Differential Diagnosis: Chronic Quadriparesis + Hypotonic Areflexia + Proximal Muscle Weakness + Lost Joint Position Sense + Horizontal & Vertical Nystagmus + Square Wave Jerks | 28F, Onset 1st Decade, Sporadic

Primary Differentials (High Probability)

1. Friedreich Ataxia (FRDA) - Most Likely

• Genetics: Autosomal recessive; GAA trinucleotide repeat expansion in intron 1 of FXN (frataxin gene, 9q13-q21). No family history is classic if both parents are silent carriers (carrier frequency ~1/100 in Europeans).
• Onset: Typically 1st-2nd decade (before age 25), median ~10 years.
• Core features: Progressive mixed sensory-cerebellar ataxia, loss of deep tendon reflexes (especially lower limbs), loss of joint position and vibration sense, nystagmus (horizontal > vertical), dysarthria, Babinski sign (extensor plantar), scoliosis/kyphosis, pes cavus, hypertrophic cardiomyopathy.
• Square wave jerks: Explicitly listed in the Bradley & Daroff textbook differential for SWJs.
• Proximal weakness: Distal > proximal as a rule, but progressive course produces generalized weakness; some patients develop significant proximal weakness late.
• Why it fits: Sporadic presentation (autosomal recessive), 1st decade onset, loss of JPS, areflexia, nystagmus, SWJs, female sex (equally affected).
• Key distinguishing features: Cardiomyopathy (ECG, Echo), scoliosis, pes cavus, extensor plantars (may be absent in hypotonic stage).
• Investigations: FXN GAA repeat PCR (99% sensitive for classic FRDA); Echo; EMG/NCS (axonal sensory neuropathy).
• Goldman-Cecil Medicine, p. 533; Adams & Victor, p. 1107

2. Ataxia with Vitamin E Deficiency (AVED)

• Gene: TTPA (alpha-tocopherol transfer protein), autosomal recessive.
• Presentation: Clinically identical to Friedreich ataxia - areflexia, JPS loss, cerebellar ataxia, nystagmus, onset in 1st-2nd decade.
• Critical distinction: Treatable - serum vitamin E is very low; supplementation arrests or reverses progression.
• Why it matters: Must not be missed. No family history is consistent with recessive inheritance.
• Goldman-Cecil Medicine: "treatable conditions with similar clinical manifestations include ... selective defect in vitamin E absorption"
• Investigation: Serum alpha-tocopherol (vitamin E level) - mandatory in every Friedreich-like case.

3. Abetalipoproteinemia (Bassen-Kornzweig Syndrome)

• Gene: MTTP (microsomal triglyceride transfer protein), autosomal recessive.
• Onset: Early childhood (1st decade), consistent with this case.
• Features: Friedreich-like ataxia + sensory neuropathy + areflexia + steatorrhea/fat malabsorption + pigmentary retinopathy + acanthocytosis on peripheral smear + extremely low serum cholesterol/triglycerides + absent apolipoprotein B.
• Nystagmus: Present in advanced disease.
• Treatable: High-dose fat-soluble vitamins (A, D, E, K).
• Investigation: Fasting lipid profile, peripheral blood smear, serum vitamin E/A levels, apoB.

4. Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay (ARSACS)

• Gene: SACS (sacsin), chromosome 13q12.
• Onset: Early childhood (12-18 months onset of gait problems), well within 1st decade.
• Features: Slowly progressive spastic ataxia (cerebellar ataxia + pyramidal spasticity) + nystagmus + distal muscle wasting + demyelinating peripheral neuropathy + retinal nerve fiber hypermyelination.
• Note: Spasticity is a hallmark in Quebec cases, but non-Quebec cases (worldwide spread) may present with absent reflexes, ophthalmoplegia, or less spasticity - making it phenotypically overlap with this case.
• SWJs: Cerebellar component produces saccadic intrusions.
• MRI: Superior cerebellar vermis atrophy, linear pontine T2 hypointensities.
• Why it fits: Quadriparesis, childhood onset, female, sporadic (recessive), cerebellar eye signs.

5. Ataxia-Oculomotor Apraxia Type 1 (AOA1) and Type 2 (AOA2)

• AOA1: APTX (aprataxin) gene. AR. Onset ~7 years. Features: cerebellar ataxia, oculomotor apraxia, areflexia, nystagmus, chorea early then disappears, hypoalbuminemia, hypercholesterolemia, axonal sensorimotor neuropathy.
• AOA2: SETX (senataxin) gene. AR. Onset 10-22 years. Features: cerebellar ataxia, oculomotor apraxia, sensorimotor neuropathy, areflexia, elevated AFP (alpha-fetoprotein) - a useful biomarker; more common in females.
• Square wave jerks / abnormal eye movements: Expected given cerebellar/brainstem involvement.
• Why it fits: Sporadic, childhood onset, female, areflexia, JPS loss, nystagmus.
• Investigation: AFP (elevated in AOA2), albumin (low in AOA1), genetic panel.

