Hereditary sensory autonomic neuropathy (HSAN) is a group of rare peripheral neuropathies where neurons and/or axons are affected. The major feature of these conditions is the loss of large myelinated and unmyelinated fibers.Myelin is an insulating layer, or sheath that forms around nerves, made up of protein and fatty substances, that allows electrical impulses to transmit along the nerve cells. If myelin is damaged, these impulses slow down. Symptoms of HSAN include diminished sensation of pain and its associated consequences of delayed healing, Charcot arthopathies, infections, osteomyelitis, and amputations.

They have been categorized into types one through five, although some children do not fit well into this classification and do not all have altered pain sensation and/or autonomic function.

• HSAN type I is the most common form of HSAN. It is caused by a mutation in the SPTLC1 gene and inherited in an autosomal dominant pattern.
• HSAN type 2 is caused by mutations in the WNK1 gene and inheritance is autosomal recessive.
• HSAN type 3 (Riley-Day syndrome or familial dysautonomia) is caused by mutations in the IKBKAP gene and inheritance is autosomal recessive.
• HSAN type 4, also called congenital insensitivity to pain with anhidrosis (CIPA), is caused by mutations in the NTRK1 gene and is an autosomal recessive disorder.
• HSAN type 5 is caused by mutations in the NGFB gene and inherited in an autosomal recessive manner.

Hereditary sensory neuropathy type 1 (HSN1) is a neurological condition characterized by nerve abnormalities in the legs and feet. Many people with this condition have tingling, weakness, and a reduced ability to feel pain and sense hot and cold. Some affected people do not lose sensation, but instead feel shooting pains in their legs and feet. As HSN1 progresses, sensory problems can affect the hands, arms, shoulders, and abdomen. In rare cases, people with this condition develop sensorineural hearing loss. Symptoms of HSN1 typically begin during a person's teens or twenties and worsen over time.

HSN1 is caused by mutations in any of several genes, depending on the form of HSN1 (HSN1A is caused by mutations in the SPTLC1 gene; HSN1B is linked to a gene located in chromosome 3; HSN1C is caused by mutations in the SPTLC2 gene; HSN1D is caused by mutations in the ATL1 gene and HSN1E is caused by mutations in DNMT1 gene. All forms of HSN1 are inherited in an autosomal dominant manner. If symptoms are treated properly, the condition does not appear to affect life expectancy.

Hereditary sensory neuropathy type IA is a condition characterized by nerve abnormalities in the legs and feet (peripheral neuropathy). Many people with this condition experience prickling or tingling sensations (paresthesias), numbness, and a reduced ability to feel pain and sense hot and cold. Some affected individuals do not lose sensation, but instead feel shooting pains in their legs and feet. As the disorder progresses, the sensory abnormalities can affect the hands, arms, shoulders, joints, and abdomen. Affected individuals may also experience muscle wasting and weakness as they get older. Weakness in the ankle muscles can make walking difficult. As the condition progresses, some people with hereditary sensory neuropathy type IA require wheelchair assistance.

Individuals with hereditary sensory neuropathy type IA typically get open sores (ulcers) on their feet or hands or infections of the soft tissue of the fingertips (whitlows) that are slow to heal. Because affected individuals cannot feel the pain of these sores, they may not seek immediate treatment. Without treatment, the ulcers can become infected and may require amputation of the surrounding area or limb.

Some people with hereditary sensory neuropathy type IA develop hearing loss caused by abnormalities of the inner ear (sensorineural hearing loss). Hearing loss typically develops in middle to late adulthood.

The signs and symptoms of hereditary sensory neuropathy type IA can begin anytime between adolescence and late adulthood. While the features of this condition tend to worsen over time, affected individuals have a normal life expectancy if signs and symptoms are properly treated.

Hereditary sensory and autonomic neuropathy type II (HSAN2) is a condition that primarily affects the sensory nerve cells (sensory neurons), which transmit information about sensations such as pain, temperature, and touch to the brain. These sensations are impaired in people with HSAN2. In some affected people, the condition may also cause mild abnormalities of the autonomic neurons, which control involuntary body functions such as heart rate, digestion, and breathing. The sensory and autonomic neurons are part of the body's peripheral nervous system, which comprises the nerves outside the brain and spinal cord. HSAN2 is considered a form of peripheral neuropathy.

The signs and symptoms of HSAN2 typically begin in infancy or early childhood. The first sign of the condition is usually numbness in the hands and feet. Soon after, affected individuals lose the ability to feel pain or sense hot and cold. In people with HSAN2, unnoticed injuries often lead to open sores (ulcers) on the hands and feet. Because affected individuals cannot feel the pain of these sores, they may not seek treatment right away. Without treatment, the ulcers can become infected and may require amputation of the affected area. People with HSAN2 often injure themselves unintentionally, typically by biting the tongue, lips, or fingers. These injuries may lead to loss of the affected areas, such as the tip of the tongue. Affected individuals often have injuries and fractures in their hands, feet, limbs, and joints that go untreated because of the inability to feel pain. Repeated injury can lead to a condition called Charcot joints, in which the bones and tissue surrounding joints are damaged.

The effects of HSAN2 on the autonomic nervous system are more variable. Some infants with HSAN2 have digestive problems such as the backflow of stomach acids into the esophagus (gastroesophageal reflux) or slow eye-blink or gag reflexes. Affected individuals may also have weak deep-tendon reflexes, such as the reflex being tested when a doctor taps the knee with a hammer.

Some people with HSAN2 lose a type of taste bud on the tip of the tongue called lingual fungiform papillae and have a diminished sense of taste.