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Charcot-Marie-Tooth (CMT) Disease - Comparative Summary

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Table 1: CMT1 - Demyelinating Forms (Autosomal Dominant)

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Table 2: CMT2 - Axonal Forms (Mostly Autosomal Dominant)

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Table 3: CMTX - X-Linked Forms

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Table 4: CMT4 - Autosomal Recessive Demyelinating Forms

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Table 5: Dejerine-Sottas Disease (DSD / Former CMT3) and HNPP

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NCS Classification Summary

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Key Skeletal Abnormalities Across CMT Types

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• Bradley and Daroff's Neurology in Clinical Practice, Ch. 106
• Goldman-Cecil Medicine International Edition, Ch. 388
• Campbell's Operative Orthopaedics 15th Ed 2026, Ch. 15 & 18

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Why Pes Cavus and Hammer/Claw Toes Develop in CMT

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1. The Fundamental Principle - Selective Muscle Weakness

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2. Mechanism of Pes Cavus (High Arch)

Step 1 - First Ray Plantarflexion (Forefoot Equinus)

• Peroneus longus (plantarflexes the 1st metatarsal) is strong, but tibialis anterior (dorsiflexes the 1st ray) is weak.
• The peroneus longus overpowers, pulling the first metatarsal head down - depressing the 1st ray into plantarflexion.
• This creates a pronated forefoot with the first ray lower than the lateral rays.

Step 2 - Long Extensors Recruited for Dorsiflexion

• Because the tibialis anterior is weak, the patient recruits the extensor hallucis longus (EHL) and extensor digitorum longus (EDL) to help dorsiflex the foot during the swing phase of gait.
• These long extensors cross the MTP joints and hyperextend the MTP joints of the toes while they pull up the forefoot.
• This repetitive recruitment accelerates MTP hyperextension and shortens the plantar fascia.

Step 3 - Plantar Fascia Contracture

• The drop of the first metatarsal head combined with long extensor recruitment causes a progressive shortening of the plantar fascia (the "windlass mechanism" in reverse).
• A shortened plantar fascia pulls the calcaneus anteriorly toward the metatarsal heads, further elevating the arch - true cavus.

Step 4 - Hindfoot Varus (Completing the Cavovarus)

• The forefoot is now rigidly pronated (supinated in some descriptions - first ray down, fifth ray up).
• With weight-bearing, the rigid pronated forefoot forces the hindfoot into compensatory varus to achieve a plantigrade foot.
• This is the "tripod" mechanism: weight is borne on the heel, 1st metatarsal head, and 5th metatarsal head only.
• Tibialis posterior (hindfoot invertor) is also relatively strong, compounding hindfoot varus.

"The neuropathic cavovarus deformity of CMT disease is caused by a combination of intrinsic and extrinsic weakness, beginning with weakness of the intrinsic foot muscles and the anterior tibial muscle, with normal strength of the posterior tibial and peroneus longus muscles."

• Campbell's Operative Orthopaedics, 15th Ed.

"Strong peroneus longus and posterior tibialis overpower tibialis anterior and peroneus brevis, resulting in hindfoot varus and a depressed first metatarsal head."

• Miller's Review of Orthopaedics, 9th Ed.

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3. Mechanism of Hammer Toes and Claw Toes

The Normal Intrinsic Balance

• Flex the MTP joint (pull the proximal phalanx down)
• Extend the IP joints (straighten the toe at the middle and end joints)

When Intrinsics Are Lost - The "Intrinsic Minus" Toe

• MTP joint: hyperextended (toe base pulled up toward dorsum)
• PIP joint: flexed (middle knuckle bent down)
• DIP joint: flexed in claw toe (tip bent down) or neutral/extended in hammer toe

"Loss of intrinsic function of the foot leads to an imbalance, allowing the extensor digitorum longus to extend the MTP joint and the flexor digitorum longus to flex the IP joints. When an extension posture of the MTP joint develops, the long extensor loses its excursion and no longer can extend the IP joints. The powerful flexors of the toe accentuate the deformity, causing flexion of the IP joints."

• Campbell's Operative Orthopaedics, 15th Ed.

Hammer Toe vs. Claw Toe - Distinction

"Claw-toe deformity results from dysfunction of the intrinsic musculature (imbalance relative to extrinsics). Often a neurogenic etiology (i.e., CMT or peripheral neuropathy)."

• Miller's Review of Orthopaedics, 9th Ed.

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4. How Cavus and Clawing Reinforce Each Other

1. High arch raises the metatarsal heads relative to the toes
2. This puts the MTP joint in a hyperextended posture even at rest
3. EDL is now always under stretch tension, accentuating MTP hyperextension
4. FDL is always pulling the toes into flexion
5. Toes claw further, pushing the metatarsal heads plantarward
6. Weight bearing on metatarsal heads worsens the arch further

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5. Why the Deformity Becomes Rigid Over Time

• Initially the foot is flexible and plantigrade with weight-bearing (correctable).
• Over years, the plantar fascia shortens and contracts permanently.
• Ligaments and joint capsules adapt to the abnormal position.
• Bone remodelling occurs (e.g., calcaneal pitch increases, metatarsal heads develop subchondral changes).
• The deformity transitions from flexible → semi-rigid → rigid, which is why early intervention is preferred.

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• Campbell's Operative Orthopaedics 15th Ed 2026, Chapter 37 (Cavovarus Foot Deformity in CMT)
• Miller's Review of Orthopaedics 9th Edition, Section 3 & 6
• Bradley and Daroff's Neurology in Clinical Practice, Chapter 106