Differential Diagnosis in Neurology

9.1. Introduction

• General Features:
  • Lumbar plexus lesions demonstrate various combinations of deficits in the iliohypogastric, ilioinguinal, genitofemoral, femoral and obturator nerve distributions
  • Weakness is seen in hip flexion, knee extension (concomitant inability to lock the leg on standing), and adduction of the thigh. Sensory loss is noted in the lower abdominal wall, inguinal, labial and scrotal areas as well as the thigh (except posteriorly) and medial lower leg
  • Absent or depressed knee jerk (quadriceps muscle) is affected
  • Sacral plexus lesions cause deficits within the innervation of the gluteal, sciatic, tibial and peroneal nerve distributions. Weakness occurs in hip extension and abduction, knee flexion and all intrinsic foot musculature. Sensory loss occurs in the posterior thigh, anterior and posterior aspects of the lower leg below the knee and most of the foot. There are diminished or absent AJs (sciatic nerve). Gluteal and sciatic nerve weakness localizes a lesion to the sacral plexus.
  • Pain is a prominent symptom; hip (L5); buttock L5, S1; proximal thigh laterally (L5); medial thigh, ilioinguinal nerve, L1, L2 roots; dorsal thigh (recurrent nerve of Spurling), the dural innervation of L5 as well as L1, L2, L3; groin S1 as well as T12, L1; top of the foot L5; sole S1.
  • Straight leg raising test causes sciatic nerve pain; reverse straight leg raising test stretches the femoral nerve (L2–L3 sensory roots are involved)
  • Valsalva maneuvers often do not elicit pain that is common with radicular lesions; minimal back pain is noted with lumbosacral plexus lesions.
  • Peroneal neuropathy with foot drop:
    • Inversion of the foot is normal
    • Toe flexion and hip abduction are normal
  • Differential signs to distinguish lumbosacral trunk lesion from a L5 radiculopathy:
    • Lumbar trunk formed primarily from L5 root with a contribution from L4; peroneal sensation is normal which favors a trunk lesion
  • Patterns of weakness is hip adductors, iliopsoas and quadriceps distinguishes a lumbar plexopathy from a femoral or obturator mononeuropathy
  • Paraspinal muscles (innervated by the primary rami) of the dorsal roots are not involved in an L5 plexopathy
  • Simultaneous involvement of the lumbar and sacral plexus is usual with external trauma; iatrogenic injury may involve individual L5 plexus components.

Anatomical relationships of lumbar and sacral plexus

• L1, L2 L3 root ventral rami are the primary roots of the lumbar plexus; contributions from T11 and T12
• They traverse the posterior portion of the psoas muscle anterior to the vertebral transverse processes:
  • Femoral nerve L2–L4 (primary roots)
    • Medial and intermediate nerve of the thigh (sensory)
    • Saphenous nerve of medial calf (sensory)
  • Obturator nerve (L2–L4)
    • Adductor muscles of the thigh
    • Medial thigh cutaneous innervation
• Muscular branches that derive directly from the plexus:
  • Psoas major (L2–L3); iliacus (L2–L3)
• Sensory nerves of the lumbar plexus:
  • Iliohypogastric (L1)
    • Anterior and lateral lower abdominal wall
  • Ilioinguinal (L1)
    • Upper medial thigh
    • Base of the penis; labia majora
  • Genitofemoral nerve (L1–L2)
    • Upper anterior thigh
    • Scrotum; labia majora
  • Lateral cutaneous nerve of thigh (L2, L3)

Sacral Plexus

• S1–S3 ventral rami are the major roots
• There is a contribution from L4–L5 and S4–S5
• Over lies the lateral sacrum and the posterior lateral pelvic wall
• Sciatic nerve (L4, L5, S1–S3)
  • Innervates hamstrings; all muscles below the knee
  • All sensation below the knee except that supplied by the saphenous nerve (medial lower leg)
  • At sciatic notch divides into common peroneal and tibial nerves
• Superior gluteal nerve (L4, L5, S1)
  • Innervates gluteus medius and minimus muscles
• Inferior gluteal nerve (L5, S1, S2)
  • Innervates gluteus maximus muscle
• Posterior femoral cutaneous (S1, S3)
  • Innervates the buttocks, perineum, posterior thigh
    • Cluneal branch innervates posterior upper thigh and inferior buttock

Differential Diagnostic Features between Root and Lumbosacral Plexus Lesions

• Positive mechanical signs favor a root lesion:
  • Straight leg raising test; reverse SLR test (traction on the femoral nerve)
  • Valsalva maneuver
  • May be positive in tumor
  • Aneurysm
  • Disc protrusion
  • Stenosis, spondylosis, spondylolysis and spondylolisthesis
• Warm, dry red foot:
  • Indicative of plexus lesions
  • Involvement of retroperitoneal lumbar sympathetic nerves (anterolateral aspects of vertebral bodies)
• Proximal leg muscle weakness > distal suggests plexus lesion
• Gluteus muscle innervation arises directly from the plexus
• Iliopsoas muscle is not involved in an obturator or femoral nerve lesion because its innervation is from the plexus directly

Lumbosacral Plexus: Areas of Injury

• Susceptible to trauma of the pelvic ring:
  • Disrupted by double fracture dislocations
  • Traction injury from dislocation of the hip joint
• Femoral nerve compressed due to position:
  • Occupies the gutter between the psoas and iliopsoas muscle above the inguinal ligament:
    • Surgical retractors (medially)
    • Injured laterally by hematoma between the iliacus fascia and the nerve
• Lumbosacral cord (of the plexus) vulnerable to compression at the:
  • Pelvic brim by the fetal head
  • Obstetric forceps
  • Aneurysm of the common iliac or hypogastric arteries in the presacral areas
• Femoral nerve compressed:
  • Angulation under the inguinal ligament
  • Prolonged flexion abduction of the thighs (dorsal lithotomy position under anesthesia)
• Fixation points of the common peroneal nerve are at the sciatic notch and fibular neck
  • More vulnerable to traction injury