Decompression of a Dorsal Arachnoid Web of the Spine: 2-Dimensional Operative Video

Arachnoid webs of the spine are a relatively rare entity with unique radiological findings, most notably the scalpel-sign on sagittal magnetic resonance imaging (MRI).1-4 To date there have been no videographic descriptions of the surgical treatment for this clinical entity. We present the case of a patient with progressive myelopathy and MRI showing a cervical and thoracic syrinx with a sharp transition point at the level of the T5 vertebral body. On computed tomography myelogram, there was preserved cerebrospinal fluid (CSF) in the ventral subarachnoid space—this space is often interrupted in ventral cord herniation, and preserved in dorsal arachnoid webs.5 A laminectomy with intradural excision of arachnoid web was offered and the patient consented for the procedure. Preoperatively, a fiducial screw was placed at T5. A T4-6 laminectomy was performed. A clearly compressive arachnoid web was encountered on exposure. Bands were dissected under an operating microscope, restoring normal CSF flow. Ventral dural defect was ruled out by passing a delicot beneath the cord and withdrawing it on the contralateral side. The patient did well and has shown improvement in myelopathic symptoms at 1- and 3-mo follow-up. Arachnoid webs of the spine can be treated effectively with a prudent, stepwise approach, and careful microsurgical technique. The neurosurgeon should consult closely with their neuroradiology colleagues to rule out other entities prior to the operation, such as ventral cord herniations, which can mimic dorsal arachnoid webs radiologically and clinically. We have received informed consent of the patient to submit this video.

The Conserved ASCL1/MASH-1 Ortholog HLH-3 Specifies Sex-Specific Ventral Cord Motor Neuron Fate in Caenorhabditis elegans

Neural specification is regulated by one or many transcription factors that control expression of effector genes that mediate function and determine neuronal type. Here we identify a novel role for one conserved proneural factor, the bHLH protein HLH-3, implicated in the specification of sex-specific ventral cord motor neurons in C. elegans. Proneural genes act in early stages of neurogenesis in early progenitors, but here, we demonstrate a later role for hlh-3. First, we document that differentiation of the ventral cord type C motor neuron class (VC) within their neuron class, is dynamic in time and space. Expression of VC class-specific and subclass-specific identity genes is distinct through development and is dependent on the VC position along the A-P axis and their proximity to the vulva. Our characterization of the expression of VC class and VC subclass-specific differentiation markers in the absence of hlh-3 function reveals that VC fate specification, differentiation, and morphology requires hlh-3 function. Finally, we conclude that hlh-3 cell-autonomously specifies VC cell fate.

The conserved ASCL1/MASH-1 ortholog HLH-3 specifies sex-specific ventral cord motor neuron fate in C. elegans

ABSTRACTNeural specification can be regulated by one or many transcription factors. Here we identify a novel role for one conserved proneural factor, the bHLH protein HLH-3, implicated in the specification of sex-specific ventral cord motor neurons in C. elegans. In the process of characterizing the role of hlh-3 in neural specification, we document that differentiation of the ventral cord type C neurons, VCs, within their motor neuron class, is dynamic in time and space. Expression of VC class-specific and subclass-specific identity genes is distinct through development and dependent on where they are along the A-P axis (and their position in proximity to the vulva). Our characterization of the expression of VC class and VC subclass-specific differentiation markers in the absence of hlh-3 function reveals that VC fate specification, differentiation, and morphology requires hlh-3 function. Finally, we conclude that hlh-3 cell-autonomously specifies VC cell fate.

Rare dorsal thoracic arachnoid web mimics spinal cord herniation on imaging

Background: Dorsal arachnoid webs (DAWs) are rare clinical entities that can mimic other conditions on magnetic resonance imaging (MRI). Here, we present a case of DAW that was misdiagnosed on MR as a ventral cord herniation. Case Description: A 35-year-old female presented with a 1-year history of lower extremity weakness and numbness. The MRI of the thoracic spine showed ventral cord displacement with syringomyelia. The computed tomography myelogram demonstrated ventral cord herniation. Intraoperatively, the patient had a dorsal thoracic web in the absence of cord herniation. Within 8 postoperative weeks, the patient had improved, and the follow-up MI showed a significant reduction in the syrinx size. Conclusion: On MR scans, DAWs may look like ventral cord herniation. However, the positive “scalpel sign” and syrinx, the absence of an arachnoid cyst on myelography, and the findings on cine MR help differentiate DAWs from ventral cord herniation.

Imaging Findings in Dorsal Thoracic Arachnoid Web and the Differential Diagnosis of “Scalpel Sign”

A dorsal thoracic arachnoid web represents an intradural extramedullary transverse band of arachnoid tissue that causes mass effect and dorsal cord indentation, and can or cannot be associated with spinal cord altered signal. On sagittal MR imaging, this focal dorsal indentation of the thoracic spinal cord resembles a scalpel with its blade pointing posteriorly (called a “scalpel sign”). Although very suggestive of dorsal thoracic arachnoid web, this sign is not specific and should be differentiated from other ventral cord displacement causes (eg, idiopathic spinal cord herniation and spinal arachnoid cyst). In idiopathic spinal cord herniation, cord tissue protrudes through a ventral dural defect, and the focal deformity can be seen along the ventral aspect of the cord on spinal axial MR imaging and with a characteristic “C sign” on sagittal MR imaging; in spinal arachnoid cysts, the marginated walls and the presence of smooth, wide scalloping of the cord surface can be identified. Recognition of these imaging findings, especially the scalpel sign, can help radiologists and clinicians make a correct diagnosis of ventral cord displacement causes and allow subsequent prompt treatment for the patient.

Caenorhabditis elegans excitatory ventral cord motor neurons derive rhythm for body undulation

The intrinsic oscillatory activity of central pattern generators underlies motor rhythm. We review and discuss recent findings that address the origin of Caenorhabditis elegans motor rhythm. These studies propose that the A- and mid-body B-class excitatory motor neurons at the ventral cord function as non-bursting intrinsic oscillators to underlie body undulation during reversal and forward movements, respectively. Proprioception entrains their intrinsic activities, allows phase-coupling between members of the same class motor neurons, and thereby facilitates directional propagation of undulations. Distinct pools of premotor interneurons project along the ventral nerve cord to innervate all members of the A- and B-class motor neurons, modulating their oscillations, as well as promoting their bi-directional coupling. The two motor sub-circuits, which consist of oscillators and descending inputs with distinct properties, form the structural base of dynamic rhythmicity and flexible partition of the forward and backward motor states. These results contribute to a continuous effort to establish a mechanistic and dynamic model of the C. elegans sensorimotor system. C. elegans exhibits rich sensorimotor functions despite a small neuron number. These findings implicate a circuit-level functional compression. By integrating the role of rhythm generation and proprioception into motor neurons, and the role of descending regulation of oscillators into premotor interneurons, this numerically simple nervous system can achieve a circuit infrastructure analogous to that of anatomically complex systems. C. elegans has manifested itself as a compact model to search for general principles of sensorimotor behaviours. This article is part of a discussion meeting issue ‘Connectome to behaviour: modelling C. elegans at cellular resolution’.