Diabetic neuropathy can affect any peripheral nerves including
sensory neurons,
motor neurons, and the
autonomic nervous system. Therefore, diabetic neuropathy has the potential to affect essentially any organ system and can cause a range of symptoms. There are several distinct syndromes based on the organ systems affected.
Sensorimotor polyneuropathy Longer nerve fibers are affected to a greater degree than shorter ones because
nerve conduction velocity is slowed in proportion to a nerve's length. In this syndrome, decreased sensation and loss of reflexes occur first in the toes on each foot, then extend upward. It is usually described as a glove-stocking distribution of numbness, sensory loss,
dysesthesia and nighttime pain. The pain can feel like burning, pricking sensation, achy or dull. A pins and needles sensation is common. Loss of
proprioception, the sense of where a limb is in space, is affected early. These patients cannot feel when they are stepping on a foreign body, like a splinter, or when they are developing a callus from an ill-fitting shoe. Consequently, they are at risk of developing
ulcers and infections on the feet and legs, which can lead to
amputation. Similarly, these patients can get multiple fractures of the knee, ankle or foot, and develop a
Charcot joint. Loss of motor function results in dorsiflexion,
contractures of the toes, and loss of the interosseous muscle function that leads to contraction of the digits, so-called
hammer toes. These contractures occur not only in the foot but also in the hand where the loss of the musculature makes the hand appear gaunt and skeletal. The loss of muscular function is progressive.
Autonomic neuropathy The
autonomic nervous system is composed of nerves serving the
heart,
lungs,
blood vessels,
bone,
adipose tissue,
sweat glands,
gastrointestinal system and
genitourinary system.
Autonomic neuropathy can affect any of these organ systems. One commonly recognized autonomic dysfunction in diabetics is
orthostatic hypotension, or becoming dizzy and possibly
fainting when standing up due to a sudden drop in blood pressure. In the case of diabetic autonomic neuropathy, it is due to the failure of the heart and arteries to appropriately adjust heart rate and vascular tone to keep blood continually and fully flowing to the
brain. This symptom is usually accompanied by a loss of
respiratory sinus arrhythmia – the usual change in heart rate seen with normal breathing. These two findings suggest autonomic neuropathy. Gastrointestinal manifestations include
gastroparesis,
nausea,
bloating, and
diarrhea. Because many diabetics take oral medication for their diabetes, absorption of these medicines is greatly affected by the delayed gastric emptying. This can lead to
hypoglycemia when an oral diabetic agent is taken before a meal and does not get absorbed until hours, or sometimes days later when there is normal or low blood sugar already. Sluggish movement of the
small intestine can cause
bacterial overgrowth, made worse by the presence of
hyperglycemia. This leads to
bloating, gas and
diarrhea. Urinary symptoms include urinary frequency, urgency,
incontinence and retention. Again, because of the retention of
urine,
urinary tract infections are frequent. Urinary retention can lead to bladder
diverticula, kidney stones, and
reflux nephropathy.
Cranial neuropathy When
cranial nerves are affected, neuropathies of the
oculomotor nerve (cranial nerve #3 or CNIII) are most common. The oculomotor nerve controls all the muscles that move the
eye except for the
lateral rectus and
superior oblique muscles. It also serves to constrict the
pupil and open the eyelid. The onset of a diabetic third nerve palsy is usually abrupt, beginning with frontal or pain around the eye and then
double vision. All the oculomotor muscles innervated by the third nerve may be affected, but those that control pupil size are usually well-preserved early on. This is because the
parasympathetic nerve fibers within CNIII that influence pupillary size are found on the periphery of the nerve (in terms of a cross-sectional view), which makes them less susceptible to ischemic damage (as they are closer to the vascular supply). The sixth nerve, the
abducens nerve, which innervates the lateral rectus muscle of the eye (moves the eye laterally), is also commonly affected but fourth nerve, the
trochlear nerve, (innervates the superior oblique muscle, which moves the eye downward) involvement is unusual.
Damage to a specific nerve of the thoracic or lumbar
spinal nerves can occur and may lead to painful syndromes that mimic
a heart attack,
gallbladder inflammation, or
appendicitis. Diabetics have a higher incidence of entrapment neuropathies, such as
carpal tunnel syndrome. ==Pathogenesis==