it has been recently suggested that cm1 in some cases may caused by tethered cord with a traction mechanism ; this may be the case also if the conus is located in a normal position due to a reduced elasticity of the filum , thick or lipomatous , and the name for this condition is occult tethered cord . as a consequence of this hypothesis , the filum section has been proposed for the treatment of cm1 in a selected group of patients by some authors   and in very case of cm1 and scoliosis by others . this pathogenesis has been deeply criticized for two reasons : while in open spina bifida the cm type 2 in a very frequent association , the association of occult spina bifida and cm1 is so rare to be anecdotal ; secondly there are some experimental studies that showed no variation in the tonsillar position after detethering in the cadaver . the clinical picture of tethering is caused by ischemia of the lower conus due to traction of the spinal cord occurring with growth and flexion [ 6 , 7 ] ; this mechanism leads to motor deficits and spasms , sensory problems , and urodynamic and anorectal abnormalities ; later on , orthopedic deformations may also occur due to the strength disequilibrium on feet and calves . the tethered cord syndrome ( tcs ) associated with cm1 , characterized by a conus positioned lower than the endplate of l2 , as well as the occult conus tethering ( traction due to a tight anelastic filum with conus at a normal level ; if it exists , is expected to cause the cauda equina symptoms by the same mechanism . the aim of the present study is to look the chiari problem from below by a caudorostral  perspective : we reviewed our series of patients operated on for a  real tethering  searching for an associated tonsillar herniation to define the incidence of cm1 in tethered cord . recently we tried to understand better the role of the two malformations on symptoms ; so we performed the detethering by first to see if the simple filum sectioning somehow influences the tonsillar position , so restoring the csf circulation with a benefit on chiari related symptoms and syringomyelia . the present study concerns 110 patients operated on for a  real tethering  ( tcs ) between 1990 and 2008 at the fondazione istituto neurologico  carlo besta  ( fincb ) of milan , italy . all the patients were submitted to a whole mr examination both before and after the surgery to document the tonsillar position and the presence of a syrinx . the conus level was l2l3 in 55 cases , l4 in 41 and l5 or lower in 14 . a terminal syringomyelia was documented in 30% , a cutaneous stigmata was observed in 86% of the cases , with a dermal sinus in 20% . all patients neurological examination were scored by the hoffman scale   from diagnosis to follow - up to have comparable data . despite the clear signs for diagnosis , all the patients were symptomatic at the time of surgery and 74% presented progressive deterioration during the long pre - operative period . in fact , the mean time span occurred between diagnosis and surgery was longer than 5  years . before being operated post - operative urodynamic tests were repeated within the first year after the surgery and later on , if they were abnormal . the steps of surgery were : resection of the subcutaneous lipoma and of the dermal sinus , laminar exposure , extended at least one space cranially than the first non - schisic segment , laminotomy performed by high speed drill , exposition the intact dural sac , opening of the dura under optic magnification , separation of the intradural lipoma from the lateral and caudal dural adhesion and from neural tissue under emg - monitoring control , near - complete lipoma removal , sectioning of the tight filum , suture of the placode , positioning of a dural graft to prevent retethering and fibrin glue in order to try and reduce csf leaks , and fixation of the laminotomic flap . a csf collection , deserving surgical revision , occurred in 18 cases and infection in 2 cases ; none of these complications caused permanent morbidity . by the described procedure detethering was obtained in the great majority of the cases , except five cases ( 4% ) , all performed without emg intraoperative monitoring . the intraoperative cranial migration of the conus was registered only recently ; it was described in 18 cases . the scored neurological outcome at diagnosis , at surgery , at discharge and at long - term follow - up is summarized in fig .  1 . transient post - operative deterioration occurred in 11 cases , while permanent post - operative surgery - related deterioration was observed in 5 ( 4% ) cases of conus lipomas ; this percentage reduced to zero with intraoperative emg monitoring.fig . 