MRI God Fails Again – Peroneal Neuropathy

Peroneal neuropathy is a symptom complex of shin weakness, atrophy and foot drop, and knee pain. A common cause is a cyst in the nerve as it wraps around fibula. However, this will be missed in 64% of MRI negative cases but seen later. Another example how MRI God is not infallible.

Wilson, Thomas J., et al.
Recognition of peroneal intraneural ganglia in an historical cohort with “negative” MRIs.” Acta Neurochirurgica (2017): 1-6.
http://link.springer.com/article/10.1007%2Fs00701-017-3130-3

Anterior view of Peroneal Nerve wrapping around knee at fibula:

  • Common Peroneal nerve comes from L4, L5, S1, and S2
  • It divides into superficial and deep peroneal nerves after wrapping about fibula
  • The superficial Peroneal Nerve innervates the peroneus longus and peroneus
    brevis muscles and sensory innervation to the lateral lower leg and the dorsum of the foot
  • The deep gives motor innervation to the foot and toe dorsiflexors, including the tibialis anterior , extensor digitorum longus , extensor hallucis longus, peroneus tertius, and extensor digitorum brevis  muscle.
  • The deep courses 3 to 4 cm along the front cortex of the fibula and then goes distally. It then goes  just anterior and medial to the intermuscular septum “between the anterior and lateral compartments as it travels with the anterior
    tibial artery.”
  • Cyst usual originate from superior tibio-fibular joint
  • This Mayo clinic study followed 11 cases with negative MRIs and found 6 of them showed cysts compression later
  • 29% with cyst had knee pain; none without cyst had it
  • with cyst, ” 71% had deep peroneal-predominant weakness, 57% had tibialis
    anterior-predominant denervation/atrophy on imaging, and 100% had degenerative changes of the Superior tibio-fibular joint”
  • 5/6 with cyst had surgical removal and improvement in condition

Comment – I am amazed at how patients and clinicians put all there faith in MRI without realizing they are fallible. Pain doesn’t show up on MRI so sources are often missed. Without a cyst, leg crossing is a common way to crush nerve and end up with foot drop.

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