Electron microscopy of human peripheral nerves of clinical relevance to the practice of nerve blocks. A structural and ultrastructural review based on original experimental and laboratory data

ObjetivoEl principal objetivo fue describir la ultraestructura normal de los nervios periféricos y aportar todos aquellos aspectos relevantes para la práctica de los bloqueos anestésicos de los nervios periféricos.MétodoA partir de muestras de nervio ciático obtenidas de pacientes y muestras de saco dural, manguitos durales y nervios del plexo braquial disecados de cadáveres frescos se estudió la estructura del nervio periférico, incluyendo los fascículos y su composición topográfica, como así también la morfología de las cubiertas que revisten el nervio. Axones mielinizados, no mielinizados, fascículos, epineuro, perineuro y endoneuro fueron estudiados por microscopía óptica incluyendo métodos de inmunohistoquímica, y por microscopía electrónica de barrido y de transmisión. La estructura del perineuro, de los capilares intrafasciculares y su implicación en la barrera hematonerviosa fueron revisados.ResultadosCada uno de los elementos anatómicos fue analizado de forma individual, y los resultados fueron correlacionados con su impacto en la práctica clínica de la anestesia regional.ConclusionesLa práctica cotidiana de técnicas de anestesia regional y la visualización ecográfica de las estructuras nerviosas nos llevan a crear interpretaciones de la anatomía de los nervios cuyas repercusiones pueden ser importantes en la futura aplicación de la técnica. A este respecto, los resultados obtenidos en estudios ultraestructurales e histológicos están dirigidos a dar respuestas a esas dudas con origen en el entorno de las técnicas de la anestesia regional (AU)

AimThe goal is to describe the ultrastructure of normal human peripheral nerves, and to highlight key aspects that are relevant to the practice of peripheral nerve block anaesthesia.MethodUsing samples of sciatic nerve obtained from patients, and dural sac, nerve root cuff and brachial plexus dissected from fresh human cadavers, an analysis of the structure of peripheral nerve axons and distribution of fascicles and topographic composition of the layers that cover the nerve is presented. Myelinated and unmyelinated axons, fascicles, epineurium, perineurium and endoneurium obtained from patients and fresh cadavers were studied by light microscopy using immunohistochemical techniques, and transmission and scanning electron microscopy. Structure of perineurium and intrafascicular capillaries, and its implications in blood–nerve barrier were revised.ResultsEach of the anatomical elements is analyzed individually with regard to its relevance to clinical practice to regional anaesthesia.ConclusionsRoutine practice of regional anaesthetic techniques and ultrasound identification of nerve structures has led to conceptions, which repercussions may be relevant in future applications of these techniques. In this regard, the ultrastructural and histological perspective accomplished through findings of this study aims at enlightening arising questions within the field of regional anaesthesia (AU)

Electron microscopy of human peripheral nerves of clinical relevance to the practice of nerve blocks. A structural and ultrastructural review based on original experimental and laboratory data.

AIM: The goal is to describe the ultrastructure of normal human peripheral nerves, and to highlight key aspects that are relevant to the practice of peripheral nerve block anaesthesia. METHOD: Using samples of sciatic nerve obtained from patients, and dural sac, nerve root cuff and brachial plexus dissected from fresh human cadavers, an analysis of the structure of peripheral nerve axons and distribution of fascicles and topographic composition of the layers that cover the nerve is presented. Myelinated and unmyelinated axons, fascicles, epineurium, perineurium and endoneurium obtained from patients and fresh cadavers were studied by light microscopy using immunohistochemical techniques, and transmission and scanning electron microscopy. Structure of perineurium and intrafascicular capillaries, and its implications in blood-nerve barrier were revised. RESULTS: Each of the anatomical elements is analyzed individually with regard to its relevance to clinical practice to regional anaesthesia. CONCLUSIONS: Routine practice of regional anaesthetic techniques and ultrasound identification of nerve structures has led to conceptions, which repercussions may be relevant in future applications of these techniques. In this regard, the ultrastructural and histological perspective accomplished through findings of this study aims at enlightening arising questions within the field of regional anaesthesia.

Cerebrospinal fluid volume and nerve root vulnerability during lumbar puncture or spinal anaesthesia at different vertebral levels.

Cerebrospinal fluid (CSF) and nerve root volumes within the lumbosacral dural sac were estimated at various vertebral levels, in an attempt to determine any possible relevance to the incidence of nerve root trauma during lumbar puncture or spinal anaesthesia. Magnetic resonance images from seven patients were studied. Volumes were calculated by semi-automatic threshold segmentation combined with manual editing of each slice. The mean dural sac volume from S1 to T12 was 42.8±5.8 ml and the mean CSF volume 34.3±5.1 ml with the mean root volume being 10.4±2.2 cm(3). The mean CSF volume per vertebral segment ranged from 4.3±0.7 ml at L5, to 5.8±2.5 ml at L1, with high inter-individual variability. The mean root volume ranged from 0.6±0.1 cm(3) at L5 to 2.4±0.5 cm(3) at T12. The conus medullaris was located at L1 in four of the five patients scanned at upper lumbar levels, and at the lower border of L2 in the other. Vulnerability to nerve root damage was expressed as the Vulnerability Index (%), being defined as the ratio of root volume to dural sac volume (CSF volume + root volume). The value ranged between 7 and 14% at L5, increasing rostrally to 30 to 43% at T12. Caution is obviously required in high punctures to avoid contact with the conus medullaris, but the cauda equina is also vulnerable to contact with more caudal punctures and had a Vulnerability Index of about 25% at L4, that increased rostrally.

