Amyotrophic lateral sclerosis (ALS) is a progressive neurological disease that affects motor neurons, which are the nerve cells that control movement.

Evidence of lower motor neuron degeneration is one of the El Escorial criteria for ALS diagnosis. Lower motor neurons are those that go from the spinal cord to a muscle, whereas upper motor neurons are situated in the brain and spinal cord and send signals to the lower motor neurons to control movement.

Several tests can be done to confirm an ALS diagnosis. A nerve conduction study, which assesses the neuropathological features of lower motor neurons, is one such test. Another, also used for neuropathological examination, involves taking a small nerve biopsy.

How a nerve biopsy is performed

A nerve biopsy is performed under local anesthesia. The surgeon makes an incision about 2 inches long at a site where the nerve is accessible and runs below the skin. The skin is retracted, and about 2 inches of the nerve is removed and sent for pathological examination, where the sample is viewed under a microscope. Special stains may be used to visualize certain areas or proteins within the sample.

The incision is closed by below-the-skin suturing so that the suture is not visible. The part of the body where the biopsy is taken needs to rest for a few days, and patients can usually resume full activity after one week.

What is seen in a nerve biopsy

Nerve cells have specific characteristics, and a nerve biopsy is used to detect abnormalities. For example, nerve fibers, or axons, are surrounded by a myelin sheath necessary for the fast transduction of nervous signals. Degeneration of the axon or the myelin sheath can be easily detected with a nerve biopsy.

In ALS, the axons of upper and lower motor neurons have often degenerated.

There are also specific markers that are useful for the diagnosis of ALS, one of which is the presence of Bunina bodies, small inclusions in the lower motor neurons present in 70-100 percent of ALS patients.

Another useful marker for ALS diagnosis is ubiquitinated inclusions. Ubiquitin is a small protein that is attached to other proteins to mark them for degradation, a normal process involved in the life cycle of proteins. This is known as ubiquitination. These kinds of inclusions are specific for ALS and occur in up to 95 percent of patients.

Hyaline conglomerate inclusions (HCI) are small inclusions that also occur in motor neurons. Although they are not specific for ALS and are also seen in other neurodegenerative diseases,  they can help confirm an ALS diagnosis when assessed in combination with other markers and clinical history.

