Ocular Myasthenia Gravis (OMG)
🔑 Key concept
- Autoimmune
NMJ disorder →
fatigable weakness
- จำกัดเฉพาะ:
- levator
palpebrae →
ptosis
- extraocular
muscles →
diplopia
- 50% เริ่มด้วย ocular symptoms
- ~15% เป็น ocular only ตลอด
🧬 Pathophysiology
- antibody-mediated:
- AChR-Ab
(main)
- MuSK
/ LRP4 (บางราย)
- NMJ
transmission ↓ → fatigability
👁️ Clinical features
(high-yield)
✔️ Triad
- Ptosis
- Ophthalmoparesis
→ diplopia
- Orbicularis
oculi weakness
✔️ Key characteristics
- Fluctuating
+ fatigable
- แย่:
- ตอนเย็น
- ใช้งานนาน (driving, reading)
- ดี:
- หลังพัก / ตอนเช้า
✔️ Ptosis clues
- unilateral
/ asymmetric
- alternating
ptosis → highly
suggestive
- signs:
- Cogan
lid twitch
- fatigability
with upgaze
- improvement
after rest
✔️ Diplopia
- binocular
- pattern:
- variable
(horizontal / vertical / oblique)
- hallmark:
- เปลี่ยน pattern ได้ (inconsistent)
❗ Important
- ❌
pupil ปกติ (no anisocoria)
→ ถ้ามี pupil involvement → คิด diagnosis อื่น
⚠️ Red flags (ไม่ใช่
OMG)
- pupil
abnormal → CN III /
compressive lesion
- fixed
deficit (ไม่ fluctuate)
- sensory
deficit / CN อื่น → brainstem lesion
⚖️ Differential diagnosis
|
Condition |
Clue |
|
CN III/IV/VI palsy |
pattern fixed |
|
Thyroid eye disease |
proptosis, lid retraction |
|
CPEO |
slow progression, no fluctuation |
|
Myopathy |
symmetric |
|
Brainstem lesion |
neuro deficit |
**CPEO: Chronic progressive external ophthalmoplegia
🧪 Diagnosis
✔️ Bedside
- Ice
pack test
- sensitivity
~80%
- specificity
~100%
✔️ Lab
- AChR-Ab
- sensitivity
OMG: ~45–60%
- specificity:
~100%
✔️ Electrophysiology
- SFEMG
→ most
sensitive (63–100%)
- RNS → low sensitivity in OMG
✔️ Additional workup
- CT
chest → thymoma
- Thyroid
function
- MRI → ถ้า atypical
📈 Risk of generalization
- ~2/3
→ กลายเป็น
generalized MG
- 78% ภายใน 1 ปี
💊 Management
✔️ Symptomatic
- eye
patch / prism
- ptosis
tape
✔️ First-line drug
- Pyridostigmine
- effect
limited (esp diplopia)
✔️ Immunotherapy
- Prednisone
→ effective
(~70%)
- steroid-sparing:
- azathioprine
- mycophenolate
✔️ Severe cases
- IVIG
/ plasmapheresis (rare in OMG)
✔️ Thymectomy
- indicated:
- thymoma
- consider:
- refractory
AChR-Ab positive
📈 Prognosis
- fluctuating
course
- ~10%
spontaneous remission
- risk
progression → GMG
🧾 Clinical pearls
- Fluctuating
+ variable pattern diplopia = OMG จนพิสูจน์ว่าไม่ใช่
- alternating
ptosis = clue สำคัญ
- pupil
ปกติ → แยกจาก CN III palsy
- ice
pack test → bedside
tool ที่ดีที่สุด
🧠 One-line summary
👉 “Fluctuating ptosis
+ variable diplopia without pupil involvement → think ocular myasthenia gravis”
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