Split hand is a phenomenon where muscles on one side of your hand weaken. It often has links with early ALS, though some other rare conditions may also cause it.

Amyotrophic lateral sclerosis (ALS) causes the progressive loss of voluntary muscle movements and breathing.

One common symptom of ALS is hand muscle weakness. The pattern of weakness is quite specific, initially affecting some hand muscles and not others.

This phenomenon, called “split hand,” can be a sign of ALS, but doctors can’t use it in isolation for a conclusive diagnosis of ALS.

The split hand phenomenon refers to a specific type of muscle wasting (atrophy). It affects your ability to pinch or to meet the thumb and forefinger.

This happens due to the atrophy of muscles in your outer thumb, also called the abductor pollicis brevis (APB), and between the thumb and forefinger, the first dorsal interosseous (FDI).

As these muscles get smaller, the muscles by the pinky finger, particularly the abductor digiti minimi (ADM) of the outer palm, stay intact. This difference leads to the term “split hand.” As the split hand progresses, you might grasp objects using only the ring and pinky fingers.

In some cases of ALS, however, both the inner and outer hand muscles waste away. This is particularly the case in bulbar onset ALS but can happen in any ALS subtype as the condition progresses.

A doctor or specialist can observe your split hand through a nerve conduction study. Doctors use a compound muscle action potential (CMAP) test. This type of measurement looks at the relative nerve conduction in hand muscles, noting the weaker ones and the stronger ones.

the split hand sign in ALS, with weakened FDI and APB muscles on one side, but intact ADM muscle on the otherShare on Pinterest
Medical Illustration by Maya Chastain

Specific calculations help determine the relative nerve conduction in the muscles involved in split hand. The two key measurements are:

  • Split hand ratio: This ratio compares the CMAP of the hand muscles, either APB/ADM or FDI/ADM.
  • Split hand index: This formula uses all three measurements, multiplying the CMAP of the APB and FDI and dividing it by the CMAP of the ADM.

Testing the muscles using F-wave persistence instead of CMAP is also possible. Although this can be a more sensitive measurement, it also requires more stimulation of the muscles. F-wave measures cell backfiring upon stimulation.

Split hand is a common feature of ALS, showing up in about half of the people with the condition. Research suggests that the split hand index might be a helpful tool for doctors in diagnosing ALS in some people, particularly in the early stages.

The split hand index only supports other evaluations for diagnosing ALS. Doctors don’t use it on its own for ALS diagnosis.

According to a 2021 literature review, the split hand index had a sensitivity of 78% for ALS, meaning it accurately identified people with ALS 78% of the time. It had a specificity of 81%, meaning it accurately identified people who did not have ALS 81% of the time.

The test has a few limitations. It might not give accurate readings depending on the amount of muscle atrophy. It also assumes that there is no compression of the nerves in the middle and outer hand because of another condition.

Split hand is also not unique to ALS. Experts see this phenomenon in other conditions, such as:

There is no single test to diagnose ALS. A doctor might run several tests on reflexes and muscle responses. Among the other tests you might receive are:

Early signs and symptoms of ALS

ALS causes the progressive loss of muscle movements. Initial symptoms might include:

  • muscle twitching (specifically in the arm, leg, shoulder, or tongue)
  • muscle cramps
  • muscle stiffness (spasticity)
  • muscle weakness (specifically in the neck, arm, or leg)
  • slurred speech
  • nasal speech
  • difficulty chewing
  • difficulty swallowing
Was this helpful?

What does ALS feel like in the hands?

Hands might feel weak in ALS but not numb or painful. You might drop things or have problems grasping objects. Symptoms are usually asymmetrical, so you might feel that one hand is weaker than the other.

Which fingers are affected by ALS?

ALS typically affects the thumb and forefinger, specifically the abductor pollicis brevis (ABD) and first dorsal interosseous (FDI).

Are there other split limb signs that suggest ALS?

Researchers suggest that a split elbow and a split leg might also indicate ALS.

Split elbow is when the bicep muscle is weaker than the tricep muscle. There is conflicting research on this in ALS.

Split leg is a foot drop from the asymmetrical weakening of ankle dorsiflexion muscles. However, foot drop is not unique to ALS.

Most people with ALS experience split hand at some point in the course of the condition. The split hand test can support an ALS diagnosis, but the presence of split hand alone is not conclusive of ALS. The test has limitations, and split hand might be a symptom of a different medical condition.