Sudden Cardiac Death Risk

How the page chooses the model

• This is the routing of the AHA combined adult/paediatric HCM SCD calculator: 0–15 years to the paediatric models (HCM Risk-Kids, PRIMaCY), 16 to both paediatric and adult, 17–80 to the adult HCM Risk-SCD, and an error above 80.
• ESC 2023 (Recommendation Table 23) agrees: validated paediatric models such as HCM Risk-Kids under 16, and HCM Risk-SCD from 16 (both Class I, Level B).
• HCM Risk-Kids was validated “up to the age of, and including, 16 years”, so at 16 both apply. The AHA calculator notes that patients aged 16 to 18 were 2% of the adult model's cohort.
• Age from dates uses completed calendar years, so a 16th or 17th birthday switches the model on that day.

HCM Risk-Kids equation (Norrish 2019, verbatim constants)

Prognostic index (PI) =
    0.2171364 x (MWT Z - 11.09)
  - 0.0047562 x (MWT Z^2 - 174.12)
  + 0.130365  x (LA diameter Z - 1.92)
  + 0.429624  x unexplained syncope   (1 = yes, 0 = no)
  + 0.1861694 x NSVT                  (1 = yes, 0 = no)
  - 0.0065555 x (maximal LVOT gradient [mmHg] - 21.8)

5-year risk [%] = (1 - 0.949437808 ^ exp(PI)) x 100

Risk group, on the unrounded value:
  low  < 4%     intermediate  4 to < 6%     high  ≥ 6%

MWT Z² is the square of the Z score centred on 174.12 (the mean of Z²), not (Z − 11.09)². exp is the natural exponential. Because of the quadratic term, estimated risk peaks at an MWT Z of about +23 (0.2171364 / (2 × 0.0047562) = 22.8) and falls with more extreme hypertrophy. The LVOT coefficient is negative: a higher gradient lowers the estimate. 0.949437808 is the 5-year baseline survival.

Measurement to Z score (as implemented by hcmriskkids.org)

BSA [m2]     = 0.1023 x weight[kg]^0.68
MWT Z        = ((MWT[mm] / 10) / BSA^0.4 - 0.58) / 0.09
LA Z, male   = (LA[mm] / (10.665 x weight[kg]^0.255) - 1) / 0.118
LA Z, female = (LA[mm] / (10.74  x weight[kg]^0.246) - 1) / 0.124

• The MWT constants (α 0.40, mean 0.58 cm, SD 0.09) equal the PHN septal-thickness row of Lopez 2017, which the paper cites. The calculator applies this septal reference to the maximal thickness in any segment.
• The weight-only BSA formula comes from the official calculator; no published source was found for it, and PHN used Haycock BSA (height and weight), so the MWT Z can differ slightly from a PHN Z score calculated with Haycock BSA, depending on the child's height.
• The LA constants come from the official calculator and could not be checked against the Neilan 2009 paper, which the model cites for LA Z scores.
• This page computes normally at an LVOT gradient of 0 mmHg; the official calculator's formula treats 0 as missing and may return a blank.

Variables (Norrish 2019, Table 1)

Maximal LV wall thickness Z score

Greatest thickness on 2D echo in the parasternal short-axis views at the mitral and papillary-muscle levels (anterior and posterior septum, lateral and posterior wall) and at the apical level (anterior and posterior septum), at baseline. Z score from the PHN reference.

Left atrial diameter Z score

Maximal LA diameter by M-mode or 2D echo in the parasternal long-axis plane. Z score from Neilan 2009.

Maximal LVOT gradient

Maximum LV outflow tract gradient at rest or with Valsalva, pulsed- and continuous-wave Doppler from the apical 3- or 5-chamber view; gradient = 4V².

Unexplained syncope

Transient loss of consciousness with no identifiable cause, at or before first evaluation.

Non-sustained VT

≥3 consecutive ventricular beats at ≥120 bpm lasting <30 s on ambulatory ECG (minimum 24 h), at or before first evaluation.

Outcome

SCD or an equivalent event (aborted cardiac arrest, appropriate ICD therapy, or sustained VT with haemodynamic compromise) within 5 years of baseline.

