computed tomography scan revealed a low-lying terminal lipomyelocystocele with holocord syrinx and partial sacral agenesis. The child was operated on successfully. This is an interesting and rare case of giant terminal lipomyelocystocoele with syrinx and a partial sacral agenesis. Terminal lipomyelocystocele should be included in the differential diagnosis of congenital lesions presenting as a lumbosacral mass and operated early.

Intramedullary tubercular abscess with syrinx formation

Intramedullary spinal cord tubercular abscess with involvement of whole cord is a rare entity that too with syrinx formation following disseminated meningitis. Accurate diagnosis requires a high index of suspicion with clinical history and imaging features for a favorable outcome. Here-in we present a similar case with tubercular etiology which was also associated with syrinx formation and has not been reported previously in the literature up to the author's knowledge.

Lumbosacral arachnoid cyst with tethered cord: A rare case report

Arachnoid cysts are cerebrospinal fluid collections in the spine that can present with neurological symptoms or be discovered accidentally. Intradural location of such cysts especially in the lumbosacral region is relatively rare. The association of such cysts with other congenital anomalies such as tethered cord lends evidence to the developmental origin of arachnoid cysts. We report a case of lumbosacral arachnoid cyst with tethered cord in a 6-year-old male child and discuss the etiopathogenesis and management options.

Charcot arthropathy of the shoulder associated with typical and atypical findings

We present a patient with a relatively rare condition: Charcot joint of the shoulder, with a rare complication, the first known example of combined neurovascular compression in this location. A 49-year-old man presented with neuropathic arthropathy of the shoulder caused by syringomyelia from a Chiari I malformation, leading to compression of both the brachial plexus and the axillary vein by mass effect from the synovitis. The brachial plexopathy resolved with surgical decompression and synovectomy, and the syringomyelia stabilized after Chiari decompression. A large acromioclavicular joint synovial cyst developed as a late complication, which was treated nonoperatively. Understanding neuropathic arthropathy can explain the spectrum of interrelated typical and atypical features in this case over long-term follow-up.

Acute respiratory failure as a first manifestation of syringomyelia

A 40 year old woman presented with a short history of acute onset of breathlessness to the ER of our hospital and after initial evaluation for acute pulmonary embolism which was ruled out after carrying out the appropriate investigations, she was diagnosed to be afflicted with syringomyelia based on her neurological symptoms and clinical findings, which was confirmed by doing an MRI scan, which was her basic diagnosis that was complicated by acute hypercapnic respiratory failure. This case is being reported to highlight syringomyelia as an unusual cause of acute respiratory failure, which manifested clinically in this patient as its first presentation and the underlying neurological diagnosis has been found to be present in very few reported cases (less than 0.01% of case reports) in the available literature as the basic disease in the absence of its classical presenting features. Problems associated with acute respiratory failure in the setting of syringomyelia are discussed.

Peripheral mechanisms for vocal production in birds - differences and similarities to human speech and singing

Song production in songbirds is a model system for studying learned vocal behavior. As in humans, bird phonation involves three main motor systems (respiration, vocal organ and vocal tract). The avian respiratory mechanism uses pressure regulation in air sacs to ventilate a rigid lung. In songbirds sound is generated with two independently controlled sound sources, which reside in a uniquely avian vocal organ, the syrinx. However, the physical sound generation mechanism in the syrinx shows strong analogies to that in the human larynx, such that both can be characterized as myoelastic-aerodynamic sound sources. Similarities include active adduction and abduction, oscillating tissue masses which modulate flow rate through the organ and a layered structure of the oscillating tissue masses giving rise to complex viscoelastic properties. Differences in the functional morphology of the sound producing system between birds and humans require specific motor control patterns. The songbird vocal apparatus is adapted for high speed, suggesting that temporal patterns and fast modulation of sound features are important in acoustic communication. Rapid respiratory patterns determine the coarse temporal structure of song and maintain gas exchange even during very long songs. The respiratory system also contributes to the fine control of airflow. Muscular control of the vocal organ regulates airflow and acoustic features. The upper vocal tract of birds filters the sounds generated in the syrinx, and filter properties are actively adjusted. Nonlinear source-filter interactions may also play a role. The unique morphology and biomechanical system for sound production in birds presents an interesting model for exploring parallels in control mechanisms that give rise to highly convergent physical patterns of sound generation. More comparative work should provide a rich source for our understanding of the evolution of complex sound producing systems.