Australasian Anaesthesia 2011 - Australian and New Zealand ...

34 Australasian Anaesthesia 2011The disappointing spinal 35ANATOMICAL VARIANTSAnatomical variants have long been touted as the cause of failed and inadequate spinal anaesthesia. Althoughdifficult to ascribe to individual cases post hoc, numerous studies have demonstrated that the anatomy of theintrathecal compartment is not simply a cylindrical structure allowing unimpeded flow of csf and introduced agents.Subarachnoid trabeculaeIntrathecal subarachnoid trabeculae have been described and are formed from arachnoid trabecula cells surroundingextracellular collagen. Arachnoid trabeculae appear to form a loose and indeed random arrangement, bridging thesubarachnoid space between the more tightly organised ‘arachnoid barrier cell’ layer and the pia mater. 11 However,they may form more organised and extensive sheets and under examination the collagen content of the spinalarachnoid trabeculae (or reticular layer) is often more substantial than that of its cranial counterpart. 12 Furthermore,the arachnoid barrier cell layer may at times come into close apposition with the pia mater, the arachnoid trabeculaebeing compacted in between, and the subarachnoid space become all but obliterated. As a result of these morphologicalcharacteristics, the free flow of exogenous substances introduced into the csf may be impeded resulting in asuboptimal block.‘subdural’ blockCertain authors refute the notion of a true potential subdural space. 11 Histologically, the meninges, from outside in,consist of the superficial periosteal dural layer, a meningeal dural layer, a dural border cell layer, arachnoid barriercell layer, arachnoid trabeculae, and ultimately the pia mater. The so called ‘subdural space’ has been argued tobe an artifactual space formed within the dural border cell layer and therefore, ‘intradural’. This comes about becauseof the existence of many cellular interconnections between the arachnoid (barrier cell layer) and the dura (bordercells). Histological evidence of blood between the cells of the dural border cells in de novo and experimental‘subdural’ haematomas seems to confirm an intradural locale. The dural border cell layer represents a weak layerof the meninges due to the paucity of intercellular connections, enlarged extracellular spaces, and lack of extracellularcollagen. 11,12 Further, the existence of dural border cells lining the capsules of subdural haematomas lends credenceto the fact that the subdural space forms as a result of tearing and disruption of cells within the dural border celllayer. This lack of a definitive true potential subdural space may explain why ‘subdural injection’ of local anaestheticcreates a block that is often patchy and high relative to dose, as the ‘subdural space’ is artificially and pathologicallycreated rather than opened up.SeptaeOther anatomical variants have also been described in the literature including the existence of a posterior septumof the cord formed from the arachnoid trabecula. In a study of human cadaver cords, the posterior septum wasdescribed as progressing from a number of filaments and strands in the cervical area to becoming more extensivecaudad to form a finely-woven, fenestrated mesh in the lumbar region of the cord. 18 The posterior septum wasnoted to occupy the majority of the posterior subarachnoid space and arose perpendicularly from the pia cordbetween the posterior nerve rootlets and even beyond their origins. Although largely fenestrated it is possible thatindividual differences in the density of openings may limit the spread of local anaesthetic throughout the subarachnoidspace. The presence of a ‘septum posticum’ has been demonstrated and was seen to occupy the midline in thelower thoracic and upper lumbar region dividing the subarachnoid space into two sections. 19 In a clinical correlate,complete unilateral anaesthesia was described following spinal injection in a 26 year old woman undergoingcaesarean section. 20 Despite an uncomplicated procedure with easy identification and aspiration of csf, sensoryand motor blockade was achieved purely on the right side to the T2 level with no demonstrable block on the left.This persisted into the recovery period after general anaesthesia was performed. The author postulated that theexistence of an imperforate posterior septum or thickening of the dorsolateral membrane surrounding posteriornerve roots might have been responsible.Csf volumeMany explicit patient and injectate factors have been postulated to affect the spread of local anaesthetics, howevernone have been shown to explain more than 50% of the variability in patient response. However, one small studydemonstrated that both block height and sensory block duration were highly correlated with csf volume (r=0.91and 0.83 respectively). 13 The diluent effect of the csf on local anaesthetics injected into the intrathecal space andthe limitation of spread of effective concentrations of these agents is a plausible and attractive explanation for thisfinding. This correlation was supported in a further study wherein removal of 5ml of csf prior to subarachnoid blockled to a higher block 14 , and a number of case reports have confirmed the presence of a large csf volume in instancesof failed spinal anaesthesia. 15,16Individual csf volumes can vary by a factor of three, and this large interindividual variability makes prediction ofextent and duration an inexact science. 