Diabetic Neuropathies
If sought, evidence of neuropathy can be found in a majority
of diabetics within 10 years of diagnosis. The forms of neuropathy
are usually divided into, by order of frequency, polyneuropathies (primarily
symmetric and sensory), autonomic neuropathies, and mononeuropathies.
Diabetic amyotrophy, an uncommon syndrome defined by the triad of pain,
severe asymmetric muscle weakness (usually of the lower extremities), and
wasting of the iliopsoas and quadriceps, falls under the heading of the
polyneuropathies. There has never been complete agreement about the
etiology diabetic neuropathy. The mononeuropathies are most likely
ischemic in nature, as suggested by the tendency toward nerves susceptible
to compression and and toward fiber subtypes with the more tenuous blood
supply. The polyneuropathies are thought to be caused by a combination
of ischemia and abnormal nerve metabolism.
Mononeuropathies
Although there is no great prevalence data, mononeuropathies
are known to be more common among diabetics than non-diabetics. One
half to one percent of diabetics are expected to have a mononeuropathy
at some point. Symptoms are generally localized to common sites of
nerve entrapment (supporting the hypothesis that nerve injury occurs when
an already tenuous blood supply is further compromised by external compression
or nerve swelling). The sights of most common involvement are shown
in the table below. This comes from a study which enrolled the 51
patients who presented to a large diabetes with mononeuropathies over the
course of 3 years.
Median, ulnar and common peroneal (lateral popliteal)
nerves are the most commonly affected nerves in all studies. Median
nerve palsies are more common in women (as in non-diabetic carpal tunnel
syndrome) while the others show no preference. Median neuropathies
usually present with dysesthesias and numbness of the radial 2/3 of the
palm and weakness of the index finger and thumb opposition and abduction.
Ulnar palsy usually present with sensory findings of numbness over the
ulnar aspect of the palm and 4th and 5th finger. Claw hand deformity
is rare. These two neuropathies are commonly acute while others are
more slow in onset. Symptoms can rarely be traced to nerve injury
above the level of the wrist. Common peroneal neuropathies almost
always present with a foot drop and usually some dorsal numbness.
Compression around the head of the fibula is usually suspected.
Diabetic cranial neuropathies usually affect CN
3 or 6. The symptoms are usually acute in onset (despite the data
above) and may or may not be accompanied by pain. Like all of the
mononeuropathies they are thought to be ischemic in nature and are usually
fairly short lived. Occulomotor palsies due to diabetes spare the
pupil 75% of the time. This supports the hypothesis about an ischemic
etiology as the most peripheral fibers in the nerve, closest to the vaso-nervorum,
are the parasympathetic fibers.
Work up of a diabetic presenting with diplopia can
be more complicated than the work up of other mononeuropathies because
of the breadth of the differential. In general, however, if diplopia
referable to a single cranial nerve occurs in isolation, the lesion can
be localized to the subarachnoid course of the 3rd or 6th nerve.
It would be very unlikely that a brainstem or cavernous sinus process could
affect a single nerve in isolation. The differential usually boils
down to an ischemic neuropathy or external compression by an aneurysm.
Inflammatory neuropathies (e.g. Lyme disease) remain in the differential
but these are far less common. Recommendations are usually that a
patient over 50 with a 3rd or 6th nerve palsy can be observed and only
imaged if symptoms progress or don’t resolve in 3 months. Patients
less than 50, or those with pupillary involvement at any age should be
imaged because aneurysmal compression moves up on the differential.
(There is debate about whether it is safe to observe patients with a incomplete
3rd nerve palsies, irrespective of age and pupil involvement).
Treatment
Like treatment for other diabetic neuropathies,
treatment is symptomatic. Improved blood sugar control is recommended
but it has never been proven to hasten functional recovery.
Interestingly, there doesn’t seem to be an association between diabetic
mononeuropathy and other forms of diabetic neuropathy.
Adam Cifu
References
Fraser, Campbell, Ewing , and Clark, Mononeuropathy in Diabetes Mellitus,
Diabetes. 1979;28:96-101.
Rowland, Lewis P. Merrit’s Testbook of Neurology, 9th edition, 669-772.
Trobe, JD. Isolated Pupil Sparing Third Nerve Palsy, Ophthalmology.
1985;92:58-61.