1neurological picture evolution neurological picture evolution at a mean follow - up longer than 5  years , late deterioration was observed up to now in four cases , in two cases due to a presacral meningocele , in the other three due to a retethering ; the global percentage of late deterioration was 3.5% . symptomatic cm1 associated to tcs was observed in six cases , while asymptomatic low lying tonsils ( llt ) in two . the global percentage of associated tcs and tonsillar descent was 7% in this large series . the two cases of llt regressed after detethering . of the symptomatic cm1 : in one case the craniovertebral decompression ( cvd ) was performed at the same time of dethethering with good outcome ; in another case , initially symptomatic for cm1 , the cvd was performed with good results on bulbar signs , but it was followed 18  months later by a detethering for the progression of a syrinx ; the results were good after the second operation ; in two cases with mild tonsillar migration the clinical picture stabilized after detethering , with no need for cvd , while in the remaining two cvd had to be performed 1  year after the detethering , to treat progressive syrinx formation and occurrence of bulbar signs ; in no case , a cranial migration of the tonsils was observed after detethering.fig . a preoperative rmi of rr , 9-month - old male , harboring a typical caudal conus lipoma : despite the relevant traction of the conus ( positioned at l4 level before surgery ) , documented intraoperatively ( c ) and solved by surgery , as documented by the significant cranial migration ( b ) toward l2 , the tonsillar position ( d ) was normal preoperatively and remained the same after surgery cranial migration of the conus medullaris . a preoperative rmi of rr , 9-month - old male , harboring a typical caudal conus lipoma : despite the relevant traction of the conus ( positioned at l4 level before surgery ) , documented intraoperatively ( c ) and solved by surgery , as documented by the significant cranial migration ( b ) toward l2 , the tonsillar position ( d ) was normal preoperatively and remained the same after surgery one of these children affected by both cm1 and tcs , a 4-year - old girl had , beside the cm1 with basilar invagination and tcs due to a filar lipoma , right hand polidattily , a kidney malformation , a morsus inversus and a vesicoureteral reflux . this cases series reflects the changes in the timing of diagnosis and surgery along the years in italy : up to mid-1990s , all the cases that were proposed for neurosurgery only when deeply symptomatic ; afterwards some asymptomatic case was identified ; due to the fear of causing adjunctive deficits our policy was to operate all the symptomatic cases and all the filar lipomas , due to their low risk of post - surgical deterioration . on the contrary all the asymptomatic conus medullaris lipomas were strictly followed by serial urodynamic testing during the first year of life . in this way , an urodynamic deterioration was observed in 12 children in the first 18  months of life ; obviously these data have no statistic value , because the test has been applied only recently , but a deterioration was discovered in more than an half of the tested children . all the cases were operated soon after the urodynamic variation , with normalization of the bladder function . the global rate of neurological deterioration before surgery in this series was higher than 50% in the long pre - operative period ; this may be assumed as the rate of deterioration of symptomatic lipoma in their  natural history  . on the contrary , the risk of early and late permanent post - surgical deterioration was low ( 4  +  3% ) , as well as the percentage of  failed  detethering ( 4% ) ; globally , surgery failed in 11% of the cases ; a result quite better than the natural history . in our experience , intraoperative multichannel emg monitoring can be carried out and requires minimal variations in anesthetic regimens . with this method , it is possible to better identify the neural structures of complex malformations , reducing the risk of surgical damage and incomplete detethering , so improving furtherly the results of surgery . retethering is a well - known cause of late deterioration ; its percentage was low in the present series . in relation to cm1 , the tonsillar descent was observed in a very low percentage of the cases ( 7% ) , despite the very low dislocation of the conus medullaris in this large series of occult spina bifida . this absence of correlation is confirmed also by the pre - operative normal position of the tonsils in all the cases with a documented intraoperative traction of the conus . no variation of the tonsilar position was obseved at mri , despite the clear cut post - operative cranial migration of the conus ( fig .  2 ) . in our opinion , both the genetic alteration found in one girl and the results of surgery on the coexisting malformations that needed separate surgery for different symptoms , orients towards the genetic origin of the rare cases of associated cm1 and tcs , rather than a simple mechanic correlation between the two .
the surgical results of this series of occult spina bifida seem better than the natural history registered in the long pre - operative period in terms of neurological deterioration . the major contribution to this result 
 is attributed to neurophysiological monitoring that lowers the risks of permanent damage and increases the percentage of effective detethering . 
 the present series of tcs , due to conus and filar lipoma , documents that cm1 is a really rare association occurring in less than 6% of the patients , despite the low position of conus . 
 the detethering procedure did not influence the tonsillar position , thus excluding the correlation between the tethering and the tonsillar descent . 
 the genetic alteration documented in a girl reinforces the hypothesis of a rare complex polymaformative picture deserving multiple procedures according to the prevailing clinical symptoms .