arachnodactyly, dolichostenomelia; spider finger; a
condition in which the hands and fingers, and often the feet and
toes, are abnormally long and slender; a characteristic of Marfan's
syndrome and Achard syndrome.
Radiologicaly the hands are imaged and measurements
When arachnodactyly is subtle, the metacarpal index can be
determined by dividing the length of each of the last four
metacarpals by the width of its midpoint and averaging the values.
Marfan's patients are often grater than 8.4, while normals are less
Other skeletal findings include high palate, long face, pectus
excavatum and recurrent joint dislocations.
Marfan syndrome, caused by a mutant gene, afflicts one in 5,000 to
one in 10,000 people, making it as common in the population as much
better known genetic conditions like sickle cell anemia and more
common than cystic fibrosis and hemophilia. Marfan's is a highly
variable but potentially deadly disorder of connective tissue
involving a defect in a protein called fibrillin.
The defect results from one of many possible mutations in a very
large gene. Only one defective gene is needed to cause the
condition, which can be passed from either parent to both boys and
girls. When a parent has the syndrome, each child faces a 50-50
chance of inheriting it. In about one quarter of the cases, however,
the mutation is not inherited but occurs spontaneously.
Although the gene involved has been identified, there is no
screening test for Marfan's, because so many different mutations can
be responsible for the syndrome. Prenatal testing is possible,
however, if a parent has Marfan's and the particular mutation
involved is identified.
Defect in gene coding for fibrillin structure
Connective tissue defect affecting multiple systems
Tall (Height exceeds 95th percentile for age)
Extremely slender build
Cardiovascular signs and conditions
Mitral Valve Prolapse
Aortic root dilatation
Congestive Heart Failure
Subacute Bacterial Endocarditis
Musculoskeletal signs and conditions
Arachnodactyly (Spider fingers)
Pectus deformity (pigeon breast or funnel breast)
High narrow palate
Arm span exceeds height
Leg length exceeds trunk length
Hyperextensible joints and ligaments
Vertebral column deformities (e.g. Kyphoscoliosis)
Ocular signs and conditions
Upward ectopia lentis
Arachnodactyly is one of the signs of Marfan's Syndrome, and refers
to the elongated fingers or spider finger characteristic
The two most serious complications are loss of vision because of
dislocation of the eyes' lenses and detached retinas, and weakening
of the aorta, rendering it prone to rupture, a usually fatal
occurrence. Both problems underscore the importance of avoiding
certain physical activities: contact sports, anything that involves
jolting movements and sports like sprinting and bicycle racing that
raise the heart rate very high.
To reduce the risk of such complications, patients are commonly
treated with a drug like propranolol (Inderal) to reduce stress on
the aorta by decreasing the force of heart-muscle contractions. They
should also undergo annual cardiovascular examinations called
echocardiograms to monitor the condition of the aorta and detect
other heart problems.
Many Marfan patients also have weakened heart valves and must take
antibiotics before undergoing dental work or any other procedure
that could spew infectious organisms into the bloodstream and cause
a cardiac infection.
Frequent eye examinations are also advisable, and patients must seek
eye treatment without delay when they notice any visual abnormality.
For patients with dislocated lenses and retinal detachment, surgery
may restore vision.
Calculating the metacarpal index Method 1
MI = (A/a + B/b + C/c + D/d) / 4
Positive if MI > 8.4
Image2 Calculating the metacarpal index
The outer- and inner diameters of the metacarpal bone is measured,
as shown below. From these measurements, the Combined Cortical
Thickness (CCT) and the Metacarpal Index (MCI) are easily calculated
CCT = L1 - L2
MCI = CCT / L1
More information from the National Marfans Association at