A timely Christmas blog post! At an MS2 session yesterday, we discussed the following practice question:
Blunt trauma to the elbow may lead to the development of:
a) Wrist drop
b) Weakness of the abductor pollicis brevis
c) Claw hand or benediction sign
d) Ulnar deviation of the hand
e) Poor pronation of the forearm
There ensued a somewhat confusing (because of my own confusion) discussion of what exactly is a benediction sign, is it different from a claw hand, what specific muscles are involved, etc. We’ll take a deep dive into that below, but first let’s just address the test question itself.
Blunt trauma to the elbow will most often injure the ulnar nerve. The ulnar nerve lies in a little groove between the medial humeral epicondyle and the olecranon. Partially flex your elbow to stretch the nerve a bit and then run your fingers in there and tap on the nerve; you’ll feel a little zinger in the medial hand. The median and radial nerves run deeper in the arm and aren’t typically injured by minor elbow (“funny bone”) trauma. So, we know that we’re looking at the answer choices for an ulnar nerve lesion.
When caused by a peripheral nerve lesion, wrist drop is due to injury of the radial nerve, so a) is out.
Abductor pollicis brevis is innervated by the median nerve, so b) is out.
Ulnar deviation of the hand (most apparent upon attempted flexion at the wrist) would also be due to a median nerve injury. Why? In a (proximal) median nerve lesion, there is unopposed action of flexor carpi ulnaris (ulnar nerve), pulling the wrist medially / to the ulnar side. In contrast, an ulnar lesion would result in unopposed action of flexor carpi radialis (median nerve) and would pull the wrist laterally / to the radial side. Thus, d) is out.
Finally, weakness of pronation is also a median nerve lesion, because pronators teres and quadratus are from the median and anterior interosseus nerves, respectively (the latter is a branch of the former). So that rules out e), leaving the answer as c) Claw hand or benediction sign.
So let’s talk about the claw hand. When attempting to open the hand with an ulnar nerve lesion, the thumb, index, and middle fingers extend normally. The 3rd and 4th lumbricals, which flex the metacarpophalangeal (MCP) joints and extend the interphalangeal (IP) joints of the ring and pinkie fingers, are weak. Unopposed by those weak muscles, the radially-innervated extensor digitorum makes the MCP joints extend and the ulnar-innervated 3rd and 4th flexor digitorum profundi make the IP joints flex, creating a claw-like appearance on the medial side of the hand:
But wait a minute! We just said that the ulnar nerve is injured at the elbow–shouldn’t the ulnar-innervated flexor digitorum profundus muscles be weak as well and NOT flex the interphalangeal joints and NOT cause clawing? Here’s where the nuances of neurology make things confusing. As described by Patten, not all sub-components of an injured nerve are affected equally. For example, sciatic nerve lesions can affect the peroneal (fibular) part of the nerve and spare the tibial part. In a proximal ulnar nerve lesion, the nerve fibers serving the flexor digitorum profundi can be spared, while the small muscles intrinsic to the hand are affected, resulting in the clawing.
Turning to the “hand of benediction”, I found conflicting descriptions in two authoritative texts. Preston and Shapiro describe it as an ulnar lesion that is synonymous with a claw hand deformity, as above and as per the test question.
However, in Brazis, the benediction sign is described as a problem in closing the hand with a median nerve lesion. The ulnar-innervated flexors of the ring and pinkie fingers function normally. The median-innervated flexors of the thumb and first two digits are weak and don’t flex:
The orthopedist who created the animation below takes the Brazis position–that the hand of benediction is actually a median nerve lesion. He also asserts that only distal ulnar lesions spare the profundi and that lesions at the elbow would affect the profundi and not cause clawing; we covered that above. But with those caveats, I still think that this animation nicely shows what median and ulnar nerve lesions do to all of these confusing little muscles in the hands; for the purpose of learning the neuroanatomy, I think it’s definitely worth 6 minutes of your time:
We see that even authorities disagree on whether the hand of benediction is an ulnar / inability to open the hand problem vs. a median / inability to close the hand problem. In my quick research of the issue, I found this neat little paper that makes, I think, a compelling argument for the ulnar localization. They first consider the question of how one peacefully greets another person or moves to bless him; it’s with an open hand–not a fist! This suggests that a hand of benediction is properly an abnormal opening of the hand and not an abnormal closing. They go on to analyze ancient paintings and other depictions of St. Peter, finding that his medial MCP joints appear to be hyperextended, as one would expect if he had an ulnar lesion. So their theory is that the hand of benediction originated with an ulnar neuropathy in the original Pontiff.
Thus it would seem that Preston and Shapiro have it right, but if any neuromuscular experts happen to be reading this, I’d certainly appreciate your comments! More important for the student of neurology, we should remember that all of the above confusion stems from the use of historical nomenclature and, in this instance, from the inherent limitations of multiple choice test questions. In real life, the patient doesn’t come to you asking for your assessment of her “hand of benediction” (with a list of choices); she comes stating that her hand doesn’t look right, or is weak, or became weak after bonking her elbow, or something along those lines. Neurological examination, including motor, reflex, and sensory functions, would make localization fairly straightforward. Then, when recording the findings, it’s best to describe them carefully, localize them to the appropriate nerve, root, etc. and develop a differential diagnosis, staying away from ambiguous terminology. The question isn’t whether the patient has a claw hand or a hand of benediction or whether those are truly synonymous. The question is whether the patient has an ulnar nerve lesion, a median nerve lesion, or some other lesion, and why.
Just to reinforce this last point about terminology, consider the evaluation of level of arousal. You’re the resident cross covering overnight and you’re called to evaluate a patient for a change in mental status. The morning progress note states that he was “obtunded”. Now, Plum and Posner do have a specific definition for that term, but the reality is that people use words such as “lethargic” and “obtunded” in varying ways; when you evaluate the patient, you can’t be sure from that one word what he truly looked like before (tip: ask the nurse!) It’s much better to describe the patient’s condition in more detail: “On exam, his eyes were closed. To gentle sternal rub and voice, he opened his eyes, made brief eye contact, and followed some simple commands, going immediately back to sleep when stimulation ceased”. With that kind of description in the chart, the next person can make an accurate assessment of whether the patient’s condition has changed.
Well, that was fun! Again, if anyone has more insight into the matter, just leave some comments below.