Amyotrophic lateral sclerosis The disorder causes muscle weakness and atrophy throughout the body caused by degeneration of the upper and lower motor neurons. Unable to function, the muscles weaken and atrophy. Affected individuals may ultimately lose the ability to initiate and control all voluntary movement, although bladder and bowel sphincters and the muscles responsible for eye movement are usually, but not always, spared. Cognitive function is generally spared for most patients although some (~5%) also have frontotemporal dementia.A higher proportion of patients (~30-50%) also have more subtle cognitive changes which may go unnoticed but are revealed by detailed neuropsychological testing. Sensory nerves and the autonomic nervous system, which controls functions like sweating, are generally unaffected but may be involved for some patients. The earliest symptoms of ALS are typically obvious weakness and/or muscle atrophy. Other presenting symptoms include muscle fasciculation (twitching), cramping, or stiffness of affected muscles; muscle weakness affecting an arm or a leg; and/or slurred and nasal speech. The parts of the body affected by early symptoms of ALS depend on which motor neurons in the body are damaged first. About 75% of people contracting the disease experience "limb onset" ALS i.e. first symptoms in the arms ("upper limb", not to be confused with "upper motor neuron") or legs ("lower limb", not to be confused with "lower motor neuron"). Patients with the leg onset form may experience awkwardness when walking or running or notice that they are tripping or stumbling, often with a "dropped foot" which drags along the ground. Arm-onset patients may experience difficulty with tasks requiring manual dexterity such as buttoning a shirt, writing, or turning a key in a lock. Occasionally, the symptoms remain confined to one limb for a long period of time or for the whole course of the illness; this is known as monomelic amyotrophy. About 25% of cases are "bulbar onset" ALS. These patients first notice difficulty speaking clearly or swallowing. Speech may become slurred, nasal in character, or quieter. Other symptoms include difficulty swallowing, and loss of tongue mobility. A smaller proportion of patients experience "respiratory onset" ALS where the intercostal muscles that support breathing are affected first. Regardless of the part of the body first affected by the disease, muscle weakness and atrophy spread to other parts of the body as the disease progresses. Patients experience increasing difficulty moving, swallowing (dysphagia), and speaking or forming words (dysarthria). Symptoms of upper motor neuron involvement include tight and stiff muscles (spasticity) and exaggerated reflexes (hyperreflexia) including an overactive gag reflex. An abnormal reflex commonly called Babinski's sign (the big toe extends upward and other toes spread out) also indicates upper motor neuron damage. Symptoms of lower motor neuron degeneration include muscle weakness and atrophy, muscle cramps, and fleeting twitches of muscles that can be seen under the skin (fasciculations). Around 15–45% of patients experience pseudobulbar affect, also known as "emotional lability", which consists of uncontrollable laughter, crying or smiling, attributable to degeneration of bulbar upper motor neurons resulting in exaggeration of motor expressions of emotion.