Population

1024 patients aged 1 to 16 years with HCM (multicentre, international; 39 centres in 17 countries [unverified]). Excluded: prior VF or sustained VT, inborn errors of metabolism, syndromic HCM (e.g. RASopathy, Friedreich ataxia), infant-onset HCM (presentation under 1 year). Validated “up to the age of, and including, 16 years”; use the adult HCM Risk-SCD model beyond that. This page applies the age limit to completed years (1 to 16), like the official calculator's whole-year age field.

How well does HCM Risk-Kids perform?

The development paper's abstract states that for every 10 ICDs implanted in patients with a 5-year risk ≥6%, 1 patient may potentially be saved from SCD at 5 years. [Unverified, full text not re-checked:] in the development cohort a ≥6% threshold identified 26 of 34 SCD events (76.5%) and a ≥4% threshold 31 of 34 (91.2%); the adult HCM Risk-SCD model applied to the same children had a C statistic of 0.67 and underestimated risk.

Guidelines

• 2023 ESC cardiomyopathy guideline, Recommendation Table 23, pp. 3561–3562 (doi:10.1093/eurheartj/ehad194), primary prevention of SCD in HCM:
  • Validated paediatric-specific risk prediction models (e.g. HCM Risk-Kids) are recommended to estimate the 5-year risk of sudden death in patients aged <16 years. Class I, Level B.
  • The HCM Risk-SCD calculator (adult model) is recommended to estimate the 5-year risk of sudden death in patients aged ≥16 years. Class I, Level B.
  • The 5-year risk of SCD should be assessed at first evaluation and re-evaluated at 1–2 year intervals or whenever clinical status changes. Class I, Level B.
  • An ICD should be considered at an estimated 5-year risk ≥6%, after detailed clinical assessment of (i) the lifelong risk of complications, (ii) competing mortality risk from the disease and comorbidities, and (iii) the impact of an ICD on lifestyle, socio-economic status and psychological health. Class IIa, Level B.
  • An ICD may be considered in individual patients at an estimated 5-year risk ≥4% to <6%, after detailed clinical assessment of the lifelong risk of complications and the impact of an ICD on lifestyle, socio-economic status and psychological health. Class IIb, Level B.
  • In the low-risk category (<4%), extensive LGE (≥15%) on CMR, or LVEF <50%, may be considered in shared decision-making about prophylactic ICD implantation, acknowledging the lack of robust data on their impact on risk estimates from HCM Risk-SCD or a validated paediatric model (e.g. HCM Risk-Kids). Class IIb, Level B (each).
  These ESC lines apply to both models: at 16 this page shows HCM Risk-Kids (inside its development range) and HCM Risk-SCD (recommended by ESC 2023 from 16).
• 2024 AHA/ACC/AMSSM/HRS/PACES/SCMR HCM guideline (doi:10.1161/CIR.0000000000001250): adult and paediatric ICD recommendations, quoted in full in the 2024 AHA/ACC section below and applied in the “2024 AHA/ACC ICD risk factors” section of the page.
• The external-validation authors cautioned that “a threshold of 6% may be inappropriate for childhood disease and expert consensus opinion would be required to determine an appropriate risk threshold” (Norrish 2022 EJPC).

Limitations (HCM Risk-Kids)

• Retrospective data with imputation of missing predictors (48.5% of patients had at least one missing [unverified]).
• Weaker performance in smaller and independent cohorts (5-year Harrell C 0.52 in a 100-patient single-centre cohort) and wide confidence intervals in external validation (23 events).
• Not validated in RASopathy (C 0.60), metabolic, syndromic or neuromuscular HCM, infant-onset HCM, under 1 year or over 16 years, or for secondary prevention.
• Does not include late gadolinium enhancement, genotype, family history of SCD or LV systolic dysfunction.
• Few patients had undergone myectomy; use cautiously after surgical gradient reduction.
• Estimated risk falls beyond an MWT Z of about +23 (inverted U); interpret extreme hypertrophy clinically.
• Intended for the baseline evaluation; serial use was not validated in the model studies. ESC 2023 recommends assessing 5-year risk at first evaluation and re-evaluating every 1–2 years or with any change in clinical status (Class I, Level B).
• Z scores from other reference databases, or a different BSA formula, shift the result.

