Table 1.

Summary of molecular findings and phenotype in patients 1 and 2

Patient	1	2
Sex	M	F
PIK3CA mutation	c.3140A > G	c.1624G > A
Protein alteration	p.(His1047Arg)	p.(Glu542Lys)
Affected cells (%)	36	52 (biopsy I) 34 (biopsy II)
Tissue analyzed	Ectopic muscle, dorsum of the left hand	I. Ectopic dorsal interosseous muscle, right side II. Ectopic extensor digitorum muscle, right side
Method for detection	Gene panel dPCR	Whole genome sequencing (WGS) Digital polymerase chain reaction (dPCR)
Description	Unilateral overgrowth of the left arm and forearm muscle, with no signs of edema neither fatty infiltration nor vascular anomaly. Swan neck‐deformity of the index finger and ulnar deviation	Bilateral overgrowth of arms and forearms muscle, with no signs of edema nor fatty infiltration
Ectopic muscles. Localization and characterization. ‐ In hand	‐ Larger fibrotic muscle mass around the extensor tendons on dorsal side of the hand ‐ Transverse ectopic muscle on the dorsal side of the proximal phalanx in the index finger ‐ Normal junctura between second and third extensor indicis communis tendons was replaced by ectopic muscle	Right hand: ‐ Fibrotic and hypertrophic muscle above the first dorsal interosseous muscle attached to the ulnar side of first metacarpal ‐ Hypertrophic abductor pollicis brevis ‐ Six accessory longitudinal muscles attached to the proximal phalanx of digiti two (ulnar), three (ulnar and radial), four (ulnar and radial) and five (radial) originating in separate tendons below the carpal tunnel. Attached to fascia in the forearm ‐ Ectopic abductor digiti minimi muscle ‐ Ectopic short flexor digiti minimi muscle ‐ Broad and extended adductor pollicis muscle inserting in fascia above fifth metacarpal ‐ Palmar aponeuroses and carpal‐ligament replaced by ectopic muscle mass to a high extent
Ectopic muscles. localization and characterization. ‐ In forearm	‐ Ectopic pale fibrotic muscle in volar forearm next to the normal flexor carpi radialis muscle. No wrist or finger movement was noted when traction was applied to the attached tendon	Bilateral in forearms: ‐ Accessory muscle mass originating deep to extensor digiti communis from middle of forearm to insertion in digit two to five via separate broad tendons. Muscle had separate nerve branches. ‐ Accessory extensor pollicis longus (EPL) muscles with tendons parallel to normal EPL Right forearm: ‐ Accessory hypertrophic muscle above the normal brachioradialis muscle ‐ Accessory abductor pollicis longus muscle
Other aberrations	‐Swan‐neck deformity of the left index finger ‐Intrinsic plus position finger two and three ‐Widening between metacarpals	‐ Broad junctura ‐ Missing extensor retinaculum ‐ Missing palmar fascia ‐ Intrinsic plus position finger two and three ‐ Abducted thumbs ‐ Widening between metacarpals
Vascular anomalies	‐	Absent digital volar arteries
Pathology	Increased perimysial and endomysial fibrosis, associated with marked fiber size variability, with scattered hypertrophic fibers and many small fibers. Occasional fibers with rimmed vacuoles were found. There were also rounded eosinophilic fibers.	The pathology differed between the two muscles. In muscle I, variation in muscle fiber size and increase of connective tissue was seen while in muscle II, eosinophilic rounded fibers were present in otherwise rather well‐preserved tissue.

F = female, M = male. The aberrations in patient 2 seemed symmetrical but because of different operations performed on the right and left hand it was not confirmed in all locations.