• FOOTDROP-EXAMINER'S FAVORITE! L5 ROOT OR PERONEAL NERVE ?

    27 September at 22:10 from atlas

    Evaluation of foot drop (FD) , or more accurately weakness or paralysis of the dorsal extensor muscles of the foot and toes is a serious Neurosurgical matter.Delay or incorrect diagnosis and thus inappropriate treatment in a functionally progressive case may detract from a better outcome for the patient.

    In a series of 303 patients with FD almost 31% were due to a peroneal neuropathy , 19.7% due to an L5 radiculopathy and other causes included limbo-scacral plexopathy etc( BalmaJ & Krivickas,L.2008)

    There are many causes for a FD and this article is not intended to be a compendium of these, but rather it is to essentially assist in the differentiation of the 2 most common causes seen in a busy Neurosurgeon's clinic: (i) L5 radiculopathy and (ii) common peroneal neuropathy.Other causes include: (i)motor cortex parasagittal brain tumour, stroke, MS,AVM etc, (ii)cord lesion , (iii) lumbosacral plexopathy including idiopathic, diabetic, infiltrative, radiotherapy etc,(iii)sciatic nerve lesion including trauma, surgery,infiltration, mono-neuritis etc ,  (v) Peripheral neuropathic/ myopathic  processes( diabetes, alcohol,drug-induced, Guillain-barre syndrome, Charcot-marie Tooth Syndrome or peroneal muscular atrophy ) , (vi) L5 radiculopathy , and (vii) peroneal nerve injury.

    For the busy Neurosurgeon (and for Surgical examination candidates!)-it is essential to be proficient in being able to deadly be able to differentiate L5 RADICULOPATHY and COMMON PERONEAL NEUROPATHY .The Neuroanatomy is complex and many clinicians can be forgiven for not remembering , as long as they do make the correct decision. In clinical practise sending the patient for urgent Neurologist consultation with NCS/EMG and MRI lumbar spine will avert a mistaken diagnosis by the surgeon. But as a Neuroanatomist I like to know the relevant way of telling these apart( and telling the Examiner at Final Year MBBS or RACS Exams-just send them to the Neurologist WILL be fraught with danger!).

    Differentiating BETWEEN an L5 radiculopathy and peroneal nerve lesion as a cause of FD can be challenging clinically even for an experienced practitioner.In so called 'obvious or classic" cases of L5 radiculopathy there will be lumbago, sciatic scoliotic list, glutalgia, posterior thigh and calf pain, alteredL5 distribution sensation and a restricted SLR and wasting of EDB .However, as we all know cases do not all present in a text book fashion, and not all patients may have all of these signs!The patient with a peroneal causation for FD may indeed also report some LBP /or non-radicular leg pain, because these symptoms are common and the patient may have an altered gait-just to further frustrate the examiner!

    One solution is to organise MR lumbar spine and Neurologist consult with electrophysiological studies-but I believe a spinal surgeon should have the Neuroanatomical skills to be reasonably capable of making the diagnosis.In an Exam situation (Final Year MBBS or RACS ) , answering the Examiner with the above solution will NOT score highly! In clinical practice, one may well make a clinical diagnosis AND still order MR, NCS, EMG & Neurologist opinion, anyway. 

    So we must go back to basics, and at times not so memorable and somewhat bland neuroanatomy!The sciatic plexus is formed from the L4-S1 ventral rami with the sciatic nerve being the largest nerve in the body.It receives contributions from L4-S3 ventral rami and exits  the sciatic plexus through the greater sciatic foramen inferior to the piriformis muscle.

    The POSTERIOR divisions of the ventral rami form the COMMON PERONEAL NERVE (CPN) and supply the short head of biceps femoris. The ANTERIOR divisions supply the 3  other hamstrings and form the TIBIAL NERVE.The SCIATIC NERVE which carries all of these divisions divides into the COMMON PERONEAL NERVE (CPN) and TIBIAL NERVE (TN) proximal to the popliteal fossa. The TIBIAL NERVE fibres run laterally in the sciatic nerve pre-bifurcation.

    The CPN becomes subcutaneous just distal to the fibular head and travels between the peroneus longs and the fibula and the divides into the DEEP PERONEAL (DP) and SUPRFICIAL PERONEAL (SP) NERVES.

    The DPN supplies TA, EDL, peroneus tertius and dorsi-flexion & toe extension. In the foot the nerve also supplies short toe extensors, EDB& EHB.It provides sensation to the inter-tarsal joints and ends as a cutaneous branch supplying the 1st web space between the 1st & 2nd toes ( Jenkins, DB.2002).

