Neurogenetics Counselling
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Direct answer
Neurogenetic counselling begins with the pedigree, not the test: a three-generation family tree with standard symbols, consanguinity marked as a double line, affected and carrier statuses shaded — the highest-yield "investigation" in genetic neurology and the only free one. Its reading determines the inheritance model — autosomal dominant with vertical transmission and male-to-male spread, autosomal recessive with sibling clustering in consanguineous unions, X-linked with carrier mothers and affected sons, mitochondrial with maternal transmission and non-Mendelian heteroplasmy, and imprinted genes whose phenotype depends on the parent of origin. From that base flow the examinable rules: anticipation in trinucleotide repeat disorders with paternal expansion in Huntington disease and maternal expansion in myotonic dystrophy (congenital disease comes through mothers), reduced penetrance and variable expressivity complicating pedigrees, germline mosaicism explaining unaffected parents of Duchenne cases, and the ethical architecture of predictive testing — adults only, counselling before consent, results in person, no testing minors for untreatable adult-onset disease, and variants of uncertain significance never over-interpreted.
What you must remember
- Pedigree mechanics: three generations minimum; squares male, circles female, filled affected, dotted carrier, slash deceased, double line consanguinity; proband arrow.
- Dominant neurogenetics with exam favourites: Huntington disease (HTT CAG, anticipation paternal), myotonic dystrophy (DMPK CTG, anticipation maternal, congenital form), spinocerebellar ataxias (paternal anticipation), CADASIL (NOTCH3), CMT1 (PMP22 duplication — also deleted in hereditary neuropathy with liability to pressure palsies).
- Recessive and consanguinity: Friedreich ataxia, Wilson disease, most metabolic disease; consanguinity raises recessive recurrence risk and is prevalent in some Indian communities — a counselling question, not a curiosity.
- X-linked nuances: Duchenne muscular dystrophy — about one-third of cases are de novo, and germline mosaicism means an unaffected mother with a previous affected son retains recurrence risk; carrier testing of sisters at appropriate age; Fabry disease affects males with variable carrier females.
- Mitochondrial rules: maternal transmission only, heteroplasmy and threshold effect explaining sibling variability (MELAS, MERRF, LHON), no paternal transmission ever.
- Imprinting: the same 15q11-13 deletion causes Prader-Willi when paternally inherited and Angelman when maternally inherited — parent of origin is the diagnosis.
- Predictive testing doctrine (Huntington model): 18 years or above, counselling sessions before consent, an accompanied, scheduled disclosure, the right to withdraw or not know, no testing of minors for untreatable adult-onset disease, prenatal and preimplantation options discussed where legal.
- VUS discipline: a variant of uncertain significance is a finding to be reclassified by data, not a diagnosis to be treated; counselling before testing must cover this possibility explicitly.
A counselling session walked through
A 26-year-old woman whose father died of Huntington disease at 50 requests testing before her marriage. The session has an architecture: the first meeting maps the family tree, confirms the father's molecular diagnosis, explains the 50% risk, and explores her reasons, support and coping — stating explicitly that she may stop at any point and that results come in person only. The second meeting rehearses both results; the third delivers the outcome with a support person present, and follow-up is arranged regardless of result, because both relief and survivor guilt are documented reactions. Contrast the next room: parents of a three-year-old with global delay are offered trio exome sequencing; the result returns a variant of uncertain significance in a candidate gene — the counsellor must stop it becoming a false diagnosis, arranging periodic reclassification review rather than treatment or an abandoned search. Two consultations, two doctrines: the at-risk adult with rights, and the investigated child whose phenotype must be matched honestly.
Where candidates slip
Marks drop on precise mechanics: quoting 50% risk then mis-hedging after germline mosaicism; assigning congenital myotonic dystrophy to paternal transmission (it is maternal); forgetting mitochondrial disease never passes from fathers; treating a VUS as actionable. The Indian dimension is real: counsellors are few and city-concentrated, testing is out-of-pocket, and requests often arrive from relatives rather than the at-risk person — the defensible answer returns to the protocol: counsel, consent, support, and the tested person's own choice.
Frequently asked questions
Which inheritance patterns show anticipation and by which parental route?
Trinucleotide repeat disorders — paternal expansion in Huntington disease and most spinocerebellar ataxias, maternal expansion in myotonic dystrophy, which causes the congenital form.
Why can an unaffected mother of a boy with Duchenne dystrophy still have another affected son?
Germline mosaicism — a proportion of her oocytes carry the variant though her blood tests normal, leaving recurrence risk above the population baseline.
How does mitochondrial inheritance differ from Mendelian patterns?
Transmission is maternal only, with heteroplasmy and a threshold effect producing variable severity among children of the same mother; affected fathers never transmit.
What rules govern predictive testing for untreatable adult-onset disease?
Adults only, structured pre-test counselling, an accompanied and scheduled result disclosure, the right to withdraw or not know, and no testing of minors.
What is a variant of uncertain significance and how is it handled?
A sequence change whose disease relevance is unresolved — it is not a diagnosis, requires periodic reinterpretation, and should be discussed as a possible outcome before testing.