17 Unfortunately, correlation between csf volume (and therefore spinal effectand necessary local anaesthetic dose) and any phenotypic patient characteristic is poor, with body mass indexbeing the best correlate identified (r=0.40). Increasing body mass index appears to negatively correlate with csfvolume, obese subjects having less csf. This finding may explain in part why obese and pregnant patients aregenerally more susceptible to the effects of subarachnoid blockade at same dose compared to their lean counterparts.Nerve rootsA further facet of human anatomy that may have a bearing on efficacy is the target nerve roots themselves and theease with which local anaesthetics can diffuse into them. Another cadaveric study demonstrated that the posteriornerve roots exhibit significant variability in diameter between individuals, with the posterior L5 nerve root arearanging from 2.33-7.71mm 2 . 21 Anterior nerve roots are generally half as large in diameter, and nerve root diametersare smaller in the thoracic region and largest in the lumbosacral region. This may explain the enhanced effect ofepidurals in the thoracic region compared to lumbar placement. Despite the dorsal nerve roots being larger, strandinginto individual components was pronounced and would serve to increase the surface area to volume ratio of nerveroots and facilitate anaesthetic action. It is therefore possible that diffusion characteristics of nerve roots maythemselves affect spinal anaesthesia, and poor effect be partly explained by large diameter posterior nerve rootsthat resist stranding.DepositionThe existence of natural curvatures within the vertebral canal can contribute to inadequate spinal blockade. Thisis a particular problem with solutions designed to be hyperbaric as their spread will be influenced by gravity andthus patient positioning and site of injection. The lumbar lordosis of the spine typically occurs at the L4 level in menand women 22 and may be imagined as the apex down which hyperbaric solutions will flow. Depending on whichside of the apex the injection occurs, anaesthetic solution, in the absence of further patient positioning, will beencouraged to flow either more cephalad or more caudad and thereby affect block efficacy. It would seem sensibleto aim to inject hyperbaric local anaesthetic solutions no lower than the L3/4 interspace in order to promote cephaladspread, unless a ‘saddle block’ is intended or patient positioning is to follow.Changes in curvatureAlterations of these natural curves may influence the effectiveness of the spinal block. Interindividual differencesin the site of the lumbar apex appear to be small but should be considered as a potential problem. The lowest pointof the thoracic spinal canal will influence the number of thoracic segments blocked. Typically this occurs at thebody of the T8 vertebra. In contrast to the lumbar apex however, the lowest thoracic level of the spinal canal doesexhibit greater individual variability and ranges over the T7-T9 vertebral bodies. 22 This may explain the differencesin cephalad spread amongst patients in which the same dose is used. Of greater import is the effect of the gravidstatus on the vertebral curvatures. MRI studies have demonstrated that in the supine position with left tilt, thepregnant patient near term has a lumbar apex in the L4/5 position and the lowest point of the thoracic spine wasmore cephalad (at T6-7) compared to non-pregnant women. 23 These findings could also account for the increasedefficacy of spinal anaesthesia in pregnant women and the potential problem of inadequate cephalad extent withhyperbaric solutions when too low an injection site is used.PATHOLOGYDural ectasia or ballooning of the lumbosacral dural sac is commonly found in conditions such as Marfan’s syndromeand is a postulated cause of spinal failure. The incidence of dural ectasia in Marfan’s syndrome is estimated to be63-92% 24,25 and, along with ectopia lentis and aortic dilatation, is one of the major manifestations of the syndrome.The presence of dural ectasia is not associated with aortic dilatation as such but is associated with back pain, andthe degree of ectasia with the severity of the pain. Similar in effect to the presence of a large csf volume, duralectasia has been described in two case reports. 26 In these two obstetric cases, continuous spinal anaesthesia failedto produce a satisfactory block for elective caesarean section despite large doses of bupivacaine, the block beingeither patchy or inadequate in its cephalad extent, and reversion to general anaesthesia was required. Postoperativecomputed tomography demonstrated dural ectasia in both cases. The authors concluded that dural ectasia resultingin a large csf volume was the cause of an unsatisfactory block.Tarlov cysts are an increasingly recognised anatomical variant most likely due to the more widespread use ofmagnetic resonance imaging. These cysts are extradural outpouchings of the meninges encasing the posteriornerve root sheaths and occur most frequently in the lumbosacral region. They may occur either de novo or as aresult of surgery or trauma. Their estimated incidence is between 4.6% 27 and 9% and although generally asymptomaticthey may give rise to pain and neurological symptoms of parasthesia, bowel, and bladder disturbance. Theycommunicate with the intrathecal compartment and as a result contain csf. Enlargement of the cyst is often dueto increasing volumes of csf within them and the cause of the mass effect and symptoms. Furthermore, they mayeventually lose communication with the intrathecal space. Their existence may be another explanation for failureto achieve satisfactory spinal anaesthesia. Entry of the spinal needle into a Tarlov cyst would lead to aspiration ofcsf and deposition of local anaesthetic into the cyst rather than the intrathecal space. The spinal anaesthetic effectwill depend on how much of the solution is able to distribute to the appropriate neural structures from the cyst. Ifthe cyst has separated entirely from the intrathecal space, a complete failure of spinal block will predictably occur.Although a postulated cause of spinal failure, to this author’s knowledge Tarlov cysts have not been described inpatients following failure of spinal anaesthesia.