6. Ataxia-Telangiectasia (A-T)

• Gene: ATM, autosomal recessive.
• Onset: 1st decade (typically age 1-3 years; symptoms worsen through childhood).
• Features: Cerebellar ataxia + oculomotor apraxia + telangiectasias (conjunctival, skin) + immunodeficiency (recurrent infections) + elevated AFP + hypotonia/areflexia as disease progresses + increased cancer risk.
• Nystagmus: Present; square wave jerks are a recognized feature.
• Note: Telangiectasias may be subtle. Absence of visible telangiectasias at age 28 makes classic A-T less likely, but ATM variants with later/milder phenotypes exist.
• Investigation: AFP (very elevated), serum immunoglobulins, ATM kinase activity in lymphocytes, genetic testing.

7. Spinocerebellar Ataxia Type 3 / Machado-Joseph Disease (SCA3/MJD) or SCA2

• Inheritance: Autosomal dominant - but de novo mutations can explain "sporadic" cases with no family history. Also, reduced penetrance or early parental death can mimic sporadic.
• SCA2: ATXN2. Prominent early areflexia is a hallmark of SCA2 (unusual for other dominant SCAs). Slow saccades + nystagmus + cerebellar ataxia + peripheral neuropathy.
• SCA3: ATXN3. Quadriparesis, cerebellar + pyramidal + extrapyramidal features. Age of onset 10-70 years; de novo cases described.
• SWJs: Explicitly listed in both SCA8 and SCA20 in the localization textbook.
• Investigation: SCA trinucleotide repeat panels (SCA1/2/3/6/7/8/10/12/17).

8. Mitochondrial Disease (POLG-Related Ataxia / MIRAS / NARP / MERRF)

• POLG1 mutations (SANDO / MIRAS): Sensory ataxia neuropathy, dysarthria, ophthalmoparesis. AR inheritance. Loss of JPS, areflexia, cerebellar degeneration, nystagmus. Can present in 1st-3rd decade.
• NARP (Neurogenic muscle weakness, Ataxia, Retinitis Pigmentosa): Mitochondrial inheritance (maternal). Proximal weakness + sensory neuropathy + ataxia + nystagmus + retinitis pigmentosa. MT-ATP6 mutation (m.8993T>G/C).
• Why to consider: Mitochondrial diseases can present sporadically (especially nuclear gene mutations or de novo mtDNA variants). Proximal weakness fits.
• Investigation: Serum/CSF lactate + pyruvate, MRI brain (Leigh syndrome pattern), muscle biopsy (ragged-red fibers, COX deficiency), mtDNA sequencing, POLG sequencing.

9. Cerebrotendinous Xanthomatosis (CTX)

• Gene: CYP27A1, autosomal recessive.
• Onset: Childhood to early adulthood.
• Features: Cerebellar ataxia + spastic paraparesis/quadriparesis + peripheral neuropathy (areflexia) + tendon xanthomas + premature cataracts + intellectual disability + white matter changes on MRI.
• Treatable: Chenodeoxycholic acid.
• Why to consider: Sporadic presentation, childhood onset, mixed upper and lower motor neuron signs, cerebellar eye signs.
• Investigation: Plasma cholestanol (elevated), urine bile alcohols.

10. Multiple Sclerosis (MS) - Relapsing or Progressive

• Particularly primary progressive MS or a cerebellar-predominant phenotype.
• Female predominance, onset possible in 1st-2nd decade (pediatric MS exists).
• Nystagmus (horizontal and vertical), cerebellar features, pyramidal/sensory deficits.
• Against: Hypotonic, areflexic quadriparesis suggests lower motor neuron/peripheral involvement - less typical of pure MS (which is upper motor neuron). Square wave jerks occur in MS with cerebellar dysfunction.
• Investigation: MRI brain/spine (plaques in periventricular, juxtacortical, infratentorial, spinal locations), CSF oligoclonal bands, VEP.

Summary Table

Recommended Initial Workup

1. Serum vitamin E (alpha-tocopherol) - rules out AVED
2. Fasting lipid profile + apolipoprotein B + peripheral smear (acanthocytes) - rules out abetalipoproteinemia
3. Serum AFP - elevated in AOA2, AT
4. Serum albumin - low in AOA1
5. FXN GAA repeat analysis (Friedreich ataxia) - single most important test
6. Plasma cholestanol - CTX
7. Serum/CSF lactate + pyruvate - mitochondrial
8. MRI brain + spine with gadolinium - cerebellar/brainstem atrophy pattern, Leigh, MS plaques, ARSACS-typical pontine changes
9. EMG/NCS - axonal sensory neuropathy (FRDA, AOA) vs. demyelinating (ARSACS, ABL)
10. ECG + Echo - cardiomyopathy (FRDA, mitochondrial)

• Whole exome sequencing / ataxia gene panel (covers SACS, SETX, APTX, POLG, CYP27A1, TTPA, MTTP, ATM, frataxin, SCAs)
• Mitochondrial DNA sequencing
• CSF oligoclonal bands (MS)
• mtDNA mutation panel (NARP: m.8993)
• Eye exam by neuro-ophthalmologist (retinal telangiectasias, RNFL hypermyelination)

Clinical Pearl