The pedigree diagram of a family is shown below. Affected individuals present with progressive external ophthalmoplegia, pigmentary retinopathy, and cardiac conduction defects. Based on the pedigree and clinical features, what is the most likely diagnosis?

Correct Answer: D. Kearns-Sayre Syndrome

Kearns-Sayre Syndrome (KSS) is a mitochondrial cytopathy caused by large deletions (typically 4.9–7.6 kb) in mitochondrial DNA, inherited in a maternal inheritance pattern. The clinical triad—progressive external ophthalmoplegia (PEO), pigmentary retinopathy, and cardiac conduction defects—is pathognomonic for KSS. The pedigree showing maternal transmission to both sons and daughters equally, with affected mothers passing the condition to all offspring, is the key discriminator. Mitochondrial mutations affect oxidative phosphorylation in high-energy tissues (skeletal muscle, cardiac muscle, retina, CNS), explaining the constellation of findings. The cardiac conduction defects (heart block, arrhythmias) are a hallmark feature requiring pacemaker insertion in many Indian patients. PEO results from mitochondrial myopathy of extraocular muscles, while retinopathy reflects photoreceptor dysfunction. Unlike nuclear-inherited disorders, KSS shows no male-to-male transmission and affects both sexes equally when inherited from an affected mother. The onset is typically before age 20, with progressive course over decades.

Why the other options are wrong

A. Friedreich Ataxia — Friedreich Ataxia is autosomal recessive (not maternal inheritance) and presents with progressive ataxia, sensory loss, and cardiomyopathy—but NOT the classic triad of PEO + pigmentary retinopathy + conduction defects. The pedigree pattern (maternal transmission to all offspring) rules out autosomal recessive inheritance. FA typically manifests in childhood with gait ataxia, not ophthalmoplegia. B. Duchenne Muscular Dystrophy — DMD is X-linked recessive, affecting primarily males with severe proximal muscle weakness and pseudohypertrophy—not the ophthalmoplegia or pigmentary retinopathy seen here. The pedigree showing affected females passing the condition to both sons and daughters equally contradicts X-linked inheritance. DMD does not cause PEO or retinopathy; cardiac involvement is dilated cardiomyopathy, not conduction defects. C. Myotonic Dystrophy — Myotonic Dystrophy is autosomal dominant (not maternal-only inheritance) and presents with myotonia, distal muscle weakness, cataracts, and testicular atrophy—not the PEO + pigmentary retinopathy + conduction defects triad. While it does cause cardiac conduction defects, the absence of myotonia and the maternal-only inheritance pattern rule it out. MD shows anticipation with paternal transmission, unlike the pedigree shown.

High-Yield Facts

Mnemonics

KSS Triad = PRC Progressive external ophthalmoplegia, Retinopathy (pigmentary), Cardiac conduction defects. Use this to recall the classic presentation instantly. Maternal Inheritance = Mitochondrial If the pedigree shows only mothers transmitting to all children (both sons and daughters equally), think mitochondrial disorder → KSS is the classic mitochondrial cytopathy with this inheritance and this triad.

NBE Trap

NBE pairs "cardiac involvement" with multiple neuromuscular disorders (DMD, myotonic dystrophy, Friedreich ataxia) to distract from the specific combination of PEO + pigmentary retinopathy + conduction defects, which is pathognomonic for KSS. Students may also confuse maternal inheritance with X-linked inheritance if they misread the pedigree.

Clinical Pearl

In Indian pediatric practice, KSS patients often present to cardiology first with syncope or arrhythmia requiring pacemaker insertion, then are recognized to have ophthalmoplegia and retinopathy on detailed examination. Maternal family history of "eye weakness" or "heart problems" is a red flag for KSS in young patients.

_Reference: Harrison Ch. 462 (Mitochondrial Diseases); OP Ghai Ch. 15 (Neuromuscular Disorders)_