Turner Syndrome (45,X) - High-Yield Viva Questions
| Question | Answer / Practical Pearls |
|---|---|
| What genetic mechanism accounts for short stature in Turner syndrome, and what are the common karyotypes? | • SHOX Gene Haploinsufficiency: The Short Stature Homeobox gene (SHOX) is located on the pseudoautosomal region (PAR1) of the X chromosome ($Xp22.3$). Normally, both alleles escape X-inactivation. Loss of one SHOX allele impairs growth plate chondrocyte hypertrophic differentiation, causing short stature and mesomelic/rhizomelic skeletal stigmata. • Cytogenetic Subtypes: - Classic Monosomy X ($45,X$): $\sim 50\%$. - Mosaicism ($45,X / 46,XX$ or $45,X / 46,XY$): $\sim 20-30\%$. - Structural X abnormalities: Isochromosome Xq ($46,X,i(Xq)$), ring X ($46,X,r(X)$), or partial deletions ($46,X,del(Xp)$). |
| What is the critical surgical imperative if Y-chromosome material (45,X/46,XY) or SRY positivity is detected? | • MANDATORY Prophylactic Bilateral Gonadectomy. • Dysgenetic streak gonads harboring Y chromosome sequences or SRY material have a $15\%\text{ to }30\%$ lifetime risk of malignant transformation into Gonadoblastoma and Dysgerminoma! VIVA TRAP: Always request SRY-specific FISH or PCR alongside standard karyotype in all Turner syndrome evaluations. |
| What congenital cardiovascular anomalies are most frequently seen in Turner syndrome? | 1. Bicuspid Aortic Valve (BAV): Most common; present in $\sim 30\%$ of patients. 2. Coarctation of the Aorta (CoA): Present in $10-15\%$, typically juxtaductal; presents with radio-femoral delay and upper limb hypertension. 3. Aortic Dilation & Dissection: Secondary to cystic medial necrosis, hypertension, and BAV flow turbulence. Quantified by the Aortic Size Index (ASI = Ascending Aorta diameter in cm / BSA in $\text{m}^2$). An $\text{ASI} > 2.5\text{ cm/m}^2$ carries extreme risk of fatal acute aortic dissection, mandating beta-blockade and elective root replacement! |
| Detail the recombinant Growth Hormone (rhGH) protocol in Turner syndrome. | • Dose: $0.045-0.050\text{ mg/kg/day}$ ($1.4\text{ mg/m}^2/\text{day}$) administered subcutaneously daily at bedtime. • Rationale for High Dosing: Girls with Turner syndrome have intrinsic chondrocyte resistance to GH due to SHOX haploinsufficiency (they are NOT GH-deficient), thus requiring supraphysiological replacement doses. • Initiation Timing: As soon as growth decelerates below the 5th percentile on normal female growth charts (typically between 4 and 6 years of age). • Monitoring: Height every 3 months; bone age and Serum IGF-1 annually (target IGF-1 $+1.5\text{ to }+2.0\text{ SD}$). |
| What is the step-by-step protocol for pubertal induction in Turner syndrome? | • Phase 1 (Estrogen Priming at Age 11-12y): - Start with transdermal $17\beta$-estradiol patch at very low dose: $6.25\text{ mcg/day}$ ($1/4\text{ of a }25\text{ mcg patch}$) continuously. - Escalated every 6-12 months over 2-3 years ($12.5 \to 25 \to 37.5 \to 50\text{ mcg/day}$) to mimic normal physiological puberty and stimulate breast development (Tanner Stage B2 $\to$ B4) and uterine enlargement. • Phase 2 (Cyclic Progestin Addition - Strict Rule): - PROGESTIN MUST BE WITHHELD FOR AT LEAST 2 YEARS OR UNTIL BREAKTHROUGH VAGINAL BLEEDING OCCURS. - Premature progestin leads to tubular breast deformity and premature epiphyseal fusion. - Once bleeding occurs: Add oral Micronized Progesterone ($100-200\text{ mg}$) or Medroxyprogesterone ($5-10\text{ mg}$) on days 1 to 12 of each calendar month. |