HCM Risk-SCD equation (O'Mahony 2014, adult)

Prognostic index (PI) =
    0.15939858 x MWT [mm]
  - 0.00294271 x MWT^2
  + 0.0259082  x LA diameter [mm]
  + 0.00446131 x maximal LVOT gradient [mmHg]
  + 0.4583082  x family history of SCD   (1 = yes, 0 = no)
  + 0.82639195 x NSVT                    (1 = yes, 0 = no)
  + 0.71650361 x unexplained syncope     (1 = yes, 0 = no)
  - 0.01799934 x age [years]

5-year risk [%] = (1 - 0.998 ^ exp(PI)) x 100

Accepted inputs (AHA calculator): age 16-80 y, MWT 10-35 mm,
LA 28-67 mm, maximal LVOT gradient 2-154 mmHg. Outside these, no result.

• Raw measurements in mm and mmHg: no Z scores and no centring. exp is the natural exponential; 0.998 is the 5-year baseline survival. Age enters as completed years.
• Coefficients as printed in the EVIDENCE-HCM methods (O'Mahony 2018) and as computed by the AHA HCM SCD Calculator's own script, which was cross-checked against Medscape's HCM Risk-SCD page. The baseline survival 0.998 is taken from the AHA script; the retrieved EVIDENCE-HCM text is illegible at that point.
• Unlike HCM Risk-Kids, the LVOT coefficient is positive (a higher gradient raises the estimate) and age lowers it. The MWT terms are quadratic, with their maximum at 0.15939858 / (2 × 0.00294271) = 27.1 mm.
• The upper input limits (LA 67 mm, LVOT gradient 154 mmHg, MWT 35 mm, age 80 years) are the “extreme clinical characteristics” defined a priori in EVIDENCE-HCM.
• Risk groups use the same unrounded 4% and 6% thresholds and the same display rounding as HCM Risk-Kids.

Variables (HCM Risk-SCD)

Quoted from the EVIDENCE-HCM methods (O'Mahony 2018), which applies the model of the 2014 ESC HCM guideline. The matching table in the 2014 ESC guideline itself (doi:10.1093/eurheartj/ehu284) could not be retrieved for this page, so its exact wording is [unverified].

Age

Age at time of evaluation (years).

Maximal LV wall thickness

MWT in the parasternal short- and long-axis plane using 2D echocardiography (mm).

Left atrial diameter

By M-mode or 2D echocardiography in the parasternal long-axis plane (mm).

Maximal LVOT gradient

Maximal instantaneous LVOT gradient at rest and with Valsalva provocation (irrespective of concurrent medical treatment) using continuous-wave Doppler (mmHg). The AHA calculator specifies rest or Valsalva, not exercise.

Family history of SCD

SCD in 1 or more first-degree relatives under 40 years of age, or SCD in a first-degree relative with confirmed HCM (post- or ante-mortem diagnosis) at any age.

Non-sustained VT

≥3 consecutive ventricular beats at ≥120 bpm and <30 s in duration on Holter monitoring, at or prior to evaluation (MDCalc wording; the EVIDENCE-HCM text is illegible here). Minimum Holter duration [unverified].

Unexplained syncope

Unexplained syncope at or prior to first evaluation.

Outcome

SCD or appropriate ICD shock within 5 years.

Population and applicability

Development: 3675 consecutive patients from six centres; 198 (5%) died suddenly or had an appropriate ICD shock over 24 313 patient-years (median 5.7 years). The AHA calculator states it is “for use in patients with hypertrophic cardiomyopathy (HCM) who are ≥ 16 years of age with > 1 major SCD risk factors…”, and that it should not be used in paediatric patients (<16 y), elite or competitive athletes, HCM with metabolic diseases (e.g. Anderson–Fabry) or syndromes (e.g. Noonan), or after aborted SCD or sustained ventricular arrhythmia (secondary prevention). “Caution should be exercised when assessing the SCD in patients following invasive reduction in left ventricular outflow tract obstruction with myectomy or alcohol septal ablation.” “Children who are 16 to 18 years of age accounted for 2% of the cohort used for the adult-based risk calculator. The low representation of this age group should be considered if calculating risk estimates for patients in this age range.” When LVEF ≤50%, an apical aneurysm or extensive LGE is present, the AHA tool adds that the estimate may not be accurate in that setting.

How well does HCM Risk-SCD perform?

The development abstract states: “For every 16 ICDs implanted in patients with ≥4% 5-year SCD risk, potentially 1 patient will be saved from SCD at 5 years.”

