Developmental regression

A child who acquired developmental milestones (motor, language, social, cognitive) and then lost them. Regression is fundamentally different from delay: delay is a slower trajectory, regression is a falling trajectory. Confirmed loss of previously fluent skills is one of the most ominous presentations in pediatric neurology and demands urgent workup. The first job is to verify the regression (video, milestone history, school reports), then characterize what was lost and when.

Three questions structure the workup:

1. What domain regressed, and over what timescale? Motor-predominant regression points to neurometabolic (lysosomal, peroxisomal, mitochondrial) and motor-system neurodegeneration. Language-predominant regression with seizures suggests Landau-Kleffner. Social/communication regression at 18-24 months suggests autistic regression (idiopathic, MECP2, Rett syndrome). Stepwise regression with intercurrent illness suggests mitochondrial or organic acidemia decompensation.
2. Always ask: is this reversible? Treatable causes go first because the cost of missing them is permanent injury. Biotinidase deficiency, vitamin B12 deficiency, cerebral folate deficiency, cerebrotendinous xanthomatosis, Wilson disease (older child), and pyridoxine-dependent epilepsy can all halt or reverse regression with replacement.
3. Age and pattern do most of the diagnostic work. Infancy (Krabbe, GM1, Tay-Sachs infantile), 6-18 months (Rett, infantile Sandhoff), early childhood (Sanfilippo, MLD late-infantile, NCL), school-age (juvenile NCL "Batten," MLD juvenile, Wilson), adolescence (NPC, juvenile Huntington, X-linked adrenoleukodystrophy cerebral form).

Treatable / reversible (test first, do not delay)

• Biotinidase deficiency (BTD): alopecia, seborrheic dermatitis, seizures, optic atrophy, ataxia, regression. On most newborn screens; missed screens or late-presenting partial deficiency are the trap. Biotin replacement is curative.
• Cobalamin / vitamin B12 deficiency (including dietary deficiency in breastfed infants of vegan mothers, cobalamin C deficiency, and intrinsic factor deficiency): hypotonia, regression, megaloblastic anemia, hyperhomocysteinemia. Hydroxocobalamin replacement.
• Cerebral folate deficiency: low CSF 5-MTHF with normal serum folate; ataxia, regression, dyskinesias. Folinic acid crosses the BBB and reverses symptoms.
• CTX (CYP27A1): chronic diarrhea, juvenile cataracts, tendon xanthomas, dementia. Chenodeoxycholic acid is disease-modifying.
• Wilson disease: school-age + adolescent presentation with dystonia, dysarthria, tremor, psychiatric features, hepatic dysfunction. KF rings, low ceruloplasmin, elevated 24-h urine copper. Chelation and zinc are disease-modifying.
• Pyridoxine-dependent epilepsy (ALDH7A1): refractory neonatal-to-infantile seizures with regression; pyridoxine trial halts seizures and improves outcome.
• GLUT1 deficiency (SLC2A1): seizures, movement disorder, regression with low CSF glucose. Ketogenic diet is treatment.

Lysosomal storage disorders (the classic neurometabolic regressions)

• Tay-Sachs disease (HEXA, AR): infantile form with regression at 6 months, cherry-red macula, exaggerated startle, hypotonia then spasticity. Late-onset forms in adolescence/adulthood with psychiatric features and motor neuron findings.
• Sandhoff disease (GM2 gangliosidoses, HEXB): clinically similar to Tay-Sachs with added organomegaly.
• GM1 gangliosidosis: coarse facies, organomegaly, cherry-red macula, regression.
• Niemann-Pick disease: type A (infantile, cherry-red macula, severe hepatosplenomegaly), type C (later, vertical supranuclear gaze palsy, splenomegaly, gelastic cataplexy, ataxia, regression). NPC is treatable with miglustat.
• Mucopolysaccharidosis type III (Sanfilippo): the MPS that presents primarily as neurodevelopmental regression with comparatively mild somatic features. Behavioral disturbance and sleep disturbance precede regression. Coarse facial features are subtler than in MPS I/II.
• Hurler (MPS I) and Hunter (MPS II): regression plus coarse facies, dysostosis multiplex, hepatosplenomegaly. ERT and HSCT available.
• Krabbe disease (GALC): infantile globoid cell leukodystrophy with irritability, hypertonia, regression, optic atrophy. HSCT before symptom onset is the only disease-modifying option.
• Metachromatic leukodystrophy (ARSA): late-infantile (most common) and juvenile forms with gait disturbance, then regression, peripheral neuropathy. HSCT and gene therapy (atidarsagene autotemcel) available.

