A 6 year old boy presents with an acute loss of ability to s…

A 6 year old boy presents with an acute loss of ability to stand. This was preceded by a dramatic onset of headache. On examination he has short stature, microcephaly and bifid right thumb. He is alert and has a power of 2/5 on his right upper limb, 5/5 on the left upper limb, 3/5 on the right lower limb and 5/5 on his left lower limb.  What is the most likely cause of his limb weaknesses?