King Syndrome

This 8-year-old girl presented with bilateral ptosis, down-slanting palpebral fissures, malar hypoplasia, mild micrognathia, and mild webbing of the neck. She also had marked lumbar lordosis and a dextroconvex thoracic scoliosis with scapular winging. There was a generalized reduction in muscle mass with proximal limb weakness, short stature, diminished deep tendon reflexes, and an awkward waddling gait.

Review of the child's past health showed that she had marked delay in gross motor development and first walked independently at the age of 23 months. Her development and intelligence were otherwise normal. During surgical release of a tethered spinal cord when she was 6 years old, she experienced malignant hyperthermia with hyperpyrexia (body temperature, 39.1°C [102.4°F]), tachypnea, tachycardia, hypertension, marked stiffening of the limbs, and a notably elevated serum creatine kinase level (13,300 U/L). She was

successfully treated with intravenous fluid, furosemide, and dantrolene sodium. Results of a muscle biopsy subsequently revealed a nonspecific myopathy.

The child had King syndrome, which is characterized by a Noonan-like facial appearance, myopathy, and susceptibility to malignant hyperthermia. The typical facial features include ptosis, down-slanting palpebral fissures, malar hypoplasia, micro-gnathia, and low-set ears.

Common clinical features of this syndrome include a nonspecific myopathy with delayed gross motor development, diminished muscle bulk, weakness, and reduced deep tendon reflexes. Short stature, kyphoscoliosis, lordosis, and cryptorchidism are also seen in most patients with King syndrome, the majority of whom are male.

King syndrome can be differentiated from Noonan syndrome by the absence of hypertelorism, epicanthic folds, lymphedema, and congenital heart defects, and by the presence of myopathy with predisposition to malignant hyperthermia. King syndrome likely results from a heterogeneous group of sporadically occurring congenital myopathies, while Noonan syndrome is an autosomal dominant, single-gene disorder.