Neurodegenerative and leukodystrophy disorders — branching viva

Opening branch — the boy with the parieto-occipital lesion

A previously well seven-year-old presents with two months of school decline, difficulty following the ball at sport, and intermittent vomiting, with an unenhanced brain MRI showing symmetric parieto-occipital white-matter change. The candidate must first recognise this as a likely cerebral X-linked adrenoleukodystrophy and name the mechanism-based classification - lysosomal and dysmyelinating, peroxisomal and inflammatory, hypomyelinating, astrocytopathic and interferonopathic - placing this disorder in the peroxisomal and inflammatory group driven by very-long-chain fatty acid accumulation and CD8 T-cell-mediated inflammation. [1] [2]

The examiner probes the management decision. The candidate states that the single addition that settles the transplant question is gadolinium contrast, because the enhancing leading edge of the inflammatory phase confirms active cerebral disease and is missed on the unenhanced scan, and that the Loes score alongside the neurological examination stages the disease. The teaching point is that allogeneic haematopoietic stem cell transplant halts the inflammation only when the burden is still low and the examination preserved, so the case is a transplant-pace emergency referred the same day. [2]

Second branch — the tiered workup and the adrenal question

The examiner asks for the investigation strategy and the immediate management. The candidate gives the magnetic-resonance-imaging-led tiered approach: a contrasted brain MRI with the Loes score, plasma very-long-chain fatty acids to confirm the ABCD1 defect, a synacthen test to exclude adrenal insufficiency with hydrocortisone replacement if deficient, and urgent referral to a transplant service. The candidate explains that adrenal insufficiency accompanies a substantial fraction of cerebral adrenoleukodystrophy, can be the first presentation, and untreated Addisonian crisis is fatal, so adrenal assessment is essential and independent of the neurological decision. [2]

The candidate then generalises to the workup for any unexplained white-matter change: targeted biochemistry matched to the imaging pattern (very-long-chain fatty acids, arylsulfatase A and urinary sulfatides, galactocerebrosidase and psychosine), magnetic resonance spectroscopy for the metabolic signature such as the N-acetylaspartate peak of Canavan disease, and a leukodystrophy gene panel or trio exome when the biochemistry is unrevealing - never accepting a degenerative label until the contrasted scan has excluded cerebral adrenoleukodystrophy. [1]

Third branch — the girl with the tigrid pattern

The case shifts to a two-year-old girl who has lost the ability to walk and most of her single words over six months, with a progressive stiff and unsteady gait, optic atrophy, and symmetric confluent periventricular white-matter change with a tigrid pattern. The candidate diagnoses metachromatic leukodystrophy, anchored on the lysosomal mechanism - arylsulfatase A deficiency with sulfatide accumulation destroying formed myelin - confirmed by low leucocyte arylsulfatase A activity with increased urinary sulfatides and molecular testing to exclude pseudodeficiency. [1]

The examiner probes the therapy. The candidate states that lentiviral haematopoietic stem cell gene therapy works when given presymptomatically or very early in the disease, whereas the benefit is limited once symptoms are established, which is why the diagnosis must be made at speed and why newborn screening and at-risk-sibling testing matter. The candidate distinguishes this from vanishing white matter - rarefied cavitating white matter with stepwise post-fever deterioration - where the immediate act is trigger avoidance because there is no curative therapy. [4] [5]

Fourth branch — the neonate identified by newborn screening

The examiner moves to a neonate identified by newborn bloodspot screening positive for X-linked adrenoleukodystrophy. The candidate explains that newborn screening lets the cerebral form be detected at its earliest, most treatable stage, and that a structured magnetic-resonance-imaging surveillance programme through the highest-risk childhood years is the safeguard. The candidate names the principle: the treatable leukodystrophies are treated before symptoms begin, and surveillance plus early therapy converts a fatal diagnosis into a halted one for the screen-positive boy. [3]

Closing synthesis

The examiner asks for the single unifying principle. The candidate states that every child with white-matter change and regression reduces to four questions: which mechanism group, is there contrast enhancement, what does the Loes score and the examination say about the transplant window, and is there a presymptomatic detection pathway. The governing rule is that a leukodystrophy is named by its magnetic-resonance-imaging pattern, the contrasted scan excludes the transplant emergency of cerebral adrenoleukodystrophy, and the treatable leukodystrophies are treated before symptoms begin. [1] [2]