Presentation. Baby M is a 3-day-old term neonate who presents with focal clonic seizures beginning on day 2 of life. Continuous EEG monitoring reveals a burst-suppression pattern — prolonged periods of near-flat voltage suppression interrupted by high-amplitude bursts of mixed epileptiform activity. This is the most severe EEG signature of a neonatal epileptic encephalopathy: the suppression reflects pathological loss of the background activity that should be present even in a sleeping neonate, and its presence (rather than the seizures alone) is what defines the encephalopathy and predicts developmental impairment.

Why this matters for the differential. Burst-suppression in a term neonate has two broad explanations that must be separated early: an acquired static insult (hypoxic-ischemic injury, stroke, infection) versus a genetic/metabolic cause. The reassuring birth history and normal MRI below push strongly toward the latter — and within the genetic causes, burst-suppression specifically points to the early-infantile developmental and epileptic encephalopathies (the Ohtahara-spectrum phenotype) rather than benign familial neonatal epilepsy.

Birth history. Reassuringly uncomplicated: spontaneous vaginal delivery at 39 weeks with APGAR scores of 8 at one minute and 9 at five minutes. Normal APGARs and absence of a sentinel event make significant perinatal hypoxic-ischemic injury unlikely — an important point, because HIE is the diagnosis most often invoked (sometimes incorrectly) for any sick seizing neonate.

Exam. Baby M demonstrates:

• Axial hypotonia
• Poor feeding requiring nasogastric supplementation
• Subtle distinctive features including a slightly broad nasal bridge and thin upper lip

The distinctive features are nonspecific here. They are worth noting but do not narrow the differential the way they would in a recognizable multiple-anomaly condition — in the neonatal epilepsies the face is usually unremarkable, and a normal-appearing infant should never lower suspicion for a channelopathy.

Initial workup.

• Metabolic: unremarkable — glucose, electrolytes, lactate, and ammonia are all within normal limits. This is a deliberate, fast, treatable-first screen: it rapidly excludes hypoglycemia, a urea-cycle ammonia crisis, and a primary lactic acidosis, the metabolic emergencies that demand immediate specific therapy. Note that normal first-line chemistries do not exclude two treatable, EEG-mimicking entities — pyridoxine-dependent (ALDH7A1) and pyridoxal-5'-phosphate-responsive epilepsy — which is why an empiric pyridoxine trial is standard in refractory neonatal seizures before anchoring on a genetic answer.
• MRI brain: no structural abnormality — no cortical malformation, no diffusion restriction to suggest ischemia, and no white matter signal change. A normal MRI is expected in the channelopathies and synaptic disorders and is itself a clue that the lesion is molecular rather than structural.
• Chromosomal microarray: normal, excluding aneuploidies and clinically significant copy number variants. CMA is the right tool to exclude a contiguous-gene deletion but is blind to single-nucleotide variants — so a normal array does not address the most likely cause here and simply moves the workup to sequence-level testing.

Family history provides important clues that must not be overlooked: the mother has a mild postural hand tremor that she has never investigated, and her maternal grandmother died of 'dementia' at age 55. Holding these in mind is the discipline this case rewards — a maternal-line tremor plus early 'dementia' is a pattern, not noise, and it will matter later.

The clinical team now faces a critical testing decision — whether to order a targeted epilepsy gene panel or proceed directly to rapid trio whole exome sequencing.

Why trio WES here. The team chooses rapid trio whole exome sequencing over a targeted epilepsy panel for a specific reason: in a critically ill neonate the speed of a confident answer changes therapy, and the trio design (proband plus both parents) lets the lab phase variants and immediately confirm de novo status. Because the great majority of severe neonatal epileptic encephalopathy is caused by de novo dominant variants, demonstrating that a variant is absent in both parents supplies PS2 — a Strong line of ACMG evidence — in the same report that finds the variant, rather than requiring a second round of parental Sanger confirmation that costs days. A panel would have been a reasonable, often cheaper alternative, but it returns a variant without parental context and is fixed to its gene list.

The key test. Rapid trio WES returns within 10 days and identifies a de novo heterozygous pathogenic variant in KCNQ2: c.740C>T, p.Ala247Val.

