Pearls & Oy-sters: Use of Muscle Ultrasound as a Clinical Tool in INPP5K-Related Muscular Dystrophy

Biallelic pathogenic variants in <i>INPP5K</i> have been associated with a rare congenital muscular dystrophy that presents with muscle weakness, short stature, intellectual disability, and early-onset cataracts. A characteristic pattern of muscle involvement has been identified on muscle MRI in a small case series, including involvement of the vasti, anterior tibialis, and peronei with relative sparing of the rectus femoris, sartorius, and gracilis muscles. This case describes a patient who initially presented in infancy with hypotonia, motor delays, and short stature. She was eventually diagnosed at almost 3 years with <i>INPP5K</i>-related muscular dystrophy after extensive workup that included multiple subspecialist evaluations, genetic testing for non-neuromuscular disorders, and a muscle biopsy. Muscle ultrasound (MUS) was performed at the end of this diagnostic journey, which demonstrated characteristic features that supported the diagnosis, including notable involvement of the vasti muscles with sparing of the rectus femoris. This case highlights how MUS can be a useful tool in the evaluation of children for neuromuscular disorders. MUS can help refine the differential and guide further steps in evaluation when performed early in the diagnostic process and may help clarify interpretation of genetic testing results when performed later.