What is the name of the condition in which the neck cannot support the head? Is it curable?

Neck so weak, it cannot support the head sufficiently. I Saw this condition in a baby of about two years old.

What is the name of the condition in which the neck cannot support the head? Is it curable?
One possible cause is Myasthenia Gravis, a neurological condition where the chemical message to the muscle from the neuron (the nerve cell from the brain) is interrupted at the synapse (the tiny space at the junction between the two). This means that the muscle doesn't get a strong enough order to move, or in this case, to be tense enough to hold the head up. MG can happen at any age, but it is rarer in young children, so this is just a long shot possible explanation.
Reply:I am sure there are several conditions that cause this. One is hypotonia.