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. Author manuscript; available in PMC: 2021 Jun 7.
Published in final edited form as: Am J Med Genet C Semin Med Genet. 2020 Jun 7;184(2):428–443. doi: 10.1002/ajmg.c.31807

TABLE 1.

Summary of neurodevelopmental, medical, and gonadal function in SCT described in the 1970’s birth cohorts

XXY (Klinefelter syndrome) N = 95 XYY (Jacobs syndrome) N = 59 XXX (triple X) N = 46
Neurodevelopmental Early developmental delays 1–7 70%
Usually mild, speech delay>motor		 80%
Usually mild, speech del>motor		 55%
Usually mild, speech del>motor
Learning disabilities 4,5,8–11 64–85% (esp. reading) ∼55% 75–100% (esp. reading)
Mean cognitive (IQ) 12–18 10–15 points lower than normal;
Verbal < nonverbal		 5–10 points lower than normal; verbal usually = nonverbal 10–15 points lower than normal; verbal < nonverbal
Behavior/Social-emotional, 13,15,17–22 Shy, social difficulties; immature, attentional problems Hyperactivity; negative mood; impulsivity 2 Shy, anxiety, social difficulties; sensory integration problems
Motor skills 6,7,13 Motor delays, coordination and motor planning problems Increased rate of balance and coordination problems 1, 12 Motor delays, coordination problems, low strength
Physical/medical Average birth size 1,2,3,23 Slightly smaller Normal Smaller
Congenital anomalies 1,2,3,13 Modest increase Rare Rare
Dysmorphisms 1,2,3,13 Minimal: Hypertelorism, epicanthal folds, clinodactyly, small head circumference No dysmorphisms reported normal head circumference Minimal: Hypertelorism, epicanthal folds, clinodactyly, small head circumference
Growth and body habitus 1,13–15,22,23 Tall stature; long legs increased growth velocity starting at 5yo; excess weight gain Tall stature, long legs thinner than XXY delayed growth spurt 1 Tall stature; increased growth velocity starting at 7yo; abdominal pain (25%)
Muscle tone 1–3,6,7 Hypotonia Hypotonia Hypotonia
Gonadal function Prepubertal gonadal function 1–3,13,17,22,24,25,26 Cryptorchidism in 10–20%; small testes <0.5ml in 65% at 6m; slow penile growth; T under assay detection limit; bone age delayed (−2 SD) Testes and penile size in the normal range; Testo concentrations normal Nothing reported in infancy, high FSH in mid-childhood; bone age delayed (−1 SD)
Puberty timing, tempo, and course 13,15,17,20,22,24 Testes enlarge to max of 10ml, high LH & FSH after Tanner 3, Testo plateaus in late puberty Early onset of testicular enlargement, normal testosterone levels Thelarche and menarche late-normal, but precocious puberty also reported
Adult function/fertility 14,18,19,21,22 Infertility, 90% with low Testo Assumed normal fertility ∼10% secondary amenorrhea; pregnancies in 9/37