Limitations (HCM Risk-SCD)

• Retrospective, multicentre cohort; patients aged 16 to 18 (2%) and 16 to 20 were under-represented.
• Does not include LGE, LVEF or LV apical aneurysm; ESC 2023 and the 2024 AHA/ACC guideline consider these separately.
• Not for use under 16, in elite or competitive athletes, in metabolic or syndromic HCM, or for secondary prevention; caution after myectomy or alcohol septal ablation.
• No result outside the AHA input ranges (for example a resting gradient below 2 mmHg or wall thickness over 35 mm).

2024 AHA/ACC recommendations for ICD placement (section 7.2)

Read from the guideline's recommendation table (doi:10.1161/CIR.0000000000001250). The page shows the rows that match the answers in the “2024 AHA/ACC ICD risk factors” section. A class supports shared decision-making; it is not a decision, and the guideline states that risk thresholds should not be the sole arbiter.

Table 8 definitions (abridged):

Family history

Sudden death definitively or likely attributable to HCM in ≥1 first-degree or close relatives ≤50 years.

Massive LVH

≥30 mm in any segment by echo or CMR; borderline values ≥28 mm can be considered at the cardiologist's discretion. Paediatric: no absolute or Z-score threshold is established; “a maximal wall thickness that corresponds to a z-score ≥20 (and >10 in conjunction with other risk factors) appears reasonable.”

Unexplained syncope

≥1 unexplained episodes of acute transient loss of consciousness, judged by history unlikely to be neurocardiogenic (vasovagal) and not attributable to LVOTO; especially within 6 months of evaluation (events beyond 5 years in the past do not appear relevant).

Systolic dysfunction

EF <50% by echo or CMR.

Apical aneurysm

Discrete thin-walled dyskinetic or akinetic segment with transmural scar or LGE of the most distal LV, independent of size. In children it is uncommon and its risk has not been studied.

Extensive LGE

≥15% of LV mass. The extent conferring risk has not been defined in children.

NSVT

≥3 beats at ≥120 bpm; more weight to frequent (≥3), longer (≥10 beats) or faster (≥200 bpm) runs over 24–48 h. Paediatric: a VT rate exceeding the baseline sinus rate by >20% is considered significant.

Figure 3 footnote

“ICD decisions in pediatric patients with HCM are based on ≥1 of these major risk factors: family history of HCM SCD, NSVT on ambulatory monitor, massive LVH, and unexplained syncope.”

How this page applies them.

• Adult rows from 16 years, paediatric rows under 16 (the age at which the AHA calculator and ESC 2023 switch to the adult model). Secondary prevention overrides everything (Class 1).
• Adult: ≥1 major factor (family history, massive LVH, syncope, apical aneurysm, EF <50%) shows 2a plus the 2a row on discussing the 5-year risk; if extensive LGE or NSVT is also present, the 2b row is added for when the decision remains uncertain. No major factor with extensive LGE or NSVT shows 2b. None of these shows 3: Harm.
• Children: ≥1 of family history, NSVT, massive LVH or syncope shows the paediatric 2a and the 2a discussion row; extensive LGE or EF <50% adds the paediatric 2b row. With none of these and no apical aneurysm, 3: Harm. An apical aneurysm alone in a child matches no row (its risk has not been studied in children).
• Massive LVH: adults, derived from the wall thickness entered for the model (≥30 mm; 28–29 mm flagged as borderline, not counted). Children, an explicit Yes/No, because no threshold is established. The MWT Z shown for context is the HCM Risk-Kids Z score (PHN reference, weight-only BSA); other references give different values.
• Family history is asked twice because the definitions differ: HCM Risk-SCD uses SCD in a first-degree relative under 40 years, or in a first-degree relative with confirmed HCM at any age; the 2024 guideline uses HCM-related sudden death in a first-degree or close relative ≤50 years. For example, HCM-related sudden death of a 45-year-old first-degree relative counts for the guideline but not for the model unless HCM was confirmed; sudden death of a 60-year-old first-degree relative with confirmed HCM counts for the model but not the guideline.
• Syncope and NSVT reuse the model answers; the guideline's syncope definition (suspected arrhythmic, especially within 6 months) is narrower than “unexplained”, so answer with the guideline wording in mind.
• Differences from the AHA online tool: the tool prints “Class 3 – No Benefit” and “EF ≤ 50%”; the guideline table says 3: Harm and EF <50%. This page follows the guideline wording.
• When an apical aneurysm, EF <50% or extensive LGE is present, the page notes that these are not predictors in either model and that their effect on the estimate is uncertain.

