What is Linear Nevus Sebaceous Syndrome (aka Schimmelpenning Syndrome)?

What Is It? 

Schimmelpenning syndrome is a rare multisystem disorder characterized by sebaceous nevus associated with other abnormalities outside the skin which most commonly affect the brain, eyes and bones. The skin lesions associated with this disorder are called sebaceous nevus because they consist of an increased number of sebaceous glands (small oil-producing glands in the skin) along with an overgrowth (hyperplasia) of the epidermis and dilated apocrine glands. Sebaceous nevus is the most common type of organoid epidermal nevi (which broadly encompass abnormally formed adnexal skin elements such as hair follicles and glands within the skin). Epidermal nevi are usually present at birth (congenital), although they might not be identified until later during childhood or after puberty. Affected individuals may also have abnormalities affecting the brain such as seizures or intellectual impairment, the eyes such as clouding (opacity) of the cornea or partial absence of tissue of the iris or retina (coloboma), and the skeleton such as spinal malformations, craniofacial defects, and deformities of the arms and legs.Schimmelpenning syndrome occurs randomly for no apparent reason (sporadically) during the formation and development of the embryo (embryogenesis), most likely due to a mutation of a gene that occurs after fertilization (postzygotic mutation) and is present in only some of the cells of the body (mosaic pattern).

How Does It Manifest?

The characteristic skin lesion that affects individuals with Schimmelpenning syndrome is a sebaceous nevus, which is a type of epidermal nevus. The scalp, neck and mid-facial area are most often affected. The arms, legs and trunk may also be affected. Sebaceous nevi are usually salmon or yellowed colored, hairless, smooth patches. Eventually (usually around puberty) they become more pronounced and may appear scaly, warty or thickened. When the scalp is involved, large lesions may be present and patchy areas of hair loss (alopecia) may occur. Sebaceous nevi can be prominent and easily noticeable at birth or be subtle and easily missed. Sometimes, sebaceous nevi do not become apparent until after puberty when they go through the above mentioned changes. The lesions, apart from their appearance, usually do not cause additional symptoms.

Neurological abnormalities often occur in individuals with Schimmelpenning syndrome include seizures, delays in attaining developmental milestones (developmental delays), intellectual impairment, damage to certain cranial nerves and abnormalities affecting certain structures of the brain. Such abnormalities include one side of the brain being larger than the other (hemimegalencephaly), malformation (dysplasia) of the certain brain vessels, absence (agenesis) of the bundle of nerves that connects the two cerebral hemispheres (corpus callosum), and defects of the folds of the brain including a smooth brain that lacks the distinctive folds (agyria), abnormally small folds (microgyria) and abnormally thickened folds (pachygyria).

Ocular abnormalities also occur in Schimmelpenning syndrome including a partial absence of tissue (coloboma) from the colored portion of the eye (iris) or the membrane lining the back of the eyes (retina), clouding (opacity) of the cornea, crossed eyes (strabismus), defects of the optic nerve and scarring degeneration or detachment of the retina. Some individuals may have a benign, yellowish-white, fatty tumor on the outer portion of the eyeball (epibulbar lipodermoid). A sebaceous nevus on the face can potentially involve structures in the eye including the eyelids and the thin, clear membrane that covers the outer surface of the eye (conjunctiva).

Affected individuals may also have skeletal malformations including abnormal curvature of the spine, dislocation of the hip, and deformities of the limbs. Craniofacial defects such as an unusually prominent forehead (frontal bossing), underdeveloped nasal and orbital bones and asymmetry of the skull may also occur. Additional skeletal malformations may include bone cysts, underdevelopment of the pelvis and incomplete formation of certain bony structures including the ankle, foot and bones of the spinal column (vertebrae).

Individuals with Schimmelpenning syndrome may also develop vitamin D-resistant rickets, a condition characterized by bow deformities of the legs, pain in the legs and progressive softening of the bone structure. In children, growth rates may be slow, ultimately resulting in short stature. Affected individuals may be prone to fractures.

According to the medical literature, sebaceus nevi are associated with an increased risk (approximately 25 % of affected individuals) of developing secondary benign skin tumors such as trichoblastoma or syringocystadenoma papilliferum, whereas malignant secondary tumors (basal cell carcinoma, squamous cell carcinoma, sebaceous carcinoma) on the basis of a sebaceous nevus are very rare but cannot be excluded.

What Causes It?

The exact cause of Schimmelpenning syndrome is unknown. The disorder is believed to be caused by a mutation in a gene that occurs after fertilization of the embryo (postzygotic mutation), most likely early during embryonic development. Affected individuals have some cells with a normal copy of this gene and some cells with the abnormal gene (mosaic pattern). This may be referred to as having two distinct cell lines in the body. The variability of symptoms associated with Schimmelpenning syndrome is due in part to the ratio of healthy cells to abnormal cells. When all cells have the abnormal gene, the condition is not compatible with life. Researchers believe that these postzygotic mutations occur randomly for no apparent reason (sporadically).


Rarediseases.org
https://www.rarediseases.org/rare-disease-information/rare-diseases/byID/1267/viewFullReport






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