    The SPN travels between peroneus longs and brevis and innervates BOTH of these PRIMARY ANKLE EVERTORS.The peroneus tertius and EDL (supplied by DP nerve) ALSO assist with ankle EVERSION . The SPN also supplies sensation to the lateral leg and dorsum of foot& toes SPARING the small area between the 1st & 2nd toes(KimuraJ 2001).

    So, weakness of ankle dorsiflexors, toe extensors and ankle EVERTORS is suggestive of a PERONEAL NERVE lesion.Patients wit an L5 RADICULOPATHY will have similar deficits as those with a COMMON PERONEAL NERVE lesion- HOWEVER, the tibialis posterior (a primary ANKLE INVERTOR) is supplied by L5 THROUGH the TIBIAL NERVE SO WILL BE SPARED IN A COMMON PERONEAL NERVE lesion BUT IT WILL BE INVOLVEDIN AN L5 RADICULOPATHY(WITH WEAKNESS OF ANKLE INVERSION).THIS IS THE ANSWER!.

    CONCLUSIONS & GUIDES TO CLINICAL EVALUATION OF FOOTDROP.

    GUIDE1: Weakness ankle DF, toe extensors and evertors MAY BE DUE TO EITHER L5 RADICULOPATHY OR COMMOM PERONEAL NERVE LESION BUT with L5 Radiculopathy ANKLE EVERSION is weak.(The Nurologis will find EMG changes in L5 innervated TP, gluteus medius and L5 paraspinal muscles).

    GUIDE 2 : If there is altered sensation strictly defined in the 1st & 2nd toe webspace (with predominant weakness of ankle DF and toe extension than eversion, then this suggests a DEEP PERONEAL NERVE lesion over L5 Radiculopathy which usually involves a more extensive area of sensation medial and dorsal foot.

    GUIDE 3:Obtain a thorough history and meticulous neurologic examination. look for signs of 'true' sciatica, Tinel sign of CPN, assess glutalgia/hip signs, knee and ankle primary pathology,vibration & single 10&20G fibre tests,weakness of ankle inversion AND eversion , as well as DF and PF.Always assess AJs with patient kneeling on a chair and use Jendrsik's  augmentation if required,and always remember alcohol & drug side-effects.

    GUIDE 4:  Recommend MR Lumbar spine (with sagittal whole spine scout localised)including pelvis, NCS/EMG and Neurologist opinion , and maybe MR knee and foot if all tests clear. I generally request NCS/EMG/MR Lumbar and MR knee.A cousre of steroids (N.B. PU and DM) and an AFO in the interim and early surgery is the goal to obtain maximal recovery.

    GUIDE 5: Consider rare causes of peripheral neuropathy, particularly in an Exam situation!SCDs for VTE prophylaxis after recent general (or L5 radicular !) surgery is described and easily overlooked(FukadaH 2006).Bariatric surgery and other causes sudden weight loss are rare causes for a FD( Elias WJ et al J Neurosurg.2006).Gynecologic stirrup  surgery positioning  ,posterior hip dislocation, ace tabular fracture and hip osteotomy are  well known causes for FD.  Remember that leprosy (Hansen's Disease still occurs, particularly at Exam times!

    GUIDE 6 : The CPN is more likely to be involved in a peripheral nerve injury because at the level of the hip the lateral fibres of the sciatic nerve are MORE vulnerable and these form the CPN, with tethering of the CPN at the fibular head by periosteum and the larger size of the CPN funiculi rendering it more susceptible.

    GUIDE7 : The usual cause with proven Common Peroneal Nerve (CPN)  responsible foot drop at the level of the  lateral knee include : fibrous bands, exostoses,tumours,cysts, ganglions, direct laceration and compression of the nerve as it passes by tethering by the tendinous origin of the peroneus longs as the CPN winds around the fibular head and passes through the peroneal tunnel(well described by McCrory P et al.Sports Med 2002)to divide over the fibular neck into Deep Peroneal Nerve(DPN) and the Supreficial Peroneal Nerve (SPN).

    So, weakness of ankle dorsiflexors, toe extensors and ankle EVERTORS is suggestive of a PERONEAL NERVE lesion.Patients wit an L5 RADICULOPATHY will have similar deficits as those with a COMMON PERONEAL NERVE lesion- HOWEVER, the tibialis posterior (a primary ANKLE INVERTOR) is supplied by L5 THROUGH the TIBIAL NERVE SO WILL BE SPARED IN A COMMON PERONEAL NERVE lesion BUT IT WILL BE INVOLVEDIN AN L5 RADICULOPATHY(WITH WEAKNESS OF ANKLE INVERSION).THIS IS THE ANSWER!

    ASSISTANT PROFESSOR MICHAEL CORONES CIME MASE

    FAIM FRACS FACS FRCS(EDIN)SN FRCSI MB BS(1st CLASS HONOURS)

    Please always follow your doctor's advice.