Hello, the symptoms you described after L4-L5 fusion - loss of balance, unable to stand without support, leg trembling, loss of sensation in big toe and tingling - are not normal post-op symptoms. This needs urgent evaluation by her operating spine surgeon. Gabapentin can cause dizziness / imbalance in some patients. Please do not start any new medicine on your own.
Next Steps
Please contact her operating surgeon immediately / visit ER today for clinical examination.
Health Tips
Avoid walking without support till evaluated and keep wound clean. Any new weakness or numbness after spine surgery should be seen as emergency.