Viral myositis

Thanks to Evie for presenting! Today we had an excellent case about a young patient coming in with acute-subacute symmetric bilateral proximal muscle weakness and paresthesias after a recent URI found to have most likely a viral myositis.

A few learning points:

1) The differential diagnosis for weakness can be broken down into 4 main groups: CNS, PNS/neuromuscular, oxygen delivery, and other.

2) Early UMN lesions may present with hyporeflexia.

3) EMG is a great modality to use to help sort out the differential for neuromuscular conditions. It can be used to localize the lesion [anterior horn vs nerve (root or axonal or demyelinating) vs NMJ vs muscle] as well as determine chronicity.

Want to know more?

Ddx Weakness:

1) CNS

  • a. CNS lesions
  • b. Spinal cord – MS/TM/ADEM, epidural compression, vascular

2) PNS

  • a. Anterior horn – ALS is the classic (also polio, west nile, lead)
  • b. Peripheral nerve
    • i. mononeuropathy & plexopathy eg. carpal tunnel
    • ii. mononeuritis multiplex eg. vasculitis
    • iii. polyneuropathy
      1. axonal eg. DM (also B12, thyroid, toxins, infectious, vasculitis, paraproteinemic)
      2. demyelinating eg. AIDP (GBS)/ CIDP (also CMT)
  • c. NMJ – Myesthenia gravis, Lambert-Eaton are classic (also west nile)
  • d. Muscle/ myopathy – can be separated into inflammatory (eg DM/PM), infectious, endocrine, toxic, metabolic/electrolytes, paraneoplastic, critical illness

3) Oxygen delivery

  • a. Remember DO2 = CO x Hgb x O2 sat so this would include cardiovascular, pulmonary, anemia etc.

4) Other

  • a. Psych

Localization of PNS lesions: Here is a great PDF created by former UCSF chiefs that breaks down using the exam and EMG to determine etiology.

Transverse myelitis: An illness script for transverse myelitis is a young patient coming in with acute-subacute symmetric motor, sensory and autonomic dysfunction usually with a clearly defined sensory level. Etiologies include 1) idiopathic 2) infectious or inflammatory 3) toxic-metabolic and 4) MS-NMO related condition. The usual workup includes MRI w gadolinium contrast and LP (looking for CSF pleocytosis or elevated IgG index). Brain imaging, and visual evoked potentials are needed to distinguish TM from MS-NMO-ADEM. The TM consortium workgroup has a nice paper about this.

Viral myositis: An illness script for viral myositis is a person coming in with a recent viral prodrome (days-2 weeks) found to have elevated CK. Some culprits include: flu, coxsackie, EBV, HSV, paraflu, adeno, echo, CMV, measles, VZV, HIV, and dengue.