⚡ Skeletal muscle fiber types
| Feature | Type I (slow) | Type II (fast) |
|---|---|---|
| Twitch | Slow | Fast |
| Primary function | Postural, sustained tension | Purposeful movement, quick bursts |
| Metabolism | Aerobic (fatty acid oxidation) | Anaerobic glycolysis |
| Energy substrate | Fatty acids | Glycogen |
| Mitochondria | Numerous | Few |
| Color | Red (high myoglobin) | White (low myoglobin) |
| Fatigue | Resistant | Rapid |
- Fiber type is determined by innervation pattern
- Type I fibers dominate in antigravity muscles (soleus, erector spinae)
- Type II fibers predominate in extraocular and hand muscles
- Conversion between types occurs with chronic low‑frequency stimulation
High‑yield: muscle biopsy shows fiber type grouping in neurogenic atrophy
🔥 Inflammatory myopathies
Polymyositis
- Autoimmune, adults
- Bilateral proximal weakness
- Endomysial CD8+ T‑cell infiltrate
- Fiber degeneration + regeneration
- Responds to immunosuppression
Dermatomyositis
- Connective tissue disorder (any age)
- Proximal weakness + heliotrope rash (eyelids)
- Periorbital edema
- Perimysial & perivascular inflammation
- Perifascicular atrophy
- ↑ risk: lung, colon, breast, ovarian cancer
Inclusion body myositis
- Onset >50 years, slowly progressive
- Asymmetrical distal weakness
- Autophagic vacuoles + inclusion bodies
- Refractory to immunosuppression
Anti‑tRNA synthetase (anti‑Jo‑1) is highly specific for inflammatory myopathies
🧬 Myasthenic syndromes
Myasthenia gravis
- Autoantibodies vs ACh receptor
- Facial & extraocular weakness
- Ptosis, diplopia
- Worsens with repeated contraction
- Associated: thymic hyperplasia / thymoma
- Tx: anticholinesterases, steroids, thymectomy
Lambert‑Eaton
- Antibodies vs presynaptic Ca²⁺ channels
- Dry mouth, proximal weakness
- Often paraneoplastic (small cell lung cancer)
- Tx: immunotherapy + treat underlying cancer
MG: respiratory failure is a leading cause of death
LEMS: weakness improves with repeated contraction (post‑tetanic potentiation)
🦵 Muscular dystrophies
Duchenne (X‑linked)
- Dystrophin gene (Xp21) – absent protein
- Onset by age 5, rapid progression
- Calf pseudohypertrophy
- Proximal girdle weakness
- Cardiac involvement (arrhythmia, failure)
- Respiratory insufficiency
- ↑ CK, muscle biopsy: variable fiber size, necrosis, fibrosis
- Gower sign (climbing up legs)
Becker (X‑linked)
- Altered (partially functional) dystrophin
- Later onset, slower progression
- Less severe than Duchenne
- Cardiac involvement rare
- Near‑normal lifespan
Dystrophin defect →
sarcolemmal instability →
fiber necrosis →
fatty / fibrotic replacement
Duchenne: elevated CK from birth, diagnosis confirmed by genetic testing or muscle immunostain
🧠 Peripheral nerve disorders
Diabetic neuropathy
- Most common neuropathy in diabetes
- Distal symmetric polyneuropathy (lower → upper)
- Pain, numbness, ulceration risk
- Skin biopsy + neurologic testing
Guillain‑Barré syndrome
- Autoimmune demyelination (Schwann cells)
- Preceded by viral infection
- Ascending paralysis, areflexia
- CSF: elevated protein (albuminocytologic dissociation)
- Nerve conduction: demyelinating pattern
- 5% fatal due to respiratory paralysis
- Tx: plasmapheresis / IVIG
GBS: weakness peaks within 4 weeks; autonomic instability may occur
🔬 Soft tissue & nerve sheath tumors
Lipoma
- Benign, most common soft tissue tumor
- Mature fat, subcutaneous trunk/extremity
Liposarcoma
- Malignant, most common adult sarcoma
- Thigh/retroperitoneum, lipoblasts
- Well‑diff: hyperchromatic cells, multivacuolated lipoblasts
- Recurrence in retroperitoneum
Dermatofibroma
- Benign dermal spindle cells
- Small red nodule, tender, mobile
Fibrosarcoma
- Malignant, thigh/limb
- Spindle cells, herringbone pattern
- Hematogenous spread (lung)
Undiff. pleomorphic sarcoma
- Older adults, extremity/retroperitoneum
- Storiform pattern, aggressive
Leiomyoma / leiomyosarcoma
- Benign: uterus/GI, rare in soft tissue
- Malignant: retroperitoneum, older women
- “Cigar‑shaped” nuclei, trichrome striations
Synovial sarcoma
- Young adults, knee
- Biphasic (epithelial + spindle) or monophasic
- Calcification common
Schwannoma
- Encapsulated, Antoni A/B areas
- Associated with NF2
Neurofibroma
- Non‑encapsulated, diffuse/plexiform
- Associated with NF1
Malignant PNST
- Young adults, major nerve trunks
- Resembles fibrosarcoma
- Recurrence + metastases common
Fibromatosis: non‑neoplastic, infiltrates muscle; deep desmoids associated with Gardner syndrome
💎 Clinical pearls & exam traps
- Dermatomyositis: adult onset = paraneoplastic workup (lung, colon, breast, ovary)
- Inclusion body myositis: refractory to steroids — don't mistake for polymyositis
- MG: anti‑AChR antibodies; thymoma in 10–15%
- LEMS: associated with small cell lung cancer — smoking history is key
- Duchenne: Gower sign, pseudohypertrophy, early death from respiratory/cardiac failure
- Becker: later onset, milder, preserved ambulation into adulthood
- GBS: albuminocytologic dissociation (CSF protein ↑, cells normal)
- Liposarcoma vs lipoma: lipoblasts, fibrous bands, larger size
- Synovial sarcoma: biphasic pattern, calcification on imaging
- NF1: neurofibromas, café‑au‑lait; NF2: schwannomas (bilateral vestibular)
Never use “anti‑Jo‑1” as a mnemonic — it is a specific autoantibody; recall its clinical utility