1021 - Facioscapulohumeral Muscular Dystrophy Variant Presenting as Chronic Tibial Pain

A 30-year-old female recreational athlete with a history of foveal hypoplasia reported bilaterally alternating antero-lateral tibial pain, ongoing for 10 years. She was initially diagnosed with Medial Tibial Stress Syndrome and later had MRI findings suggesting Tibial Stress fractures at our sports medicine clinic. Initial response to conservative treatment was positive. However, symptoms persisted over the next two years despite modified activities. There was concern for Chronic Exertional Compartment Syndrome due to occasional “tingling” in her toes with exertion. Prior to considering compartment testing, laboratory markers were ordered and notable for CK of 1166. An EMG/NCS was performed but without overt evidence of myopathy or polyneuropathy. Repeat clinical examination was remarkable for bilateral deltoid and ankle evertor weakness, but normal tibialis anterior strength. Thus, an underlying low-grade myopathy was suspected.

Discussions:

While common causes of tibial pain were considered, several features of her presentation suggested another etiology.

Her exam was consistent with a humeral-peroneal distribution of weakness. This pattern is noted in Facioscapulohumeral Dystrophy (FSHD), particularly in the scapulohumeroperoneal variant. In FSHD, serum CK may be elevated 3-5 times normal but not exceeding 1500 IU/L. Conversely, this is not a defining feature in the other etiologies mentioned. Central retinal abnormalities are also described in FSHD. Interestingly, she was undergoing separate work-up for foveal hypoplasia.

The pathophysiology of FSHD involves aberrant DUX4 gene re-expression. In FSHD1, this is due to contractions in the D4Z4 sequence on locus 4q35. In FSHD2, there is mutation to SMCHD1, a suppressor of DUX4.

Conclusions:

Tibial pain is common among athletes. When presenting atypically, it is essential to consider neuromuscular etiologies. This case presented phenotypically consistent with an uncommon FSHD variant. The pathophysiology of FSHD is complex which may explain variations. This patient was ultimately referred to genetic counseling for further investigation.