Mechanism — and why it predicts the drug. The variant sits in the S4 voltage-sensor domain of the Kv7.2 subunit. Kv7.2 and Kv7.3 co-assemble as heterotetramers to carry the neuronal M-current, a slow, non-inactivating potassium current that acts as a brake on repetitive firing near threshold. Most KCNQ2-DEE variants act by a dominant-negative loss-of-function mechanism: the mutant subunit still folds and incorporates into the channel but poisons the tetramer, so even one mutant allele removes far more than half of M-current. With the brake gone, neurons fire in runs — clinically, seizures. This is the mechanistic basis for treatment:

• Dominant-negative LOF variants respond preferentially to sodium-channel blockers (carbamazepine, oxcarbazepine, phenytoin), which reduce the downstream hyperexcitability rather than acting on the channel itself. In a prospective series, carbamazepine or phenytoin controlled seizures in the majority of KCNQ2-encephalopathy infants, and earlier seizure control tracked with milder cognitive outcome (Pisano 2015).
• The mirror-image caveat: a minority of KCNQ2 variants are gain-of-function (e.g., p.Arg201Cys, p.Arg201His). These cause a distinct, often more severe phenotype, are not reliably helped by sodium-channel blockers, and have been treated with potassium-channel blockers such as 4-aminopyridine. Same gene, opposite mechanism, opposite drug — which is exactly why the molecular diagnosis, not just the gene name, drives management.

ACMG classification: Pathogenic. Reported in ClinVar as pathogenic by multiple independent submitters. The evidence codes are worth reading as a logic chain, not a checklist:

• PS2 — confirmed de novo (the trio's payoff)
• PS3 — functional studies show the predicted M-current deficit, linking genotype to a real biophysical defect
• PM1 — in the critical S4 voltage-sensor hotspot
• PM2 — absent from gnomAD (consistent with a severe, non-reproducing phenotype)
• PP3 — concordant in silico prediction

Two Strong plus a Moderate already clear the Pathogenic threshold; PM1/PP3 reinforce rather than carry the call. Note that PS3 and PP3 are partly correlated (both speak to deleteriousness), so they should be weighted thoughtfully rather than treated as fully independent.

Management. Carbamazepine is initiated as a mechanism-matched first-line agent rather than escalating broad-spectrum anticonvulsants. Seizures resolve completely within 48 hours — the kind of rapid, complete response that both treats the infant and corroborates the diagnosis.

Second diagnosis (family). The retained family-history clue now pays off. The mother's tremor and the grandmother's early 'dementia' prompt targeted FMR1 testing, revealing an FMR1 premutation of 89 CGG repeats (normal <45, gray zone 45–54, premutation 55–200, full mutation >200). The premutation — not the full mutation — is what causes FXTAS via a toxic gain-of-function of the expanded CGG-containing mRNA (RNA toxicity), explaining the mother's early tremor and the grandmother's likely FXTAS misattributed to dementia. Critically, this is mechanistically unrelated to Baby M's KCNQ2 disorder, and Baby M tests negative for an FMR1 expansion.

The teaching point is a trap to avoid: a confirmed diagnosis in the proband can anchor the team and shut down inquiry into a relative's unexplained symptoms. Here, two independent molecular diagnoses coexist in one family — and the maternal premutation carries its own counseling implications (risk of FXPOI, and risk of CGG expansion to a full mutation in future pregnancies) entirely separate from the index seizure case.

Presentation. Alex is a 14-year-old previously healthy teenager referred to the neurogenetics clinic with an 18-month history of progressive gait unsteadiness, frequent falls, and declining school performance. His parents first noticed difficulty during football practice when he began stumbling without apparent cause. Over the following months, his handwriting deteriorated and his speech became mildly slurred. The progressive trajectory over the better part of two years is itself a discriminator: it argues against a static or acute process and toward a degenerative, often inherited, disorder.

Neurological exam — reading the localization.

• Broad-based gait ataxia with inability to perform tandem walking
• Mild cerebellar dysarthria
• Bilateral dysmetria on finger-to-nose testing
• Deep tendon reflexes absent at the knees and ankles bilaterally
• Vibration sense and proprioception diminished in both feet, indicating posterior column involvement

The most informative finding is the combination of cerebellar signs with absent reflexes plus large-fiber sensory loss. Pure cerebellar disease leaves reflexes intact; areflexia here signals a coexisting sensory/peripheral neuropathy and dorsal root ganglion (posterior column) involvement. A mixed cerebellar-plus-sensory-axonal picture with areflexia in a teenager is a specific, almost fingerprint pattern. (Many of these patients also show extensor plantar responses — areflexia with upgoing toes is a classic, seemingly paradoxical combination that points hard at this diagnosis.)

Systemic exam — looking outside the nervous system.

• Musculoskeletal: bilateral pes cavus (high-arched feet) and mild thoracolumbar scoliosis — skeletal signatures of a long-standing neuropathy with imbalanced foot-muscle pull
• Cardiac: grade II/VI systolic murmur at the apex — a deliberate prompt to image the heart, because cardiac involvement reframes both prognosis and surveillance
• MRI brain: mild superior cerebellar vermis atrophy but no focal lesions, demyelination, or structural malformations. Importantly, in this disorder the dominant pathology is in the spinal cord and dorsal root ganglia, not the cerebellum — so a near-normal brain MRI is expected and does not argue against the diagnosis.

Family history is significant: Alex's parents are first cousins of Pakistani origin, and his older sibling, aged 17, has recently developed similar gait difficulties. Consanguinity raises the prior probability of a rare autosomal recessive condition by making the child homozygous-by-descent for a shared ancestral allele; an affected sibling with unaffected parents fits recessive inheritance and effectively excludes a dominant or X-linked pattern.

The consanguinity plus the mixed cerebellar-sensory-areflexic phenotype now focuses the recessive ataxia differential, which must be evaluated systematically before committing to a test.

The diagnostic pitfall. WES is ordered first, a defensible choice given the genuinely broad recessive-ataxia differential and consanguinity. It returns negative — no pathogenic or likely pathogenic variant in any ataxia gene. The decisive move is interpreting that negative correctly: a negative exome does not exclude Friedreich ataxia, because the disease is caused not by a coding point mutation but by an expanded GAA repeat in the first intron of FXN. Short-read exome sequencing both targets coding exons (often skipping deep-intronic regions) and, more fundamentally, cannot span or accurately count long, homogeneous repeat tracts — the reads are shorter than the expansion and map ambiguously. The phenotype here is too characteristic to abandon on a test that is mechanistically blind to the relevant mutation type.

The key test. Targeted FXN GAA analysis by repeat-primed PCR confirms the diagnosis: Alex is homozygous for GAA expansions of roughly 850 and 920 repeats (normal alleles <33 GAA; intermediate/premutation 34–65; pathogenic full expansions in the hundreds-to-~1000+ range). RP-PCR is the right tool because it detects and approximately sizes large expansions that standard PCR (which fails to amplify across them) and short-read sequencing miss; Southern blot or long-read sequencing are alternatives.

Mechanism — why the repeat causes the disease. The intronic GAA expansion does not change the frataxin protein sequence; it silences transcription of FXN (through heterochromatin formation/R-loop effects), so patients make too little of a normal protein. Frataxin is a mitochondrial protein essential for iron-sulfur cluster biogenesis; its deficiency impairs key respiratory-chain and aconitase enzymes and promotes oxidative stress. This loss-of-expression mechanism explains two clinical facts: longer GAA tracts cause earlier, more severe disease (more silencing), and the most vulnerable tissues are the high-energy, post-mitotic ones — dorsal root ganglia, spinocerebellar tracts, and cardiomyocytes.

Diagnosis: Friedreich ataxia, confirmed. Echocardiography reveals concentric left ventricular hypertrophy consistent with early hypertrophic cardiomyopathy. This is not incidental: cardiac, not neurological, disease is the leading cause of premature death in FRDA, which is why the murmur on the screening exam earned an echo.

Management.

• Omaveloxolone (Skyclarys) — the first FDA-approved therapy for Friedreich ataxia (approved February 2023, for patients ≥16). It activates the NRF2 antioxidant transcription factor to counter the oxidative stress of frataxin deficiency. In the registrational MOXIe trial it produced a modest but statistically significant slowing of decline on the modified Friedreich Ataxia Rating Scale versus placebo (Lynch 2021) — a disease-modifying, not curative, effect, and a realistic expectation to set with the family.
• Cardiac surveillance with annual echocardiography and ECG.
• Physical therapy for gait and balance.
• Genetic counseling for autosomal recessive inheritance — both parents are obligate carriers, with a 25% recurrence risk in each future pregnancy.

Cascade testing. The 17-year-old sibling undergoes targeted FXN testing (the correct, inexpensive, mechanism-specific test — not a repeat exome). He is confirmed homozygous for the same expansion. Catching him at an earlier stage allows surveillance and treatment to begin before advanced cardiomyopathy is established, illustrating why cascade testing within a family should use the known molecular target rather than re-running a broad, blind assay.

Presentation. Priya is a 5-year-old girl referred from the cerebral palsy clinic for genetic evaluation. She was diagnosed with spastic diplegic cerebral palsy at age 2 based on gross motor delay, lower-extremity hypertonia, and persistent toe-walking. The early CP label is understandable — these features are real and common in CP — but the diagnosis was made on a cross-sectional snapshot, and CP is fundamentally a diagnosis about trajectory. The story only becomes diagnostic when the course is observed over time.

Birth history is unremarkable: term, uncomplicated spontaneous vaginal delivery, normal APGARs, no perinatal risk factors and no evidence of hypoxic-ischemic encephalopathy. CP usually has an identifiable antenatal or perinatal risk profile; its absence here weakens (without alone excluding) an acquired static etiology and should lower the threshold to look for a mimic.

Red flags — and why each one breaks the CP definition. The referring neurologist is concerned about features inconsistent with CP:

• Progressive course — Priya walked independently with a posterior walker at age 3 but has declined to wheelchair dependence over 18 months. CP is by definition a non-progressive disorder arising from a one-time, static lesion of the developing brain. Genuine loss of previously acquired motor skills (regression) is incompatible with CP and mandates a search for a degenerative, metabolic, or treatable genetic cause.
• Diurnal fluctuation — function is markedly better in the morning after sleep and worsens through the day. This sleep-benefit pattern is the near-pathognomonic clue to dopa-responsive dystonia (DRD/Segawa): a partially compensated dopamine-synthesis deficit is replenished overnight and progressively exhausted with activity. No static brain injury fluctuates by time of day, so this single historical detail is worth more than any structural finding.
• Normal MRI — brain MRI at age 2 showed no periventricular leukomalacia, malformation, or any structural lesion. Most spastic diplegia from prematurity or HIE has an MRI correlate; a normal scan in a 'CP' child shifts probability toward a molecular cause and identifies the subgroup with the highest genetic-testing yield.
• Family history — her 8-year-old brother has mild persistent toe-walking with normal cognition and no diagnosis. A milder same-sign relative suggests an inherited, dominant trait with variable expressivity rather than two independent perinatal injuries.

Taken together, progression + diurnal fluctuation + normal MRI + an affected relative is not atypical CP — it is a coherent argument against CP and for a treatable genetic mimic.

Why treat before sequencing. With diurnal fluctuation and regression, the pretest probability of DRD is high enough that the most efficient and humane next step is a therapeutic trial of low-dose levodopa/carbidopa (~1 mg/kg/day of levodopa) rather than waiting on genetics. In DRD a small dose produces a large, sustained effect because the defect is in dopamine synthesis, not in the neurons or receptors themselves — the nigrostriatal pathway is structurally intact and merely substrate-starved, so replacing the downstream product restores function. (The same trial doubles as a safety net: levodopa-responsiveness is also seen in the dopamine-synthesis enzyme deficiencies, so a robust response is meaningful even before the exact gene is known.)

Response. Within two weeks Priya shows marked improvement in lower-extremity tone and motor control; by one month she walks independently for the first time in over a year. A response this complete and durable is, in practice, near-diagnostic of DRD and contrasts sharply with the partial, dose-limited, often dyskinesia-prone response seen when levodopa is tried in Parkinson disease.

Genetic confirmation. Sequencing of GCH1 reveals a heterozygous pathogenic missense variant, c.607G>A, p.Val203Ile. GCH1 encodes GTP cyclohydrolase 1, the rate-limiting enzyme for tetrahydrobiopterin (BH4) synthesis; BH4 is the obligate cofactor for tyrosine hydroxylase, the rate-limiting step in dopamine production. A single defective allele lowers BH4 enough to throttle striatal dopamine — a dominant-negative / haploinsufficiency effect on a rate-limiting cofactor — which is why a heterozygous variant suffices to cause disease.

ACMG classification: Pathogenic, read as a chain of independent evidence:

• PS3 — functional studies show markedly reduced GTP cyclohydrolase 1 activity (genotype tied to a measured enzyme defect)
• PS4 — recurrently reported in unrelated DRD families (case-level enrichment)
• PM2 — absent from gnomAD
• PP1 — co-segregates with disease in this family (present in the affected brother)
• PP3 — concordant in silico prediction

Two Strong + one Moderate + two Supporting comfortably meet the Pathogenic threshold. The strongest, most orthogonal lines here are PS3 (biology) and PS4/PP1 (human genetic evidence) — they corroborate from different directions, which is what makes the call robust rather than circular.

Cascade testing and a counseling nuance. The brother carries the same variant and improves on levodopa, confirming his toe-walking was undiagnosed DRD. GCH1-DRD is autosomal dominant with reduced penetrance that is notably sex-biased — females are affected more often and more severely than males, so an obligate male carrier can be mildly affected or even asymptomatic. This skewed penetrance can mimic skipped generations or an apparently 'sporadic' case and is a classic counseling pitfall; family members should be assessed (and offered the levodopa trial) on the basis of carrier status, not on how striking their symptoms appear.

The big-picture lesson. A meaningful minority of children carrying an 'idiopathic CP' label have an identifiable genetic etiology, and a subset of those — DRD foremost among them — are treatable. DRD is often called one of the most rewarding diagnoses in child neurology precisely because an inexpensive, well-tolerated medication can convert lifelong disability into near-normal function. The case's enduring message is procedural: when a CP diagnosis shows progression, diurnal fluctuation, a normal MRI, or a suggestive family history, reopen the differential.

For detailed coverage of GCH1 and other genetic dystonias, see the Genetic Dystonias module.