A 6-month-old female infant born to consanguineous parents in Tamil Nadu presents with progressive neurological deterioration, seizures, and developmental regression over the past 3 months. On examination, there is marked hypotonia, cherry-red spot on the macula, and hepatosplenomegaly. Serum acid phosphatase and hexosaminidase A are markedly reduced. Which of the following is the most likely diagnosis?

Explanation

## Diagnosis: GM2 Gangliosidosis (Tay-Sachs Disease) ### Clinical Presentation Analysis **Key Point:** Tay-Sachs disease is an autosomal recessive lysosomal storage disorder caused by deficiency of **hexosaminidase A (Hex-A)**, leading to accumulation of GM2 ganglioside primarily in the central nervous system. ### Pathognomonic Clinical Triad The three cardinal findings present in this case are: 1. **Cherry-red spot on the macula** — pathognomonic sign caused by accumulation of GM2 ganglioside in retinal ganglion cells, creating a contrast with the surrounding pale retina 2. **Progressive neurological deterioration with seizures** — GM2 accumulation in neurons causes progressive loss of function 3. **Developmental regression** — loss of previously acquired milestones (developmental delay) ### Biochemical Findings | Feature | Tay-Sachs (GM2) | GM1 Gangliosidosis | Metachromatic Leukodystrophy | Krabbe Disease | |---------|-----------------|-------------------|------------------------------|----------------| | **Enzyme defect** | Hexosaminidase A | β-Galactosidase | Arylsulfatase A | Galactocerebrosidase | | **Substrate** | GM2 ganglioside | GM1 ganglioside | Sulfatide | Galactocerebroside | | **Cherry-red spot** | Yes (classic) | Yes (can occur) | No | No | | **Hepatosplenomegaly** | Mild/absent | Yes (prominent) | No | No | | **Hex-A level** | ↓↓ Markedly reduced | Normal | Normal | Normal | | **Acid phosphatase** | ↑ Elevated | ↑ Elevated | Normal | Normal | | **White matter involvement** | Minimal | Minimal | Severe (demyelination) | Severe (demyelination) | | **Age of onset** | 3–6 months | Birth–6 months | 12–18 months | 3–6 months | ### High-Yield Diagnostic Clue **High-Yield:** The combination of **cherry-red spot + marked reduction in hexosaminidase A + early infantile neurological decline** is diagnostic of Tay-Sachs disease. The **reduced acid phosphatase** is a secondary finding reflecting lysosomal dysfunction. ### Mnemonic: CHERRY-RED SPOT Disorders **Mnemonic: CRS** — **C**erebral storage (Tay-Sachs, GM1), **R**etinal ganglion cell involvement (cherry-red spot), **S**ialylated substrate (gangliosides) Diseases with cherry-red spot: - **Tay-Sachs** (GM2 gangliosidosis) — Hex-A deficiency - **GM1 gangliosidosis** — β-galactosidase deficiency - **Sialidosis** — neuraminidase deficiency - **LINCL** (late infantile neuronal ceroid lipofuscinosis) ### Clinical Pearl **Clinical Pearl:** Tay-Sachs disease has a **higher incidence in Ashkenazi Jewish populations** (1 in 3,600 births), but consanguinity in this case (South Indian family) suggests autosomal recessive inheritance in a different population. Carrier screening and prenatal diagnosis are available via enzyme assay or genetic testing. ### Pathophysiology: Why These Findings? 1. **Hex-A deficiency** → GM2 ganglioside accumulates in lysosomes 2. **Neuronal accumulation** → progressive neuronal dysfunction and death → developmental regression and seizures 3. **Retinal ganglion cell storage** → opacification of ganglion cell layer → cherry-red spot (fovea appears red because underlying choroid is visible) 4. **Macrophage infiltration** → mild hepatosplenomegaly ```mermaid flowchart TD A[Hexosaminidase A deficiency]:::outcome --> B[GM2 ganglioside accumulation in lysosomes]:::outcome B --> C[Neuronal dysfunction]:::action C --> D[Developmental regression & seizures]:::urgent B --> E[Retinal ganglion cell involvement]:::action E --> F[Cherry-red spot on macula]:::outcome B --> G[Macrophage infiltration]:::action G --> H[Mild hepatosplenomegaly]:::outcome ``` [cite:Robbins 10e Ch 5; Harrison 21e Ch 356]