References

• Norrish G, Ding T, Field E, et al. Development of a novel risk prediction model for sudden cardiac death in childhood hypertrophic cardiomyopathy (HCM Risk-Kids). JAMA Cardiol 2019;4(9):918-927. doi:10.1001/jamacardio.2019.2861
• Norrish G, et al. External validation of the HCM Risk-Kids model for predicting sudden cardiac death in childhood hypertrophic cardiomyopathy. Eur J Prev Cardiol 2022;29(4):678-686. doi:10.1093/eurjpc/zwab181
• Norrish G, et al. Relationship between maximal left ventricular wall thickness and sudden cardiac death in childhood onset hypertrophic cardiomyopathy. Circ Arrhythm Electrophysiol 2022;15(5):e010075. doi:10.1161/CIRCEP.121.010075
• Lopez L, Colan S, Stylianou M, et al. PHN Normal Echocardiogram Database Z scores. Circ Cardiovasc Imaging 2017;10(11):e006979. doi:10.1161/CIRCIMAGING.117.006979
• Neilan TG, Pradhan AD, King ME, Weyman AE. Derivation of a size-independent variable for scaling of cardiac dimensions in a normal paediatric population. Eur J Echocardiogr 2009;10(1):50-55. doi:10.1093/ejechocard/jen110
• Boleti O, Roussos S, Norrish G, et al. Sudden cardiac death in childhood RASopathy-associated hypertrophic cardiomyopathy: validation of the HCM Risk-Kids model. Int J Cardiol 2023;393:131405. doi:10.1016/j.ijcard.2023.131405
• Norrish G, et al. Performance of the PRIMaCY sudden death risk prediction model for childhood hypertrophic cardiomyopathy. Europace 2023;25(11):euad330. doi:10.1093/europace/euad330
• Wilkin, Khraiche, Panaioli, et al. Independent external evaluation of pediatric hypertrophic cardiomyopathy risk scores in predicting severe ventricular arrhythmias. Circ Arrhythm Electrophysiol 2025;18(3):e012932. doi:10.1161/CIRCEP.124.012932
• Miron A, Lafreniere-Roula M, Steve Fan CP, et al. A validated model for sudden cardiac death risk prediction in pediatric hypertrophic cardiomyopathy. Circulation 2020;142(3):217-229. doi:10.1161/CIRCULATIONAHA.120.047235
• Arbelo E, Protonotarios A, Gimeno JR, et al. 2023 ESC Guidelines for the management of cardiomyopathies. Eur Heart J 2023;44(37):3503-3626 (Recommendation Table 23, pp. 3561-3562). doi:10.1093/eurheartj/ehad194
• Ommen SR, Ho CY, Asif IM, et al. 2024 AHA/ACC/AMSSM/HRS/PACES/SCMR Guideline for the Management of Hypertrophic Cardiomyopathy. Circulation 2024;149(23):e1239-e1311. doi:10.1161/CIR.0000000000001250
• O'Mahony C, Jichi F, Pavlou M, et al. A novel clinical risk prediction model for sudden cardiac death in hypertrophic cardiomyopathy (HCM risk-SCD). Eur Heart J 2014;35(30):2010-2020. PMID 24126876. doi:10.1093/eurheartj/eht439
• O'Mahony C, Jichi F, Ommen SR, et al. International external validation study of the 2014 European Society of Cardiology guidelines on sudden cardiac death prevention in hypertrophic cardiomyopathy (EVIDENCE-HCM). Circulation 2018;137(10):1015-1023. doi:10.1161/CIRCULATIONAHA.117.030437
• Elliott PM, Anastasakis A, Borger MA, et al. 2014 ESC Guidelines on diagnosis and management of hypertrophic cardiomyopathy. Eur Heart J 2014;35(39):2733-2779. doi:10.1093/eurheartj/ehu284
• AHA HCM SCD Calculator: professional.heart.org/en/guidelines-and-statements/hcm-risk-calculator (equation, input ranges, applicability text and age routing read from the tool, October 2026).
• HCM Risk-Kids online calculator: hcmriskkids.org (conversion formulas read from its page source, October 2026).