Peroxisomal

• X-linked adrenoleukodystrophy (ABCD1): cerebral form in boys 4-10 years presents with behavioral and academic regression, then visual and auditory deficits, gait, spasticity. Adrenal insufficiency may precede or follow neurologic disease. HSCT halts cerebral disease if done early. This is the "do not miss" leukodystrophy.
• Zellweger spectrum disorders: mostly infantile, with regression in milder forms.

Mitochondrial

• Leigh syndrome: stepwise regression with intercurrent illness, brainstem and basal ganglia T2 lesions, lactic acidosis.
• MELAS, MERRF, POLG-related disorders: regression as part of multisystem mitochondrial disease.

Neurodegeneration with brain iron accumulation (NBIA)

• PKAN (PANK2): dystonia, dysarthria, regression, eye-of-the-tiger sign on MRI (T2 hypointensity with central hyperintensity in globus pallidus).
• PLAN (PLA2G6): infantile neuroaxonal dystrophy or atypical NAD; regression, ataxia, dystonia.

Single-gene neurogenetic regressions

• Rett syndrome (MECP2): girls; normal development to 6-18 months, then loss of purposeful hand use with hand stereotypies (wringing, washing), loss of language, gait apraxia, deceleration of head growth, breathing dysrhythmia. The most recognizable regression syndrome in girls.
• MECP2 duplication syndrome: boys; regression, recurrent infections, progressive spasticity.
• Angelman syndrome: less classically "regression," but loss of language milestones with ataxia, seizures, happy demeanor.

Acquired

• Landau-Kleffner syndrome: acquired epileptic aphasia. Previously normal child loses receptive language with abnormal EEG (CSWS / electrical status epilepticus in sleep). Distinct from autistic regression.
• Autoimmune encephalitis (anti-NMDA receptor, others), subacute sclerosing panencephalitis (post-measles), HIV encephalopathy, prion disease.

1. Confirm the regression. Milestone history, school reports, home video. Video review is sometimes more informative than the office exam.
2. MRI brain (with spectroscopy if mitochondrial suspected). Pattern recognition is everything: periventricular and parieto-occipital T2 in X-ALD; frontal T2 in Canavan disease and Alexander; subcortical sparing in MLD; basal ganglia / brainstem in Leigh; symmetric atrophy with cerebellar involvement in NCL.
3. Treatable-cause screen, ordered urgently:
   • Biotinidase activity (if regression + seizures + alopecia / rash)
   • Vitamin B12, homocysteine, methylmalonic acid (consider cobalamin C deficiency)
   • CSF studies: glucose (GLUT1), 5-MTHF (cerebral folate deficiency), lactate, neurotransmitters
   • VLCFAs (X-ALD) and plasmalogens (peroxisomal biogenesis)
   • Ceruloplasmin, 24-h urine copper, slit-lamp for KF rings (Wilson, school age and older)
   • Cholestanol (CTX)
4. EEG, including overnight EEG if regression with possible seizures (Landau-Kleffner, electrical status epilepticus in sleep).
5. Metabolic screen: plasma amino acids, urine organic acids, acylcarnitines, lactate, ammonia, urine GAGs / oligosaccharides (MPS / LSD screen).
6. Targeted genetic testing. MECP2 in girls with Rett-like regression. Methylation studies if Angelman / Prader-Willi suspected. Lysosomal enzyme panel (white-cell enzymes) if LSD suspected, then confirmatory sequencing.
7. Exome sequencing when first-pass testing is negative; rapid trio exome in inpatient settings with active regression.