A split hand has a deep V-shaped cleft running down the center of the palm, with one or more middle fingers missing. The remaining fingers often sit on either side of the gap, giving the hand a lobster claw-like appearance. This is the congenital form, called split-hand/foot malformation (SHFM), which affects roughly 1 in 90,000 births. There’s also a neurological “split hand sign” seen in adults with ALS, which looks completely different. Both are covered below.
The Congenital Split Hand
In SHFM, the central part of the hand fails to develop properly during the first trimester. The bones of the middle fingers, along with portions of the palm itself, are partially or completely absent. What remains is a deep midline cleft that can extend from the fingertips all the way down through the palm. The depth and width of this cleft vary widely from person to person.
On either side of the cleft, you may see one, two, or three remaining fingers. These fingers sometimes appear fused together, a feature called syndactyly. In mild cases, only one central finger is missing and the cleft is shallow. In more severe presentations, the hand may have just one or two functional digits on each side of a wide gap. X-rays of affected hands typically reveal missing finger bones and missing palm bones (metacarpals), not just shortened or stunted ones.
The condition usually affects both hands, and the feet are often involved too. A split foot looks similar: a deep midline cleft with missing toes, sometimes with the remaining toes fused together. The severity doesn’t always match between hands and feet, or even between the left and right sides of the same person. One hand might have a mild cleft with four fingers while the other has a deep split with only two digits.
Variations in Appearance
There are at least six genetic types of SHFM, and each can look somewhat different. Some forms produce the classic deep cleft with a clean separation. Others come with additional features like duplicated thumbs, thumbs with an extra joint, or significant webbing between the remaining fingers. In one type, patients may have hands reduced to just one or two fingers total.
SHFM doesn’t always appear on its own. When it occurs as part of a broader syndrome called EEC (ectrodactyly-ectodermal dysplasia-clefting), the split hand comes alongside other visible features: sparse or thin hair, dry skin due to absent or reduced sweat glands, abnormal or missing teeth, and a cleft lip or palate. People with EEC syndrome cannot sweat normally, which means their bodies struggle to regulate temperature. The skin may appear dry or thin, and the nails are often brittle or malformed.
Some types of SHFM are also associated with hearing loss, which occurs in about 35% of patients with one particular genetic form.
What a Split Hand Can and Cannot Do
Despite its appearance, a congenital split hand is often more functional than people expect. Many individuals develop a strong pinch grip between the remaining digits on either side of the cleft. Children born with the condition adapt early, learning to grasp, hold, and manipulate objects using the fingers they have. The deeper the cleft and the fewer the remaining digits, the more challenging fine motor tasks become, but many people with SHFM live without significant functional limitations.
When surgery is considered, the primary goals are improving pinch strength and grip rather than creating a cosmetically typical-looking hand. Surgeons may close or narrow the cleft, separate fused fingers to increase range of motion, or reposition digits to improve opposition (the ability to touch the thumb to another finger). These procedures are typically done in early childhood.
The Neurological “Split Hand” in ALS
There’s a completely different condition also called “split hand,” and it looks nothing like the congenital form. This is a clinical sign seen in people with amyotrophic lateral sclerosis (ALS), where the muscles on the thumb side of the hand waste away while the muscles on the pinky side stay relatively preserved.
In a healthy hand, the fleshy pad at the base of your thumb (the thenar eminence) and the fleshy pad along the pinky side of your palm (the hypothenar eminence) are roughly similar in bulk. In ALS, the thumb-side muscles shrink noticeably. The web space between the thumb and index finger becomes hollow and sunken. The index finger and thumb lose strength and coordination, making it hard to pinch, turn keys, or button shirts. Meanwhile, the pinky side of the hand looks relatively normal.
This lopsided wasting creates a visual “split” between the two sides of the hand. It’s not a structural deformity but a pattern of muscle loss. The hand itself is fully formed, with all five fingers present. What changes is the muscle volume underneath the skin, giving the thumb side a flattened, wasted appearance while the pinky side retains its normal contour.
Why the ALS Split Hand Matters
This pattern of wasting is distinctive enough to help neurologists identify ALS early. Researchers have developed a numerical score called the split hand index that compares the electrical activity of the thumb-side muscles to the pinky-side muscles. In one study, this index correctly identified ALS with 72% sensitivity and 94% specificity, meaning it rarely flags someone who doesn’t have the disease.
The split hand sign matters because many conditions cause hand weakness and muscle loss, including nerve compression in the neck, carpal tunnel syndrome, and other motor neuron diseases. What makes ALS different is this specific pattern: the thumb side wastes first and fastest, while the pinky side is relatively spared. In most other conditions that cause hand wasting, both sides tend to be affected more evenly, or the pattern follows the path of a compressed nerve rather than this thumb-versus-pinky split.