Knobloch Syndrome 1
Patient Information
Characteristics
Background and History
Knobloch syndrome is a rare hereditary disorder of the brain and eyes first described in 1971.
Clinical Correlations
The brain may not be properly formed and in many patients the rear of the skull is not fully closed with a sac-like protrusion of brain tissue (occipital encephalocele). Some individuals seem to function normally and do not have the skull defect whereas others have problems with cognition and mental functioning. However, imaging of the brain with MRIs may reveal a variety of defects. Gait can be unsteady (ataxia) and epilepsy may occur.
The eyes have a variety of abnormalities. The majority of individuals are highly nearsighted. Cataracts are common and the lens may be dislocated in some eyes. The most serious problem is due to degeneration of the retina which frequently detaches. Many young children have poor vision and even night blindness. Unfortunately the vision loss is often progressive and blindness can occur before the age of 20 years. Nystagmus (dancing of the eyes), and strabismus (crossing of the eyes) are often present.
Genetics
Inheritance
This is an autosomal recessive condition that requires the presence of changes (mutations) in both copies of a specific gene. The parents of an affected individual which carry a single changed copy are clinically normal but confer a risk of 25% for Knobloch syndrome to each offspring.
Pedigree
Autosomal recessive
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.
Diagnosis and Prognosis
The skull defect is usually visible at birth but sometimes can be subtle if only the overlying skin is involved. An ophthalmologist is needed to diagnose the ocular changes in this syndrome. Unfortunately, no treatment is available for the brain defects and, while most retinal detachments can be repaired, such surgery is usually unsuccessful in Knobloch syndrome.
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