Thread by @AaronLBerkowitz

I had no idea how popular #neuroanatomy #tweetorials would be! Thanks for the❤️!

Here's innervation of the leg/foot & approach to FOOT DROP

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Just like you can localize most peripheral upper extremity weakness by just knowing 5 nerves (see prior tweetorial), you can localize most peripheral lower extremity weakness by just knowing 5 nerves!

Just 5 nerves for each extremity- no neurophobia needed!

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Which 5?

Femoral
Obturator
Sciatic
Peroneal
Tibial

And the sciatic is really just the peroneal and tibial bound together!

And the leg/foot are much easier than the hand because the foot does much less intricate movements than the hand!

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The FEMORAL nerve runs from the lumbar plexus in the pelvis under the inguinal canal to the leg and innervates just TWO muscles, both involved with the FEMUR (femoral/femur):

- Iliopsoas (flexes leg at hip)
- Quadriceps (extends knee--so it supports the patella reflex)

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FEMORAL also does sensation over anterior/medial thigh and medial leg foot by way of saphenous (mnemonic saPHenous from Femoral)

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So a FEMORAL neuropathy can cause

Hip flexion weakness and knee extension weakness including diminished/lost knee reflex

(rarely isolated knee extension weakness if affected in inguinal canal distal to innervation of psoas)

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Causes of femoral neuropathy: surgery or trauma to pelvis / hip, pelvic malignancy, femoral cath procedures, psoas hematoma

*Pearl–unilateral leg weakness in anticoagulated patient? Look for psoas hematoma with CT ab/pelvis* I've seen pts get lumbar MRI and this dx missed!

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OBTURATOR nerve innervates the adductors of the leg, and can be affected by pelvic trauma/surgery or injury during childbirth, causing isolated leg adduction weakness (i.e., difficulty moving leg toward midline)

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* Pearl *

Femoral AND obturator are both served by L2-3-4. So if either nerve affected test actions of other!

If BOTH femoral AND obturator appear affected, consider lumbosacral plexus issue or polyradiculopathy

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SCIATIC = peroneal and tibial bound together.

Sciatic runs down posterior leg, and innervates hamstrings before dividing into peroneal and tibial posterior to the knee.

Let's first talk about peroneal and tibial and then come back to sciatic

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PERONEAL and TIBIAL share innervation of the lower leg/foot:

PERONEAL: toP of foot–Dorsiflexion and Eversion (and toe extensors)

TIBIAL: Bottom of foot–Plantarflexion and Inversion (and toe flexors

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Mnemonics:

toP of foot: Peroneal
Bottom of foot: tiBial

PED: peroneal eversion/dorisflexion
TIP: tibial inversion/plantarflexion

PerOneal: uP and Out
tIbial: down and In

Or memorize one: other does the opposite movements!

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And what does it mean to evert/invert the foot?

When examining patients, I say:

Eversion = rotate ankle outward as if looking for gum on the outer sole of your shoe

Inversion = rotate ankle in as if looking for gum on your instep

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It's rare to see tibial neuropathy, but peroneal=most common mononeuropathy in the lower extremity, usually due to compression at fibular head (usually from leg crossing–did you just uncross your legs?)

This causes foot drop (weak dorsiflexion) with weak EVERSION.

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But foot drop can also be cause by more proximal lesions:

- Sciatic (which can cause loss of all peroneal/tibial function = weak dorsi/plantarflexion and e/inversion, BUT often preferentially affects JUST peroneal!)

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- L5 radiculopathy: L5 supplies eversion and dorsiflexion but also inversion, but NOT plantarflexion so you find a foot drop that looks like peroneal neuropathy PLUS inversion weakness, consider L5 radic

(L5 radic can also cause hip abduction weakness)

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But just like it's hard to assess intrinsic hand muscles in wrist drop (see prior tweetorial), it can be hard to test inversion in foot drop...So help the patient by passively dorsiflexing the foot and see if they can invert in this position!

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Here's a table I made for the different causes of foot drop and affected muscles from amazon.com/Lange-Clinical…

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Note foot drop can also be presenting sign of ALS–don't alarm your patients, it's usually from leg crossing and gets better when the patient stops crossing legs! But if you see foot drop with upper motor neuron signs in the same limb, fasciculations, etc, consider EMG/NCS

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Now what about the sciatic?

It's peroneal +tibial.

It can be injured in pelvic/hip trauma, prolonged compression of buttock (toilet seat neuropathy, chronic bed-bound state), misplaced gluteal injection.

"Sciatica" is usually from S1 compression, not sciatic compression

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Neuro geek pearl

Tibial part of sciatic does all hamstring muscles (knee flexion) except one: short head of biceps femoris done by peroneal

Remember sciatic neuropathy can preferentially affect peroneal component, mimicking peroneal neuropathy....

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The only way to distinguish is EMG of short head of biceps femoris (can't be isolated clinically).

If not affected, peroneal neuropathy at usual place: fibular head.

If affected, sciatic neuropathy!

(common RITE/ Neuro boards question!)

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Now we've been talking about leg/foot WEAKNESS, but one more useful nerve to know for lower extremity sensory loss

Lateral femoral cutaneous, which does lateral thigh sensation. Compression/injury of this nerve leads to meralgia paresthetica (outer thigh numbness)

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Confession: I can't draw lumbosacral plexus from scratch like brachial plexus. if it looks like > 1 of affected nerves of lower extremity, consider radiculopathy or plexopathy and consider EMG and/or lumbar MRI and/or CT ab/pelvis for plexus. thoughts, @CrystalYeoMDPhD ?

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