Ischemic monomelic neuropathy (IMN) is an infrequent problem that usually occurs after acute arterial occlusion or low blood flow to an extremity. IMN is primarily a problem of the distal nerves in an extremity; it is best thought of as a term referring to multiple distal focal mononeuropathies involving the sensory and motor branches. (See Etiology.)
The condition usually involves axonal nerve injury and is not a demyelinating process. Reversible motor conduction block in the forearm has been reported early in the course of IMN, but this block does not occur later in the disease. The muscle, skin, bone, and other tissues are spared. There have been reported cases of ischemic changes such as pallor, coolness, and even gangrene. The nerves in the distal limb are most affected. (See Etiology and Presentation.)
IMN can occur after surgical procedures (for example, vascular surgery involving the thoracoabdominal aorta and its caudal arterial channels), as a result of the establishment of arteriovenous fistulas for hemodialysis access, or because of the development of arterial emboli or of thrombi. (See Etiology.)
As with all medical conditions, explaining the problems associated with nerve injury can help patients to cope with issues related to the disorder and with rehabilitation. Patients should be instructed to care meticulously for their insensate skin areas in order to prevent skin breakdown. (See Treatment.)
The primary causes for IMN in the arm include any problem that causes a hypercoagulable state, as well as thoracic outlet obstruction (with angiographic confirmation), trauma or laceration of the brachial artery, intra-arterial injection, and cellulitis. A shunt for an arteriovenous fistula also can be a culprit; most cases have been described in patients with diabetes, thus confounding the development of IMN superimposed on diabetic polyneuropathy.
Risk factors include any problems that interrupt and cause acute arterial occlusion. These factors can include hypercoagulable states, methysergide (a vasoconstrictor used for migraine), cannulization of the femoral artery, aortic occlusion from arteriosclerotic vascular disease, abdominal aortic aneurysms, peripheral vascular disease, low cardiac output, an intra-aortic balloon pump (IABP),  or prolonged tourniquet time in surgery. The limb that has been cannulated for the IABP or for cardiopulmonary bypass is particularly susceptible to IMN.
A study by Biancari et al concluded that patients who, at the time of CABG, have diabetes, extracardiac arteriopathy, and a reduced glomerular filtration rate are at risk for developing late lower limb ischemia. The study involved more than 1300 patients who underwent coronary artery bypass grafting (CABG) between 1990 and 2006. The investigators found that during a mean follow-up period of 7.1 years, vascular procedures for lower limb ischemia were performed on 111 patients, with 25 major amputations taking place.  The specific incidence of IMN is not noted in this study.
In chronic IMN, the etiology is peripheral vascular disease. Upper and lower extremity IMN can be related to noniatrogenic and iatrogenic causes.
Occurrence in the United States
The frequency of IMN's occurrence is unclear because IMN has never been studied prospectively, and recognition of the problem varies. Given these limitations, IMN appears to be a rare condition, but it may be underreported. The Mayo Clinic reported only 32 patients with IMN from 1962-1987. Honet and colleagues, in a study reviewing the initial use of the IABP in the early 1970s, reported a surprisingly high incidence of IMN.  Out of 39 patients who survived the initial use of the IABP, 6 of them were referred for leg problems that were diagnosed as IMN. Current use of the IABP is unlikely to have an incidence nearly this high.
IMN can also occur after the establishment of arteriovenous fistulas for hemodialysis access, but again, the frequency of such incidences is unclear and unusual.
Sex- and age-related demographics
Male-to-female ratios have not been clearly documented. The risk for arteriosclerosis is slightly higher in males; therefore, the incidence of IMN is slightly greater in males. Because they are at higher risk for arteriosclerosis, elderly individuals have a much higher risk for IMN.
Mortality is not associated with IMN; however, morbidity is significant. Depending on the severity of the injury, patients can be left with neurologic deficits that interfere with function and cause significant pain.
The prognosis depends on the amount of initial nerve damage. Functional improvement in the leg is expected because the nerve regenerates at 1 in/month (2.5 cm/mo). Weakness of the intrinsics is common in the leg, and weakness of ankle dorsiflexors and plantar flexors also is relatively common. Sensory loss in the foot and in the leg up to the midcalf is frequent as well. Pain in the areas of neurologic deficit is reported almost universally. This pain can be persistent or reversible, depending on the severity of the injury. Occasionally, there is weakness of knee extensors and flexors. Sensation in the foot and distal leg can remain permanently impaired. Contractures of the paralyzed or weakened joints are frequent. Residual pain may be the most troubling problem following nerve regeneration and rehabilitation.
The prognosis for the arm is similar to that for the leg. The nerve regenerates at 1 in/month (2.5 cm/mo). In the hand, weakness of the thenar and intrinsic muscles has been reported. This weakness coexists with sensory loss, which exists in all fingers and can extend proximally into the palm and hand. Contractures of weakened joints, especially those of the fingers, are possible.
The extent of disability from the above neurologic dysfunctions depends on the amount of nerve injury and may alter hand function or the ability to walk. For some patients, the pain associated with IMN is the most disabling aspect of the disease.
What would you like to print?