axonopathy

We have a child now 5 yrs age. He started walking at around 1 yr of age but with an abnormal gait and continued to do so till 4 yrs of age though he was required to be made to stand(he was unable to get up on his own). Since 4 yrs age he is unable to walk without support, requires support and splints and knee cage to walk i.e. progressive weakness involving the lower limbs. he has normal IQ, no h/o seizures. His investigations are:
1) Lactate, pyruvate, CPK all normal.
2) SMN and NAIP gene no deletions seen ( Bombay hospital)
3) Muscle biopsy (NIMHANS) S/O retarted maturation/arrest of the muscle fibers and compensatory change in the adjacent ones- SMA II
4) EMG NCV (Hinduja hosp) e/o diffuse motor axonopathy probably at the level of peripheral nerve.
5) ANA negative
6) Immunoglobulins normal.
7) Anti glycolipid antibody- normal
8) Hexosaminidase- normal.
9) Heavy metals lead arsenic thallium and mercury all normal.
10) SMN 1 gene (sir gangaram hospital) no deletions seen
11) repeat EMG NCV is
--B/L sural SNAPs are normal
-- B/L `H` reflexes are absent
--B/L tibials and left peroneal nerve are inexcitable.
--right peroneal nerve shows mild increase in distal latency with significant drop in CMAP amplitude as well as moderate reduction in conduction velocity and absent `F` waves
--reduction in CMAP amplitude of right median nerve.

---impresionE/O predominantly AXONOPATHY affecting predominantly the motor fibres bilaterally.


on examination-
there is no hypertrophy of the muscles.
fasciculation is seen in the tongue. reflexes in the lower limbs are absent.

what could be the diagnosis?
is it possible to have sma despite the SMN gene being normal?
i want to know more on axonopathy and its treatment.