Lumbosacral Plexopathy


This is a clinical diagnosis supported by neurophysiological (electrophysiology NCS/EMG) tests

Clinical features:

Weakness and sensory loss in lumbosacral distribution that spares the paraspinal muscles and is usually asymmetric in a distribution localizing to the plexus.

Lumbar plexopathy:
  • Loss of knee reflex
  • Weakness: hip flexors, hip adductors, knee extensors,
  • Reverse straight leg sign may occur: pain in anterior thigh with thigh hyperextension
Sacral plexopathy:
  • Loss of ankle reflex
  • Weakness of: hip extensors, hip abductors, ankle plantar flexors
  • S2,3 sensory loss
  • Lasegue’s sign: pain in posterior thigh during straight leg raise

Findings on investigations:

Electrophysiology NCS/EMG:


  • More sensitive than CMAP
  • Normal conduction velocity and distal latency
  • Decreased amplitude in affected nerve (may be normal initially).


  • Indicates more severe injury
  • Decreased amplitude (however normal side to side differences are common, >50% difference is significant)

F-wave: nonspecific
H-reflex: not helpful

  • Fibrillations and Positive Sharp Waves PSW in denervated muscles.
  • Iliopsoas must be evaluated for lumbar plexus
  • Gluteal muscles must be evaluated for sacral plexus
  • If reinnervation has occurred: MUAP shows decreased recruitment, long duration, increased amplitude, polyphasia
  • Paraspinal muscles are normal (dorsal rami supply these, distinguishes this form radiculopathies)

Pattern by nerve:

Femoral nerve:

SNAP: Decreased amplitude in femoral nerve

  • involved muscles, quadriceps, sartorius, iliacus, pectinous
  • Spared muscles: paraspinal muscles
Obturator nerve:

SNAP: Decreased amplitude in obturator nerve

  • Involved muscles, adductor longus, adductor brevis, adductor magnus (dual supply), gracilis, obturator internis
  • Spared muscles: paraspinal muscles
Superior gluteal nerve (pure motor):


  • Involved muscles, gluteus medius and gluteus minimus, tensor fascia lata
  • Spared muscles: paraspinal muscles
Inferior gluteal nerve (pure motor):


  • Involved muscles, gluteus maximus
  • Spared muscles: paraspinal muscles
Sciatic nerve (peroneal):

SNAP: decreased amplitude in peroneal nerve

  • involved muscles, short head of biceps femoris, tibialis anterior, extensor digitorum brevis EDB, peroneus teritus, brevis and longus
  • Spared muscles: paraspinal muscles
Sciatic nerve (tibial):

SNAP: decreased amplitude in tibial nerve

  • Involved muscles, long head of biceps femoris, semitendinosus, semimembranosus, adductor magnus (dual supply), plantaris, popliteus, gastrocnemius, soleus, tibialis posterior, flexor digitorum longus, flexor hallucis longus
  • Spared muscles: paraspinal muscles

Investigations to consider:

Fasting blood glucose, HbA1c
HIV serology
CT abdomen and pelvis: retroperitoneal tumour or haematoma
MRI lumbosacral plexus: tumour infiltration
CTA abdomen: abdominal aortic aneursym
NCS/EMG: rule out radiculopathy
CSF analysis: raised protein in some cases e.g. diabetic amyotrophy

Causes of lumbosacral plexopathy:

Diabetic amyotrophy
Idiopathic Lumbosacral plexitis a.k.a. lumbosacral plexitis a.k.a. idiopathic neuralgic amyotrophy a.k.a. Lumbosacral plexopathy
HIV lumbosacral plexopathy
Compressive/trauma/radiation injury:

  • Retroperitoneal hematoma
  • Retroperitoneal tumor
  • Radiation lumbosacral plexopathy
  • Abdominal aortic aneurysm


Related articles: