Three integrative virtual patient cases that thread across multiple neurogenetics modules — neonatal seizures, progressive ataxia, and a cerebral palsy mimic. Each case challenges learners to synthesize clinical reasoning, genetic testing strategy, variant interpretation, and precision treatment in realistic clinical scenarios.
Tags: Clinical Decision-Making · Neurogenetics
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 voltage suppression interrupted by high-amplitude bursts of epileptiform activity. The birth history is reassuringly uncomplicated: spontaneous vaginal delivery at 39 weeks with APGAR scores of 8 at one minute and 9 at five minutes, ruling out significant perinatal hypoxic-ischemic injury. On examination, Baby M demonstrates axial hypotonia, poor feeding requiring nasogastric supplementation, and subtle distinctive features including a slightly broad nasal bridge and thin upper lip. Initial metabolic workup is unremarkable: glucose, electrolytes, lactate, and ammonia are all within normal limits, making acute metabolic decompensation unlikely. MRI of the brain shows no structural abnormality — no cortical malformation, no evidence of ischemic injury, and no white matter signal change. Chromosomal microarray returns normal, excluding aneuploidies and clinically significant copy number variants. The family history, however, 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. 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.
Key Points
Rapid trio whole exome sequencing (Baby M plus both parents) returns within 10 days and identifies a de novo heterozygous pathogenic variant in KCNQ2: c.740C>T, p.Ala247Val. This missense variant is located in the S4 voltage sensor domain of the Kv7.2 potassium channel subunit and has been functionally characterized as a variant that alters channel gating — at the single-channel level, the variant increases conductance and shifts voltage dependence of activation, but at the whole-cell level this paradoxically enhances neuronal excitability because the altered Kv7.2 channels fail to provide the sustained repolarizing M-current that normally dampens repetitive firing. The net effect at the network level is hyperexcitability and seizures. Given these altered channel-level properties, the variant has been termed a gain-of-function variant in the literature — though this characterization always depends on the level of analysis (channel biophysics vs. whole-cell current vs. neuronal firing vs. network behavior). The variant is reported in ClinVar as pathogenic by multiple independent submitters. ACMG classification is Pathogenic based on the following evidence codes: PS2 (confirmed de novo in a patient with the disease and no family history), PS3 (well-established functional studies demonstrate altered channel biophysics with network-level hyperexcitability), PM1 (located in the critical S4 voltage sensor domain), PM2 (absent from population databases including gnomAD), and PP3 (multiple computational tools predict a deleterious effect). Based on clinical experience that KCNQ2 encephalopathy variants with this biophysical profile respond well to sodium channel blockers, carbamazepine is initiated rather than conventional anticonvulsants. Seizures resolve completely within 48 hours, confirming the genotype-guided treatment approach. Separately, the mother's tremor and the grandmother's early-onset dementia prompt targeted FMR1 testing, which reveals that the mother carries an FMR1 premutation with 89 CGG repeats (normal <45, intermediate/gray zone 45–54, premutation 55–200). This explains the mother's tremor as early FXTAS and the grandmother's probable fragile X-associated tremor/ataxia syndrome. Baby M tests negative for FMR1 expansion. This case illustrates dual genetic findings within a single family — an increasingly recognized phenomenon in clinical neurogenetics.
Key Points
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. Neurological examination reveals broad-based gait ataxia with inability to perform tandem walking, mild cerebellar dysarthria, and bilateral dysmetria on finger-to-nose testing. Deep tendon reflexes are absent at the knees and ankles bilaterally — a striking finding in a teenager with otherwise upper motor neuron signs. Vibration sense and proprioception are diminished in both feet, indicating posterior column involvement. Musculoskeletal examination demonstrates bilateral pes cavus (high-arched feet) and mild thoracolumbar scoliosis. Cardiac auscultation reveals a grade II/VI systolic murmur at the apex. MRI brain shows mild superior cerebellar vermis atrophy but no focal lesions, demyelination, or structural malformations. 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 with frequent tripping. The consanguinity strongly suggests autosomal recessive inheritance, and the progressive ataxia differential must now be systematically evaluated.
Key Points
Whole exome sequencing is ordered as the initial genetic test given the broad ataxia differential and consanguinity. The WES report returns negative — no pathogenic or likely pathogenic variants are identified in any known ataxia gene. However, the clinical team recognizes that WES cannot detect trinucleotide repeat expansions, and Alex's phenotype is highly suggestive of Friedreich ataxia. Targeted FXN GAA repeat analysis by repeat-primed PCR (RP-PCR) is ordered, and the result confirms the diagnosis: Alex is homozygous for GAA expansions with 850 and 920 repeats on the two alleles respectively (normal alleles contain fewer than 33 GAA repeats; premutation alleles 34–65; full mutation greater than 66). The diagnosis of Friedreich ataxia is confirmed. Echocardiography reveals concentric left ventricular hypertrophy consistent with early hypertrophic cardiomyopathy — the leading cause of premature death in Friedreich ataxia. Management is initiated with omaveloxolone (Skyclarys), the first FDA-approved therapy for Friedreich ataxia (approved February 2023). Omaveloxolone is an NRF2 pathway activator that reduces oxidative stress caused by frataxin deficiency and has demonstrated slowing of neurological decline in clinical trials. Cardiac surveillance with annual echocardiography and ECG is established. Physical therapy is prescribed for gait training and balance. Genetic counseling addresses autosomal recessive inheritance with 25% recurrence risk for future pregnancies. Alex's older sibling undergoes targeted FXN testing and is confirmed to carry the same homozygous GAA expansion — importantly, early treatment is initiated before significant cardiomyopathy develops, which may improve long-term cardiac outcomes.
Key Points
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, increased tone in the lower extremities, and persistent toe-walking. Her birth history is unremarkable: she was born at term following an uncomplicated spontaneous vaginal delivery with normal APGAR scores, and there were no perinatal risk factors including no evidence of hypoxic-ischemic encephalopathy. The referring child neurologist has become concerned about several features inconsistent with the CP diagnosis. First, Priya's motor symptoms have been clearly progressive — she was able to walk independently with a posterior walker at age 3, but over the past 18 months her mobility has declined steadily, and she now requires a wheelchair for all distances. Second, her family reports a striking diurnal fluctuation pattern: Priya's tone and motor function are noticeably better in the morning after sleep and worsen progressively throughout the afternoon and evening. Third, MRI of the brain performed at age 2 was entirely normal with no evidence of periventricular leukomalacia, cortical malformation, or any structural lesion. Fourth, her older brother, aged 8, has mild persistent toe-walking but is otherwise functional with normal cognition and no formal diagnosis. The progressive course and diurnal fluctuation are fundamentally inconsistent with cerebral palsy, which is by definition a non-progressive motor disorder arising from a static brain lesion.
Key Points
Based on the clinical suspicion for dopa-responsive dystonia, the neurology team initiates a therapeutic trial of low-dose levodopa/carbidopa at 1 mg/kg/day of levodopa. The response is dramatic and sustained: within two weeks, Priya demonstrates marked improvement in lower extremity tone and motor function. By one month, she is walking independently for the first time in over a year — a transformative clinical outcome. Genetic confirmation is obtained through sequencing of GCH1, which reveals a heterozygous pathogenic missense variant: c.607G>A, p.Val203Ile. This variant in GTP cyclohydrolase 1 disrupts the rate-limiting enzyme in tetrahydrobiopterin (BH4) synthesis, leading to deficiency of dopamine in the basal ganglia. ACMG classification is Pathogenic based on: PS3 (well-established in vitro functional studies demonstrate significantly reduced GTP cyclohydrolase 1 enzyme activity), PS4 (the variant has been reported in multiple unrelated families with dopa-responsive dystonia in the literature), PM2 (absent from gnomAD population database), PP1 (variant segregates with disease in the family — present in affected brother), and PP3 (computational tools uniformly predict deleterious effect). The combination of two Strong (PS3, PS4), one Moderate (PM2), and two Supporting (PP1, PP3) criteria meets the ACMG threshold for Pathogenic classification. Priya's 8-year-old brother undergoes targeted testing, is confirmed to carry the same GCH1 variant, and begins levodopa therapy with similar improvement in his toe-walking. GCH1-related dopa-responsive dystonia is autosomal dominant with incomplete penetrance, particularly in males — explaining the brother's milder presentation. This case powerfully illustrates why an estimated 20–30% of children with idiopathic cerebral palsy have an underlying genetic cause, and why treatable conditions such as dopa-responsive dystonia must never be missed. DRD is widely considered one of the most rewarding diagnoses in child neurology because of the dramatic and lifelong response to inexpensive, well-tolerated levodopa therapy. For detailed coverage of GCH1 and other genetic dystonias, see the Genetic Dystonias module.
Key Points
1. A 5-day-old neonate presents with multifocal seizures refractory to phenobarbital and levetiracetam. EEG shows burst-suppression. Brain MRI is normal. Metabolic workup is unremarkable. The team considers ordering genetic testing. What is the MOST important advantage of rapid trio WES over a singleton epilepsy gene panel in this scenario?
The key advantage of trio WES (proband + both parents) is the ability to identify de novo variants — variants present in the child but absent in both parents. In neonatal epileptic encephalopathy, the majority of causative variants arise de novo, and confirming de novo status provides strong ACMG evidence for pathogenicity (PS2, a Strong evidence criterion). A singleton gene panel can identify the same variants but cannot determine whether they are de novo without separate parental testing, which adds time and complexity. Both trio WES and gene panels can detect variants in channelopathy genes; neither can detect repeat expansions. The speed of de novo confirmation in a critically ill neonate can directly impact treatment decisions.
2. A neonate with KCNQ2 epileptic encephalopathy is started on phenobarbital, which worsens the seizures. The team switches to carbamazepine based on the genetic diagnosis, and seizures resolve within 48 hours. Which concept does this case BEST illustrate?
This exemplifies precision medicine in epilepsy: identifying the specific genetic cause (KCNQ2 channelopathy) directly guides treatment selection. KCNQ2 encephalopathy variants disrupt the M-current that normally dampens repetitive neuronal firing. Sodium channel blockers (carbamazepine, oxcarbazepine, phenytoin) are specifically effective because they reduce the hyperexcitability downstream of the M-current deficit. Phenobarbital and other GABAergic agents are often ineffective or can paradoxically worsen seizures in KCNQ2 encephalopathy. This is distinct from pharmacogenomics (which concerns drug metabolism variants) — here the genetic diagnosis determines which drug mechanism is appropriate for the specific disease pathophysiology.
3. A 16-year-old with progressive gait ataxia, absent knee reflexes, pes cavus, and scoliosis has a normal whole exome sequencing result. His echocardiogram shows concentric left ventricular hypertrophy. His parents are first cousins. The neurologist orders targeted FXN GAA repeat testing, which reveals 750 and 900 GAA repeats on the two alleles. Which therapeutic intervention has been FDA-approved for this condition?
Omaveloxolone (Skyclarys) was approved by the FDA in February 2023 as the first treatment for Friedreich ataxia in patients aged 16 and older. It works by activating the NRF2 transcription factor, which upregulates antioxidant gene expression to counteract the oxidative stress caused by frataxin deficiency in mitochondria. Clinical trials demonstrated a modest but statistically significant slowing of neurological decline as measured by the modified Friedreich Ataxia Rating Scale (mFARS). The case also reinforces that WES cannot detect the GAA trinucleotide repeat expansion responsible for ~96% of Friedreich ataxia cases — targeted repeat testing is required.
4. A 4-year-old girl diagnosed with 'spastic diplegic cerebral palsy' at age 2 has been declining functionally — she previously walked with a walker but now needs a wheelchair. Her brain MRI is normal. Her mother notes she is 'a different child in the morning versus the evening.' Before ordering any genetic testing, the treating neurologist should:
The combination of progressive motor decline (incompatible with true CP, which is non-progressive), normal brain MRI, and clear diurnal fluctuation (better in morning, worse by evening) are red flags pointing strongly to dopa-responsive dystonia (DRD/Segawa disease). A therapeutic trial of low-dose levodopa should be started immediately and should NOT be delayed while awaiting genetic testing. The dramatic and sustained response to levodopa is itself virtually diagnostic. DRD is considered one of the most rewarding diagnoses in child neurology because an inexpensive, well-tolerated medication can transform a wheelchair-dependent child into an independently walking one. Approximately 20-30% of children with idiopathic CP have an underlying genetic cause, and DRD is the quintessential treatable mimic.
5. During trio WES for a critically ill neonate, the laboratory identifies a de novo pathogenic STXBP1 variant in the infant AND separately notes that the mother carries an FMR1 premutation (85 CGG repeats). This dual finding illustrates which important principle in clinical neurogenetics?
Dual or multiple genetic findings within a single family are increasingly recognized as comprehensive genomic testing becomes standard practice, occurring in approximately 5-7% of families. In this scenario, the infant's seizures are caused by the de novo STXBP1 variant (completely independent of the mother's FMR1 status), while the mother's FMR1 premutation (55-200 CGG repeats) confers her own set of risks — fragile X-associated tremor/ataxia syndrome (FXTAS), fragile X-associated primary ovarian insufficiency (FXPOI), and risk of transmitting a full mutation expansion to future children. Each finding must be evaluated independently. The key clinical lesson is that establishing one genetic diagnosis in a family should not prevent investigation of unexplained symptoms in other family members.
6. A clinical geneticist is evaluating a child with neonatal epileptic encephalopathy. The WES report shows a de novo KCNQ2 missense variant classified as a VUS. The variant is in the S4 voltage-sensor domain, absent from gnomAD, and computational tools predict it is deleterious. What additional evidence would MOST likely upgrade this variant to Likely Pathogenic or Pathogenic?
The variant already has de novo status (PS2 — Strong), location in a critical functional domain (PM1 — Moderate), absence from population databases (PM2 — Moderate), and computational predictions of deleteriousness (PP3 — Supporting). Under ACMG criteria, this combination may reach Likely Pathogenic but could be strengthened further. Well-established functional studies (PS3 — Strong) demonstrating that the variant alters Kv7.2 channel biophysics (e.g., shifted voltage-dependence, altered conductance, disrupted M-current) would provide the additional Strong evidence needed to confidently classify the variant as Pathogenic. This highlights the interplay between clinical genomics and functional biology in variant interpretation.