An anatomical study of the intradural space.

We have previously postulated that it is possible to accidentally insert an epidural needle or catheter into the substance of the dura during attempted epidural block, creating an intradural space. It appears that injection of local anaesthetic into an intradural space leads to an initially inadequate neuraxial block but further doses may produce an extensive life-threatening block. In the laboratory, 54 samples of human thoraco-lumbar dura were obtained from six cadavers and prepared for scanning electron microscopy. Images from these dehydrated specimens were assessed for the presence of spaces within the dura, and attempts were made to insert an epidural catheter, under optical microscopy, into the substance of the dura in 32 cases. Electron microscopy revealed the concentric laminae that compose the dura and the presence of artefactual spaces between some of these. It was possible to insert an epidural catheter into the substance of the dura in eight specimens, creating intradural spaces which remained following catheter removal. If this represents the clinical situation, it may help to explain previously reported cases of atypical neuraxial block and their associated radiological findings.

Blood patches may cause scarring in the epidural space: two case reports.

The use of epidural blood patches has become standard treatment for postdural puncture headache. Two cases suggest that there may occasionally be incomplete resorption of the blood patch, resulting in scarring in the epidural space that can be visualised using postpartum contrast injection and fluoroscopy. Both patients had a previous caesarean delivery during which they had suffered inadvertent dural punctures followed by epidural blood patches. When subsequently presenting for repeat caesarean delivery, both had inadequate epidural anaesthesia. Epidural insertion was then repeated, resulting in successful anaesthesia in one patient but a total subdural block with delayed apnoea and unconsciousness in the other. Distortion of epidural anatomy by fibrosis was considered to be a possible cause.

The intradural space: the fourth place to go astray during epidural block.

BACKGROUND: Considerable uncertainty exists regarding accidental injection of local anaesthetic into the 'subdural space' during attempted epidural block. A whole range of clinical findings, from excessively high to failed blocks has been reported although many of these findings appear difficult to explain on the basis of our current knowledge of the anatomy. The existence of another, adjacent space, the intradural space, is postulated. METHODS: Our study of atypical epidural blocks using contrast injection and radiographic screening has now obtained data on 130 patients, and results were reviewed retrospectively, searching for contrast flowing into the subdural region. RESULTS: Radiographic studies have revealed 10 patients with an unusual dense localised collection of contrast in a space previously unrecognised by anaesthetists. Clinical presentation was of inadequate neuraxial block, which could eventually be corrected by top-up doses, but with the possible risk of developing a high block. Late radiographic pictures revealed contrast escaping from the mass into the epidural, subdural or subarachnoid spaces. CONCLUSIONS: A review of electron microscopy studies suggested that a 'secondary' subdural space could be opened up by trauma in the distal layers of the dura. Our findings suggest that injection into this 'intradural' space can occur, resulting in an initially inadequate neuraxial block with limited spread. Further volumes of local anaesthetic can be expected to produce satisfactory block, probably as a result of escape to the epidural space. However, late spread to the subdural or subarachnoid space may occur.

Epidural abscess in an obstetric patient.

Epidural abscess is a rare complication of epidural block and occasionally presents in the post partum period. A case is described where a thoracolumbar abscess presented with backache and headache 10 days after an apparently uneventful block for labour and caesarean section. The abscess was treated medically with a satisfactory outcome. The literature is reviewed in order to assess several recent reports of infectious complications of epidural block in obstetric patients.

Potential intrathecal leakage of solutions injected into the epidural space following combined spinal epidural anaesthesia.

A combined spinal epidural anaesthetic (CSE), by design, produces a deliberate multicompartment block across a breached dural membrane. Since the lateral holes of the epidural catheter may lie in close proximity to the dural puncture site, a bolus solution of drug injected via the epidural catheter has the potential to leak through the dural puncture into the subarachnoid space. The aim of this study was to determine the incidence of intrathecal leak by performing an epidurogram. Fifteen patients undergoing surgery with a CSE anaesthetic using a 16 gauge Tuohy/26 gauge pencil point needle were studied. Within three hours of catheter insertion, 12 ml of contrast (iohexol 300 mg/ml) was injected via the epidural catheter under fluoroscopic control with screen recording and exposure of lateral and anteroposterior X-ray plates. All films were later reviewed for evidence of intrathecal spread. We did not observe any evidence of intrathecal spread of contrast. However, caution should be observed during administration of an intraoperative bolus dose of analgesic agent via a catheter inserted as part of a combined spinal epidural anaesthetic technique, particularly with the use of hydrophilic opiods.

Bilateral trigeminal nerve palsy during an extensive lumbar epidural block.

A rare case of trigeminal nerve blockade arising in the course of obstetric lumbar epidural anaesthesia is described. There was extensive bilateral spread of nerve-block up to the C4 level with respiratory distress after top-up for caesarean section, and subsequent epidurography revealed high epidural spread of contrast. The mechanism of the trigeminal nerve palsy was the source of some controversy, particularly as to whether intracranial spread of local anaesthetic had occurred, possibly following accidental subdural or subarachnoid injection.

Why obstetric epidurals fail: a study of epidurograms.

In a study of the factors involved in the occasional failure of continuous obstetric epidural blockade, contrast injections through epidural catheters and radiographic screening were undertaken in 35 postpartum patients. The two major causes of inadequate block were found to be transforaminal escape of the catheter tip, and persistent unilateral block associated with an obstructive barrier in the epidural space. Recommendations for overcoming these problems are discussed.