Which of the following is true regarding congenital subglottic stenosis?

Congenital subglottic stenosis It is due to abnormal thickening of cricoid cailage or fibrous tissue seen below the vocal cords. Child may remain asymptomatic till upper respiratory infection causes dyspnoea and stridor. Cry is normal as in laryngomalacia. Diagnosis:- Diagnosis is made when subglottic diameter is less than 4 mm in full-term neonate (normal 4.5-5.5 mm) or 3 mm in premature neonate (normal 3.5 mm). Many cases of congenital stenosis improve as the larynx grows but some may require surgery. Ref:- Dhingra; pg num:-295