Woman-in-a-barrel ?

I need your help in a Case:

27 yo woman with irrelevant past medical history
She came to the ER because of weakness in both arms since 6 days.
She refers discomfort in the cervical region in the beggining but not now
Doesn't have a history of upper respiratory tract infections or gastrointestinal infections or recent vaccination.
The Neurological exam reveals:
Normal MM- Folstein
Cranial Nerves Normal (All)
Severe bilateral weakness of the arms ( brachial 1/5  ante-brachial 1/5  hands 2/5 - Lucile-Daniels Scale), The estiloradial reflex is present in both arms, but others are disminished (but not absent). The sensitivity, propioception and vibration are normal). No atrofia was observed
The strength of the legs is normal, and also the rest of the neurological exploration

Imaging
Routina radiography of the cervical spine is normal
Cervical MRI - Normal (Revised by me, one neuroradiologist, and one neurosurgeon)
Cerebral MRI - Normal
Electrodiagnostic:
Sensorial Neuroconduction is normal in the median - radial and cubital nerves bilateral
Motor neuroconduction of the nerves median - radial - cubial - musculocutaneous and axilar shows prolongued latencies, low voltage and disminished conduction velocity.
The study was interpreted as a motorn neuropathy afecting C5-C6 myotomas (predominantely)

LCR analysis - normal (proteins, glucose, acelular, oppening pressure), Negative Gram stain, Negative Indian Ink, Negative Culture, VDRL was also negative
Thyroid test (TSH, free-T4), CPK level, Rheumatoid Factor, Antinuclear antibodies, anti-DNAds, ELISA-HIV, sodium, potasium, calcium All Normal or Negative

This woman is presenting as a Man (Woman) in-a-barrel syndrome (bilateral brachial diplegia).
But until know I have not found the etiology
According to literature we have rule out cerebral and spinal (cervial) or brainstem lesions (like watersheds infarcts, pontine myelinolysis, transverse myelitis)
The patient is otherwise in very good health. She continues until today in the hospital with no other signs or symptoms only the bilateral diplegia that until is unexplained

Can anybody please help me with this case?
Thanks in advances