Even though the family may not have a history of Sandhoff disease, it is possible for two individuals to have a child with the disease. Since Sandhoff disease was only discovered in 1968, there are years the disease has gone undetected because of misdiagnoses. Pathophysiology
Biallelic pathogenic variants in the HEXB gene cause Sandhoff disease. The gene provides instructions for making a protein crucial to the enzymes beta-hexosaminidase A and beta-hexosaminidase B, which function in nerve cells to break down fatty substances, complex sugars, and molecules that are linked to sugars. In particular, beta-hexosaminidase A breaks down a fatty compound called GM2 ganglioside. Mutations in the HEXB gene disrupt the activity of these enzymes, preventing the breakdown of GM2 ganglioside and other molecules.As a result, progressive damage caused by the resulting buildup of GM2 ganglioside leads to the destruction of nerve cells, causing the signs and symptoms associated with Sandhoff disease. Diagnosis
Sandhoff disease can be detected through the following procedures (before it is apparent through physical examination): a biopsy removing a sample of tissue from the liver, genetic testing, molecular analysis of cells and tissues (to determine the presence of a genetic metabolic disorder), enzyme assay, and occasionally a urinalysis to determine if the above-noted compounds are abnormally stored within the body. For a child to suffer from this disease, both parents must be carriers, and both must transmit the mutation to the child.
Persistent tunica vasculosa lentis is a congenital ocular anomaly. It is a form of persistent hyperplastic primary vitreous (PHPV). It is a developmental disorder of the vitreous. It is usually unilateral and first noticed in the neonatal period. It may be associated with microphthalmos, cataracts, and increased intraocular pressure. Elongated ciliary processes are visible through the dilated pupil. A USG B-scan confirms diagnosis in the presence of a cataract. See also
Persistent fetal vasculature
Tunica vasculosa lentis

References

http://www.djo.harvard.edu/files/1568.pdf
Wright KW, Speigel PH, Buckley EG. Pediatric Ophthalmology and Strabismus. Springer; US, New York: 2003. p. 17.